{"id":1606,"date":"2015-04-23T22:38:08","date_gmt":"2015-04-23T22:38:08","guid":{"rendered":"https:\/\/courses.candelalearning.com\/oschemtemp\/?post_type=chapter&#038;p=1606"},"modified":"2016-10-27T17:16:10","modified_gmt":"2016-10-27T17:16:10","slug":"molecular-and-ionic-compounds-2","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-chem-atoms-first\/chapter\/molecular-and-ionic-compounds-2\/","title":{"raw":"Molecular and Ionic Compounds","rendered":"Molecular and Ionic Compounds"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\nBy the end of this section, you will be able to:\r\n<ul>\r\n \t<li>Define ionic and molecular (covalent) compounds<\/li>\r\n \t<li>Predict the type of compound formed from elements based on their location within the periodic table<\/li>\r\n \t<li>Determine formulas for simple ionic compounds<\/li>\r\n<\/ul>\r\n<\/div>\r\nIn ordinary chemical reactions, the nucleus of each atom (and thus the identity of the element) remains unchanged. Electrons, however, can be added to atoms by transfer from other atoms, lost by transfer to other atoms, or shared with other atoms. The transfer and sharing of electrons among atoms govern the chemistry of the elements. During the formation of some compounds, atoms gain or lose electrons, and form electrically charged particles called ions (Figure\u00a01).\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"879\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/887\/2015\/04\/23211101\/CNX_Chem_02_06_NaCation1.jpg\" alt=\"Figure\u00a0A shows a sodium atom, N a, which has a nucleus containing 11 protons and 12 neutrons. The atom\u2019s surrounding electron cloud contains 11 electrons. Figure\u00a0B shows a sodium ion, N a superscript plus sign. Its nucleus contains 11 protons and 12 neutrons. The ion\u2019s electron cloud contains 10 electrons and is smaller than that of the sodium atom in figure A.\" width=\"879\" height=\"355\" data-media-type=\"image\/jpeg\" \/> Figure\u00a01. (a) A sodium atom (Na) has equal numbers of protons and electrons (11) and is uncharged. (b) A sodium cation (Na+) has lost an electron, so it has one more proton (11) than electrons (10), giving it an overall positive charge, signified by a superscripted plus sign.[\/caption]\r\n\r\nYou can use the periodic table to predict whether an atom will form an anion or a cation, and you can often predict the charge of the resulting ion. Atoms of many main-group metals lose enough electrons to leave them with the same number of electrons as an atom of the preceding noble gas. To illustrate, an atom of an alkali metal (group 1) loses one electron and forms a cation with a 1+ charge; an alkaline earth metal (group 2) loses two electrons and forms a cation with a 2+ charge, and so on. For example, a neutral calcium atom, with 20 protons and 20 electrons, readily loses two electrons. This results in a cation with 20 protons, 18 electrons, and a 2+ charge. It has the same number of electrons as atoms of the preceding noble gas, argon, and is symbolized Ca<sup>2+<\/sup>. The name of a metal ion is the same as the name of the metal atom from which it forms, so Ca<sup>2+<\/sup> is called a calcium ion.\r\n\r\nWhen atoms of nonmetal elements form ions, they generally gain enough electrons to give them the same number of electrons as an atom of the next noble gas in the periodic table. Atoms of group 17 gain one electron and form anions with a 1\u2212 charge; atoms of group 16 gain two electrons and form ions with a 2\u2212 charge, and so on. For example, the neutral bromine atom, with 35 protons and 35 electrons, can gain one electron to provide it with 36 electrons. This results in an anion with 35 protons, 36 electrons, and a 1\u2212 charge. It has the same number of electrons as atoms of the next noble gas, krypton, and is symbolized Br<sup>\u2212<\/sup>. (A discussion of the theory supporting the favored status of noble gas electron numbers reflected in these predictive rules for ion formation is provided in a later chapter of this text.)\r\n\r\nNote the usefulness of the periodic table in predicting likely ion formation and charge (Figure\u00a02). Moving from the far left to the right on the periodic table, main-group elements tend to form cations with a charge equal to the group number. That is, group 1 elements form 1+ ions; group 2 elements form 2+ ions, and so on. Moving from the far right to the left on the periodic table, elements often form anions with a negative charge equal to the number of groups moved left from the noble gases. For example, group 17 elements (one group left of the noble gases) form 1\u2212 ions; group 16 elements (two groups left) form 2\u2212 ions, and so on. This trend can be used as a guide in many cases, but its predictive value decreases when moving toward the center of the periodic table. In fact, transition metals and some other metals often exhibit variable charges that are not predictable by their location in the table. For example, copper can form ions with a 1+ or 2+ charge, and iron can form ions with a 2+ or 3+ charge.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"879\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/887\/2015\/04\/23211102\/CNX_Chem_02_06_IonCharges1.jpg\" alt=\"Group one of the periodic table contains L i superscript plus sign in period 2, N a superscript plus sign in period 3, K superscript plus sign in period 4, R b superscript plus sign in period 5, C s superscript plus sign in period 6, and F r superscript plus sign in period 7. Group two contains B e superscript 2 plus sign in period 2, M g superscript 2 plus sign in period 3, C a superscript 2 plus sign in period 4, S r superscript 2 plus sign in period 5, B a superscript 2 plus sign in period 6, and R a superscript 2 plus sign in period 7. Group six contains C r superscript 3 plus sign and C r superscript 6 plus sign in period 4. Group seven contains M n superscript 2 plus sign in period 4. Group eight contains F e superscript 2 plus sign and F e superscript 3 plus sign in period 4. Group nine contains C o superscript 2 plus sign in period 4. Group ten contains N i superscript 2 plus sign in period 4, and P t superscript 2 plus sign in period 6. Group 11 contains C U superscript plus sign and C U superscript 2 plus sign in period 4, A g superscript plus sign in period 5, and A u superscript plus sign and A u superscript 3 plus sign in period 6. Group 12 contains Z n superscript 2 plus sign in period 4, C d superscript 2 plus sign in period 5, and H g subscript 2 superscript 2 plus sign and H g superscript 2 plus sign in period 6. Group 13 contains A l superscript 3 plus sign in period 3. Group 14 contains C superscript 4 negative sign in period 2. Group 15 contains N superscript 3 negative sign in period 2, P superscript 3 negative sign in period 3, and A s superscript 3 negative sign in period 4. Group 16 contains O superscript 2 negative sign in period 2, S superscript 2 negative sign in period 3, S e superscript 2 negative sign in period 4 and T e superscript 2 negative sign in period 5. Group 17 contains F superscript negative sign in period 2, C l superscript negative sign in period 3, B r superscript negative sign in period 4, I superscript negative sign in period 5, and A t superscript negative sign in period 6. Group 18 contains H e in period 1, N e in period 2, A r in period 3, K r in period 4, X e in period 5 and R n in period 6.\" width=\"879\" height=\"535\" data-media-type=\"image\/jpeg\" \/> Figure\u00a02. Some elements exhibit a regular pattern of ionic charge when they form ions.[\/caption]\r\n\r\n<div class=\"textbox examples\">\r\n<h3>Example 1:\u00a0<strong>Composition of Ions<\/strong><\/h3>\r\nAn ion found in some compounds used as antiperspirants contains 13 protons and 10 electrons. What is its symbol?\r\n\r\n[reveal-answer q=\"386703\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"386703\"]\r\n\r\nBecause the number of protons remains unchanged when an atom forms an ion, the atomic number of the element must be 13. Knowing this lets us use the periodic table to identify the element as Al (aluminum). The Al atom has lost three electrons and thus has three more positive charges (13) than it has electrons (10). This is the aluminum cation, Al<sup>3+<\/sup>.\r\n\r\n[\/hidden-answer]\r\n<h4><strong>Check Your Learning<\/strong><\/h4>\r\nGive the symbol and name for the ion with 34 protons and 36 electrons.\r\n\r\n[reveal-answer q=\"644629\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"644629\"]Se<sup>2\u2212<\/sup>, the selenide ion[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox examples\">\r\n<h3>Example 2:\u00a0<strong>Formation of Ions<\/strong><\/h3>\r\nMagnesium and nitrogen react to form an ionic compound. Predict which forms an anion, which forms a cation, and the charges of each ion. Write the symbol for each ion and name them.\r\n\r\n[reveal-answer q=\"31643\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"31643\"]\r\n\r\nMagnesium\u2019s position in the periodic table (group 2) tells us that it is a metal. Metals form positive ions (cations). A magnesium atom must lose two electrons to have the same number electrons as an atom of the previous noble gas, neon. Thus, a magnesium atom will form a cation with two fewer electrons than protons and a charge of 2+. The symbol for the ion is Mg<sup>2+<\/sup>, and it is called a magnesium ion.\r\n\r\nNitrogen\u2019s position in the periodic table (group 15) reveals that it is a nonmetal. Nonmetals form negative ions (anions). A nitrogen atom must gain three electrons to have the same number of electrons as an atom of the following noble gas, neon. Thus, a nitrogen atom will form an anion with three more electrons than protons and a charge of 3\u2212. The symbol for the ion is N<sup>3\u2212<\/sup>, and it is called a nitride ion.\r\n\r\n[\/hidden-answer]\r\n<h4><strong>Check Your Learning<\/strong><\/h4>\r\nAluminum and carbon react to form an ionic compound. Predict which forms an anion, which forms a cation, and the charges of each ion. Write the symbol for each ion and name them.\r\n\r\n[reveal-answer q=\"219108\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"219108\"]Al will form a cation with a charge of 3+: Al<sup>3+<\/sup>, an aluminum ion. Carbon will form an anion with a charge of 4\u2212: C<sup>4\u2212<\/sup>, a carbide ion.[\/hidden-answer]\r\n\r\n<\/div>\r\nThe ions that we have discussed so far are called <strong>monatomic ions<\/strong>, that is, they are ions formed from only one atom. We also find many <strong>polyatomic ions<\/strong>. These ions, which act as discrete units, are electrically charged molecules (a group of bonded atoms with an overall charge). Some of the more important polyatomic ions are listed in Table 1. <strong>Oxyanions<\/strong> are polyatomic ions that contain one or more oxygen atoms. At this point in your study of chemistry, you should memorize the names, formulas, and charges of the most common polyatomic ions. Because you will use them repeatedly, they will soon become familiar.\r\n<table id=\"fs-idp278664880\" class=\"span-all\" summary=\"This table has three columns labeled \u201ccharge,\u201d \u201cname,\u201d and \u201cformula.\u201d Ammonium has a charge of positive 1 and the formula N H subscript 4 superscript plus sign. Acetate has a charge of negative 1 and the formula C subscript 2 H subscript 3 O subscript 2 superscript negative sign. Cyanide has a charge of negative 1, and the formula C N superscript negative sign. Hydroxide has a charge of negative 1 and the formula O H superscript negative sign. Nitrate has a charge of negative 1 and the formula N O subscript 3 superscript negative sign. Nitrite has a charge of negative 1 and the formula N O subscript 2 superscript negative sign. Perchlorate has a charge of negative 1 and the formula C l O subscript 4 superscript negative sign. Chlorate has a charge of negative 1 and the formula C l O subscript 3 superscript negative sign. Chlorite has a charge of negative 1 and the formula C l O subscript 2 superscript negative sign. Hypochlorite has a charge of negative 1 and the formula C l O superscript negative sign. Permanganate has a charge of negative 1 and the formula M n O subscript 4 superscript negative sign. Hydrogen carbonate, or bicarbonate has a charge of negative 1 and the formula H C O subscript 3 superscript negative sign. Carbonate has a charge of negative 2 and the formula C O subscript 3 superscript 2 negative sign. Peroxide has a charge of negative 2 and the formula O subscript 2 superscript 2 negative sign. Hydrogen sulfate, or bisulfate, has a charge of negative 1 and the formula H S O subscript 4 superscript negative sign. Sulfate has a charge of negative 2 and the formula S O subscript 4 superscript 2 negative sign. Sulfite has a charge of negative 2 and the formula S O subscript 3 superscript 2 negative sign. Dihydrogen phosphate has a charge of negative 1 and the formula H subscript 2 P O subscript 4 superscript negative sign. Hydrogen phosphate has a charge of negative 2 and the formula H P O subscript 4 superscript 2 negative sign. Phosphate has a charge of negative 3 and the formula P O subscript 4 superscript 3 negative sign.\">\r\n<thead>\r\n<tr>\r\n<th colspan=\"7\">Table 1. Common Polyatomic Ions<\/th>\r\n<\/tr>\r\n<tr>\r\n<th width=\"10%\">Charge<\/th>\r\n<th width=\"20%\">Name<\/th>\r\n<th width=\"10%\">Formula<\/th>\r\n<th width=\"10%\"><\/th>\r\n<th width=\"10%\">Charge<\/th>\r\n<th width=\"30%\">Name<\/th>\r\n<th width=\"10%\">Formula<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr>\r\n<td>1+<\/td>\r\n<td>ammonium<\/td>\r\n<td>[latex]{\\text{NH}}_{4}{}^{+}[\/latex]<\/td>\r\n<td><\/td>\r\n<td>1\u2212<\/td>\r\n<td>permanganate<\/td>\r\n<td>[latex]{\\text{MnO}}_{4}{}^{-}[\/latex]<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>1\u2212<\/td>\r\n<td>acetate<\/td>\r\n<td>[latex]{\\text{C}}_{2}{\\text{H}}_{3}{\\text{O}}_{2}{}^{-}[\/latex]<\/td>\r\n<td><\/td>\r\n<td>1\u2212<\/td>\r\n<td>hydrogen carbonate, or bicarbonate<\/td>\r\n<td>[latex]{\\text{HCO}}_{3}{}^{-}[\/latex]<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>1\u2212<\/td>\r\n<td>cyanide<\/td>\r\n<td>[latex]\\text{CN}^-[\/latex]<\/td>\r\n<td><\/td>\r\n<td>2\u2212<\/td>\r\n<td>carbonate<\/td>\r\n<td>[latex]{\\text{CO}}_{3}{}^{2-}[\/latex]<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>1\u2212<\/td>\r\n<td>hydroxide<\/td>\r\n<td>[latex]\\text{OH}^-[\/latex]<\/td>\r\n<td><\/td>\r\n<td>2\u2212<\/td>\r\n<td>peroxide<\/td>\r\n<td>[latex]{\\text{O}}_{2}{}^{2-}[\/latex]<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>1\u2212<\/td>\r\n<td>nitrate<\/td>\r\n<td>[latex]{\\text{NO}}_{3}{}^{-}[\/latex]<\/td>\r\n<td><\/td>\r\n<td>1\u2212<\/td>\r\n<td>hydrogen sulfate, or bisulfate<\/td>\r\n<td>[latex]{\\text{HSO}}_{4}{}^{-}[\/latex]<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>1\u2212<\/td>\r\n<td>nitrite<\/td>\r\n<td>[latex]{\\text{NO}}_{2}{}^{-}[\/latex]<\/td>\r\n<td><\/td>\r\n<td>2\u2212<\/td>\r\n<td>sulfate<\/td>\r\n<td>[latex]{\\text{SO}}_{4}{}^{2-}[\/latex]<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>1\u2212<\/td>\r\n<td>perchlorate<\/td>\r\n<td>[latex]{\\text{ClO}}_{4}{}^{-}[\/latex]<\/td>\r\n<td><\/td>\r\n<td>2\u2212<\/td>\r\n<td>sulfite<\/td>\r\n<td>[latex]{\\text{SO}}_{3}{}^{2-}[\/latex]<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>1\u2212<\/td>\r\n<td>chlorate<\/td>\r\n<td>[latex]{\\text{ClO}}_{3}{}^{-}[\/latex]<\/td>\r\n<td><\/td>\r\n<td>1\u2212<\/td>\r\n<td>dihydrogen phosphate<\/td>\r\n<td>[latex]{\\text{H}}_{2}{\\text{PO}}_{4}{}^{-}[\/latex]<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>1\u2212<\/td>\r\n<td>chlorite<\/td>\r\n<td>[latex]{\\text{ClO}}_{2}{}^{-}[\/latex]<\/td>\r\n<td><\/td>\r\n<td>2\u2212<\/td>\r\n<td>hydrogen phosphate<\/td>\r\n<td>[latex]{\\text{HPO}}_{4}{}^{2-}[\/latex]<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>1\u2212<\/td>\r\n<td>hypochlorite<\/td>\r\n<td>[latex]\\text{ClO}^-[\/latex]<\/td>\r\n<td><\/td>\r\n<td>3\u2212<\/td>\r\n<td>phosphate<\/td>\r\n<td>[latex]{\\text{PO}}_{4}{}^{3-}[\/latex]<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nNote that there is a system for naming some polyatomic ions; <em>-ate<\/em> and <em>-ite<\/em> are suffixes designating polyatomic ions containing more or fewer oxygen atoms. <em>Per-<\/em> (short for \u201chyper\u201d) and <em>hypo-<\/em> (meaning \u201cunder\u201d) are prefixes meaning more oxygen atoms than <em>-ate<\/em> and fewer oxygen atoms than <em>-ite<\/em>, respectively. For example, perchlorate is [latex]{\\text{ClO}}_{4}{}^{-}[\/latex], chlorate is [latex]{\\text{ClO}}_{3}{}^{-}[\/latex], chlorite is [latex]{\\text{ClO}}_{2}{}^{-}[\/latex] and hypochlorite is ClO<sup>-<\/sup>. Unfortunately, the number of oxygen atoms corresponding to a given suffix or prefix is not consistent; for example, nitrate is [latex]{\\text{NO}}_{3}{}^{-}[\/latex] while sulfate is [latex]{\\text{SO}}_{4}{}^{2-}.[\/latex] This will be covered in more detail in the next module on nomenclature.\r\n\r\nThe nature of the attractive forces that hold atoms or ions together within a compound is the basis for classifying chemical bonding. When electrons are transferred and ions form, <strong>ionic bonds<\/strong> result. Ionic bonds are electrostatic forces of attraction, that is, the attractive forces experienced between objects of opposite electrical charge (in this case, cations and anions). When electrons are \u201cshared\u201d and molecules form, <strong>covalent bonds<\/strong> result. Covalent bonds are the attractive forces between the positively charged nuclei of the bonded atoms and one or more pairs of electrons that are located between the atoms. Compounds are classified as ionic or molecular (covalent) on the basis of the bonds present in them.\r\n<h2>Ionic Compounds<\/h2>\r\nWhen an element composed of atoms that readily lose electrons (a metal) reacts with an element composed of atoms that readily gain electrons (a nonmetal), a transfer of electrons usually occurs, producing ions. The compound formed by this transfer is stabilized by the electrostatic attractions (ionic bonds) between the ions of opposite charge present in the compound. For example, when each sodium atom in a sample of sodium metal (group 1) gives up one electron to form a sodium cation, Na<sup>+<\/sup>, and each chlorine atom in a sample of chlorine gas (group 17) accepts one electron to form a chloride anion, Cl<sup>\u2212<\/sup>, the resulting compound, NaCl, is composed of sodium ions and chloride ions in the ratio of one Na<sup>+<\/sup> ion for each Cl<sup>\u2212<\/sup> ion. Similarly, each calcium atom (group 2) can give up two electrons and transfer one to each of two chlorine atoms to form CaCl<sub>2<\/sub>, which is composed of Ca<sup>2+<\/sup> and Cl<sup>\u2212<\/sup> ions in the ratio of one Ca<sup>2+<\/sup> ion to two Cl<sup>\u2212<\/sup> ions.\r\n\r\nA compound that contains ions and is held together by ionic bonds is called an <strong>ionic compound<\/strong>. The periodic table can help us recognize many of the compounds that are ionic: When a metal is combined with one or more nonmetals, the compound is usually ionic. This guideline works well for predicting ionic compound formation for most of the compounds typically encountered in an introductory chemistry course. However, it is not always true (for example, aluminum chloride, AlCl<sub>3<\/sub>, is not ionic).\r\n\r\nYou can often recognize ionic compounds because of their properties. Ionic compounds are solids that typically melt at high temperatures and boil at even higher temperatures. For example, sodium chloride melts at 801 \u00b0C and boils at 1413 \u00b0C. (As a comparison, the molecular compound water melts at 0 \u00b0C and boils at 100 \u00b0C.) In solid form, an ionic compound is not electrically conductive because its ions are unable to flow (\u201celectricity\u201d is the flow of charged particles). When molten, however, it can conduct electricity because its ions are able to move freely through the liquid (Figure\u00a03).\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"881\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/887\/2015\/04\/23211104\/CNX_Chem_02_06_NaClMolten1.jpg\" alt=\"This figure shows three photos connected by right-facing arrows. The first shows a light bulb as part of a complex lab equipment setup. The light bulb is not lit. The second photo shows a substances being heated or set on fire. The third shows the light bulb again which is lit.\" width=\"881\" height=\"373\" data-media-type=\"image\/jpeg\" \/> Figure\u00a03. Sodium chloride melts at 801 \u00b0C and conducts electricity when molten. (credit: modification of work by Mark Blaser and Matt Evans)[\/caption]\r\n\r\n<div class=\"textbox\">\r\n\r\nWatch this video to see a mixture of salts melt and conduct electricity.\r\n\r\nhttps:\/\/youtu.be\/ePzEVPDyJV8\r\n\r\n<\/div>\r\nIn every ionic compound, the total number of positive charges of the cations equals the total number of negative charges of the anions. Thus, ionic compounds are electrically neutral overall, even though they contain positive and negative ions. We can use this observation to help us write the formula of an ionic compound. The formula of an ionic compound must have a ratio of ions such that the numbers of positive and negative charges are equal.\r\n<div class=\"textbox examples\">\r\n<h3>Example 3:\u00a0<strong>Predicting the Formula of an Ionic Compound<\/strong><\/h3>\r\nThe gemstone sapphire (Figure\u00a04) is mostly a compound of aluminum and oxygen that contains aluminum cations, Al<sup>3+<\/sup>, and oxygen anions, O<sup>2\u2212<\/sup>. What is the formula of this compound?\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"325\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/887\/2015\/04\/23211106\/CNX_Chem_02_06_Sapphire1.jpg\" alt=\"This is a photograph of a ring with a sapphire set in it.\" width=\"325\" height=\"248\" data-media-type=\"image\/jpeg\" \/> Figure\u00a04. Although pure aluminum oxide is colorless, trace amounts of iron and titanium give blue sapphire its characteristic color. (credit: modification of work by Stanislav Doronenko)[\/caption]\r\n\r\n[reveal-answer q=\"960063\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"960063\"]\r\n\r\nBecause the ionic compound must be electrically neutral, it must have the same number of positive and negative charges. Two aluminum ions, each with a charge of 3+, would give us six positive charges, and three oxide ions, each with a charge of 2\u2212, would give us six negative charges. The formula would be Al<sub>2<\/sub>O<sub>3<\/sub>.\r\n\r\n[\/hidden-answer]\r\n<h4><strong>Check Your Learning<\/strong><\/h4>\r\nPredict the formula of the ionic compound formed between the sodium cation, Na<sup>+<\/sup>, and the sulfide anion, S<sup>2\u2212<\/sup>.\r\n\r\n[reveal-answer q=\"974651\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"974651\"]Na<sub>2<\/sub>S[\/hidden-answer]\r\n\r\n<\/div>\r\nMany ionic compounds contain polyatomic ions (Table 1) as the cation, the anion, or both. As with simple ionic compounds, these compounds must also be electrically neutral, so their formulas can be predicted by treating the polyatomic ions as discrete units. We use parentheses in a formula to indicate a group of atoms that behave as a unit. For example, the formula for calcium phosphate, one of the minerals in our bones, is Ca<sub>3<\/sub>(PO<sub>4<\/sub>)<sub>2<\/sub>. This formula indicates that there are three calcium ions (Ca<sup>2+<\/sup>) for every two phosphate [latex]\\left({\\text{PO}}_{4}{}^{3-}\\right)[\/latex] groups. The [latex]{\\text{PO}}_{4}{}^{3-}[\/latex] groups are discrete units, each consisting of one phosphorus atom and four oxygen atoms, and having an overall charge of 3-. The compound is electrically neutral, and its formula shows a total count of three Ca, two P, and eight O atoms.\r\n<div class=\"textbox examples\">\r\n<h3>Example 4:\u00a0<strong>Predicting the Formula of a Compound with a Polyatomic Anion<\/strong><\/h3>\r\nBaking powder contains calcium dihydrogen phosphate, an ionic compound composed of the ions Ca<sup>2+<\/sup> and [latex]{\\text{H}}_{2}{\\text{PO}}_{4}{}^{-}[\/latex]. What is the formula of this compound?\r\n\r\n[reveal-answer q=\"807500\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"807500\"]\r\n\r\nThe positive and negative charges must balance, and this ionic compound must be electrically neutral. Thus, we must have two negative charges to balance the 2+ charge of the calcium ion. This requires a ratio of one Ca<sup>2+<\/sup> ion to two [latex]{\\text{H}}_{2}{\\text{PO}}_{4}{}^{-}[\/latex] ions. We designate this by enclosing the formula for the dihydrogen phosphate ion in parentheses and adding a subscript 2. The formula is Ca(H<sub>2<\/sub>PO<sub>4<\/sub>)<sub>2<\/sub>.\r\n\r\n[\/hidden-answer]\r\n<h4><strong>Check Your Learning<\/strong><\/h4>\r\nPredict the formula of the ionic compound formed between the lithium ion and the peroxide ion, [latex]{\\text{O}}_{2}{}^{2-}[\/latex] (Hint: Use the periodic table to predict the sign and the charge on the lithium ion.)\r\n\r\n[reveal-answer q=\"571395\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"571395\"]Li<sub>2<\/sub>O<sub>2<\/sub>[\/hidden-answer]\r\n\r\n<\/div>\r\nBecause an ionic compound is not made up of single, discrete molecules, it may not be properly symbolized using a <em>molecular<\/em> formula. Instead, ionic compounds must be symbolized by a formula indicating the <em>relative numbers<\/em> of its constituent cations. For compounds containing only monatomic ions (such as NaCl) and for many compounds containing polyatomic ions (such as CaSO<sub>4<\/sub>), these formulas are just the empirical formulas introduced earlier in this chapter. However, the formulas for some ionic compounds containing polyatomic ions are not empirical formulas. For example, the ionic compound sodium oxalate is comprised of Na<sup>+<\/sup> and [latex]{\\text{C}}_{2}{\\text{O}}_{4}{}^{2-}[\/latex] ions combined in a 2:1 ratio, and its formula is written as Na<sub>2<\/sub>C<sub>2<\/sub>O<sub>4<\/sub>. The subscripts in this formula are not the smallest-possible whole numbers, as each can be divided by 2 to yield the empirical formula, NaCO<sub>2<\/sub>. This is not the accepted formula for sodium oxalate, however, as it does not accurately represent the compound\u2019s polyatomic anion, [latex]{\\text{C}}_{2}{\\text{O}}_{4}{}^{2-}[\/latex].\r\n<h2>Molecular Compounds<\/h2>\r\nMany compounds do not contain ions but instead consist solely of discrete, neutral molecules. These <strong>molecular compounds<\/strong> (covalent compounds) result when atoms share, rather than transfer (gain or lose), electrons. Covalent bonding is an important and extensive concept in chemistry, and it will be treated in considerable detail in a later chapter of this text. We can often identify molecular compounds on the basis of their physical properties. Under normal conditions, molecular compounds often exist as gases, low-boiling liquids, and low-melting solids, although many important exceptions exist.\r\n\r\nWhereas ionic compounds are usually formed when a metal and a nonmetal combine, covalent compounds are usually formed by a combination of nonmetals. Thus, the periodic table can help us recognize many of the compounds that are covalent. While we can use the positions of a compound\u2019s elements in the periodic table to predict whether it is ionic or covalent at this point in our study of chemistry, you should be aware that this is a very simplistic approach that does not account for a number of interesting exceptions. Shades of gray exist between ionic and molecular compounds, and you\u2019ll learn more about those later.\r\n<div class=\"textbox examples\">\r\n<h3>Example 5:\u00a0<strong>Predicting the Type of Bonding in Compounds<\/strong><\/h3>\r\nPredict whether the following compounds are ionic or molecular:\r\n<ol>\r\n \t<li>KI, the compound used as a source of iodine in table salt<\/li>\r\n \t<li>H<sub>2<\/sub>O<sub>2<\/sub>, the bleach and disinfectant hydrogen peroxide<\/li>\r\n \t<li>CHCl<sub>3<\/sub>, the anesthetic chloroform<\/li>\r\n \t<li>Li<sub>2<\/sub>CO<sub>3<\/sub>, a source of lithium in antidepressants<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"262423\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"262423\"]\r\n<ol>\r\n \t<li>Potassium (group 1) is a metal, and iodine (group 17) is a nonmetal; KI is predicted to be ionic.<\/li>\r\n \t<li>Hydrogen (group 1) is a nonmetal, and oxygen (group 16) is a nonmetal; H<sub>2<\/sub>O<sub>2<\/sub> is predicted to be molecular.<\/li>\r\n \t<li>Carbon (group 14) is a nonmetal, hydrogen (group 1) is a nonmetal, and chlorine (group 17) is a nonmetal; CHCl<sub>3<\/sub> is predicted to be molecular.<\/li>\r\n \t<li>Lithium (group 1A) is a metal, and carbonate is a polyatomic ion; Li<sub>2<\/sub>CO<sub>3<\/sub> is predicted to be ionic.<\/li>\r\n<\/ol>\r\n[\/hidden-answer]\r\n<h4><strong>Check Your Learning<\/strong><\/h4>\r\nUsing the periodic table, predict whether the following compounds are ionic or covalent:\r\n<ol>\r\n \t<li>SO<sub>2<\/sub><\/li>\r\n \t<li>CaF<sub>2<\/sub><\/li>\r\n \t<li>N<sub>2<\/sub>H<sub>4<\/sub><\/li>\r\n \t<li>Al<sub>2<\/sub>(SO<sub>4<\/sub>)<sub>3<\/sub><\/li>\r\n<\/ol>\r\n[reveal-answer q=\"341117\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"341117\"]\r\n<ol>\r\n \t<li>molecular<\/li>\r\n \t<li>ionic<\/li>\r\n \t<li>molecular<\/li>\r\n \t<li>ionic<\/li>\r\n<\/ol>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Key Concepts and Summary<\/h3>\r\nMetals (particularly those in groups 1 and 2) tend to lose the number of electrons that would leave them with the same number of electrons as in the preceding noble gas in the periodic table. By this means, a positively charged ion is formed. Similarly, nonmetals (especially those in groups 16 and 17, and, to a lesser extent, those in Group 15) can gain the number of electrons needed to provide atoms with the same number of electrons as in the next noble gas in the periodic table. Thus, nonmetals tend to form negative ions. Positively charged ions are called cations, and negatively charge ions are called anions. Ions can be either monatomic (containing only one atom) or polyatomic (containing more than one atom).\r\n\r\nCompounds that contain ions are called ionic compounds. Ionic compounds generally form from metals and nonmetals. Compounds that do not contain ions, but instead consist of atoms bonded tightly together in molecules (uncharged groups of atoms that behave as a single unit), are called covalent compounds. Covalent compounds usually form from two nonmetals.\r\n\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>Exercises<\/h3>\r\n<ol>\r\n \t<li>Using the periodic table, predict whether the following chlorides are ionic or covalent: KCl, NCl<sub>3<\/sub>, ICl, MgCl<sub>2<\/sub>, PCl<sub>5<\/sub>, and CCl<sub>4<\/sub>.<\/li>\r\n \t<li>Using the periodic table, predict whether the following chlorides are ionic or covalent: SiCl<sub>4<\/sub>, PCl<sub>3<\/sub>, CaCl<sub>2<\/sub>, CsCl, CuCl<sub>2<\/sub>, and CrCl<sub>3<\/sub>.<\/li>\r\n \t<li>For each of the following compounds, state whether it is ionic or covalent. If it is ionic, write the symbols for the ions involved:\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>NF<sub>3<\/sub><\/li>\r\n \t<li>BaO,<\/li>\r\n \t<li>(NH<sub>4<\/sub>)<sub>2<\/sub>CO<sub>3\u00a0<\/sub><\/li>\r\n \t<li>Sr(H<sub>2<\/sub>PO<sub>4<\/sub>)<sub>2<\/sub><\/li>\r\n \t<li>IBr<\/li>\r\n \t<li>Na<sub>2<\/sub>O<\/li>\r\n<\/ol>\r\n<\/li>\r\n \t<li>For each of the following compounds, state whether it is ionic or covalent, and if it is ionic, write the symbols for the ions involved:\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>KClO<sub>4 \u00a0<\/sub><\/li>\r\n \t<li>MgC<sub>2<\/sub>H<sub>3<\/sub>O<sub>2 \u00a0<\/sub><\/li>\r\n \t<li>H<sub>2<\/sub>S<\/li>\r\n \t<li>Ag<sub>2<\/sub>S<\/li>\r\n \t<li>N<sub>2<\/sub>Cl<sub>4 \u00a0<\/sub><\/li>\r\n \t<li>Co(NO<sub>3<\/sub>)<sub>2<\/sub><\/li>\r\n<\/ol>\r\n<\/li>\r\n \t<li>For each of the following pairs of ions, write the symbol for the formula of the compound they will form:\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>Ca<sup>2+<\/sup>, S<sup>2- \u00a0<\/sup><\/li>\r\n \t<li>[latex]{\\text{NH}}_{4}{}^{+}[\/latex], [latex]{\\text{SO}}_{4}{}^{2-}[\/latex]<\/li>\r\n \t<li>Al<sup>3+<\/sup>, Br<sup>- \u00a0<\/sup>(d) Na<sup>+<\/sup>, [latex]{\\text{HPO}}_{4}{}^{2-}[\/latex] \u00a0(e) Mg<sup>2+<\/sup>, [latex]{\\text{PO}}_{4}{}^{3-}[\/latex]<\/li>\r\n<\/ol>\r\n<\/li>\r\n \t<li>For each of the following pairs of ions, write the symbol for the formula of the compound they will form:\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>K<sup>+<\/sup>, O<sup>2-<\/sup><\/li>\r\n \t<li>[latex]{\\text{NH}}_{4}{}^{+}[\/latex], [latex]{\\text{PO}}_{4}{}^{3-}[\/latex]<\/li>\r\n \t<li>Al<sup>3+<\/sup>, O<sup>2-<\/sup><\/li>\r\n \t<li>Na<sup>+<\/sup>, [latex]{\\text{CO}}_{3}{}^{2-}[\/latex]<\/li>\r\n \t<li>Ba<sup>2+<\/sup>, [latex]{\\text{PO}}_{4}{}^{3-}[\/latex]<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"265027\"]Show Selected Answers[\/reveal-answer]\r\n[hidden-answer a=\"265027\"]\r\n\r\n1.\u00a0Ionic: KCl, MgCl<sub>2<\/sub>; Covalent: NCl<sub>3<\/sub>, ICl, PCl<sub>5<\/sub>, CCl<sub>4<\/sub>\r\n\r\n3.\u00a0(a) covalent; (b) ionic, Ba<sup>2+<\/sup>, O<sup>2-<\/sup>; (c) ionic, [latex]{\\text{NH}}_{4}{}^{+}[\/latex], [latex]{\\text{CO}}_{3}{}^{2-};[\/latex] (d) ionic, Sr<sup>2+<\/sup>, [latex]{\\text{H}}_{2}{\\text{PO}}_{4}{}^{-};[\/latex] (e) covalent; (f) ionic, Na<sup>+<\/sup>, O<sup>2-<\/sup>\r\n\r\n5.\u00a0(a) CaS; (b) (NH<sub>4<\/sub>)<sub>2<\/sub>CO<sub>3<\/sub>; (c) AlBr<sub>3<\/sub>; (d) Na<sub>2<\/sub>HPO<sub>4<\/sub>; (e) Mg<sub>3<\/sub> (PO<sub>4<\/sub>)<sub>2<\/sub>\r\n\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<h2>Glossary<\/h2>\r\n<strong>covalent bond:\u00a0<\/strong>attractive force between the nuclei of a molecule\u2019s atoms and pairs of electrons between the atoms\r\n\r\n<strong>covalent compound:\u00a0<\/strong>(also, molecular compound) composed of molecules formed by atoms of two or more different elements\r\n\r\n<strong>ionic bond:\u00a0<\/strong>electrostatic forces of attraction between the oppositely charged ions of an ionic compound\r\n\r\n<strong>ionic compound:\u00a0<\/strong>compound composed of cations and anions combined in ratios, yielding an electrically neutral substance\r\n\r\n<strong>molecular compound:\u00a0<\/strong>(also, covalent compound) composed of molecules formed by atoms of two or more different elements\r\n\r\n<strong>monatomic ion:\u00a0<\/strong>ion composed of a single atom\r\n\r\n<strong>oxyanion:\u00a0<\/strong>polyatomic anion composed of a central atom bonded to oxygen atoms\r\n\r\n<strong>polyatomic ion:\u00a0<\/strong>ion composed of more than one atom","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<ul>\n<li>Define ionic and molecular (covalent) compounds<\/li>\n<li>Predict the type of compound formed from elements based on their location within the periodic table<\/li>\n<li>Determine formulas for simple ionic compounds<\/li>\n<\/ul>\n<\/div>\n<p>In ordinary chemical reactions, the nucleus of each atom (and thus the identity of the element) remains unchanged. Electrons, however, can be added to atoms by transfer from other atoms, lost by transfer to other atoms, or shared with other atoms. The transfer and sharing of electrons among atoms govern the chemistry of the elements. During the formation of some compounds, atoms gain or lose electrons, and form electrically charged particles called ions (Figure\u00a01).<\/p>\n<div style=\"width: 889px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/887\/2015\/04\/23211101\/CNX_Chem_02_06_NaCation1.jpg\" alt=\"Figure\u00a0A shows a sodium atom, N a, which has a nucleus containing 11 protons and 12 neutrons. The atom\u2019s surrounding electron cloud contains 11 electrons. Figure\u00a0B shows a sodium ion, N a superscript plus sign. Its nucleus contains 11 protons and 12 neutrons. The ion\u2019s electron cloud contains 10 electrons and is smaller than that of the sodium atom in figure A.\" width=\"879\" height=\"355\" data-media-type=\"image\/jpeg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a01. (a) A sodium atom (Na) has equal numbers of protons and electrons (11) and is uncharged. (b) A sodium cation (Na+) has lost an electron, so it has one more proton (11) than electrons (10), giving it an overall positive charge, signified by a superscripted plus sign.<\/p>\n<\/div>\n<p>You can use the periodic table to predict whether an atom will form an anion or a cation, and you can often predict the charge of the resulting ion. Atoms of many main-group metals lose enough electrons to leave them with the same number of electrons as an atom of the preceding noble gas. To illustrate, an atom of an alkali metal (group 1) loses one electron and forms a cation with a 1+ charge; an alkaline earth metal (group 2) loses two electrons and forms a cation with a 2+ charge, and so on. For example, a neutral calcium atom, with 20 protons and 20 electrons, readily loses two electrons. This results in a cation with 20 protons, 18 electrons, and a 2+ charge. It has the same number of electrons as atoms of the preceding noble gas, argon, and is symbolized Ca<sup>2+<\/sup>. The name of a metal ion is the same as the name of the metal atom from which it forms, so Ca<sup>2+<\/sup> is called a calcium ion.<\/p>\n<p>When atoms of nonmetal elements form ions, they generally gain enough electrons to give them the same number of electrons as an atom of the next noble gas in the periodic table. Atoms of group 17 gain one electron and form anions with a 1\u2212 charge; atoms of group 16 gain two electrons and form ions with a 2\u2212 charge, and so on. For example, the neutral bromine atom, with 35 protons and 35 electrons, can gain one electron to provide it with 36 electrons. This results in an anion with 35 protons, 36 electrons, and a 1\u2212 charge. It has the same number of electrons as atoms of the next noble gas, krypton, and is symbolized Br<sup>\u2212<\/sup>. (A discussion of the theory supporting the favored status of noble gas electron numbers reflected in these predictive rules for ion formation is provided in a later chapter of this text.)<\/p>\n<p>Note the usefulness of the periodic table in predicting likely ion formation and charge (Figure\u00a02). Moving from the far left to the right on the periodic table, main-group elements tend to form cations with a charge equal to the group number. That is, group 1 elements form 1+ ions; group 2 elements form 2+ ions, and so on. Moving from the far right to the left on the periodic table, elements often form anions with a negative charge equal to the number of groups moved left from the noble gases. For example, group 17 elements (one group left of the noble gases) form 1\u2212 ions; group 16 elements (two groups left) form 2\u2212 ions, and so on. This trend can be used as a guide in many cases, but its predictive value decreases when moving toward the center of the periodic table. In fact, transition metals and some other metals often exhibit variable charges that are not predictable by their location in the table. For example, copper can form ions with a 1+ or 2+ charge, and iron can form ions with a 2+ or 3+ charge.<\/p>\n<div style=\"width: 889px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/887\/2015\/04\/23211102\/CNX_Chem_02_06_IonCharges1.jpg\" alt=\"Group one of the periodic table contains L i superscript plus sign in period 2, N a superscript plus sign in period 3, K superscript plus sign in period 4, R b superscript plus sign in period 5, C s superscript plus sign in period 6, and F r superscript plus sign in period 7. Group two contains B e superscript 2 plus sign in period 2, M g superscript 2 plus sign in period 3, C a superscript 2 plus sign in period 4, S r superscript 2 plus sign in period 5, B a superscript 2 plus sign in period 6, and R a superscript 2 plus sign in period 7. Group six contains C r superscript 3 plus sign and C r superscript 6 plus sign in period 4. Group seven contains M n superscript 2 plus sign in period 4. Group eight contains F e superscript 2 plus sign and F e superscript 3 plus sign in period 4. Group nine contains C o superscript 2 plus sign in period 4. Group ten contains N i superscript 2 plus sign in period 4, and P t superscript 2 plus sign in period 6. Group 11 contains C U superscript plus sign and C U superscript 2 plus sign in period 4, A g superscript plus sign in period 5, and A u superscript plus sign and A u superscript 3 plus sign in period 6. Group 12 contains Z n superscript 2 plus sign in period 4, C d superscript 2 plus sign in period 5, and H g subscript 2 superscript 2 plus sign and H g superscript 2 plus sign in period 6. Group 13 contains A l superscript 3 plus sign in period 3. Group 14 contains C superscript 4 negative sign in period 2. Group 15 contains N superscript 3 negative sign in period 2, P superscript 3 negative sign in period 3, and A s superscript 3 negative sign in period 4. Group 16 contains O superscript 2 negative sign in period 2, S superscript 2 negative sign in period 3, S e superscript 2 negative sign in period 4 and T e superscript 2 negative sign in period 5. Group 17 contains F superscript negative sign in period 2, C l superscript negative sign in period 3, B r superscript negative sign in period 4, I superscript negative sign in period 5, and A t superscript negative sign in period 6. Group 18 contains H e in period 1, N e in period 2, A r in period 3, K r in period 4, X e in period 5 and R n in period 6.\" width=\"879\" height=\"535\" data-media-type=\"image\/jpeg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a02. Some elements exhibit a regular pattern of ionic charge when they form ions.<\/p>\n<\/div>\n<div class=\"textbox examples\">\n<h3>Example 1:\u00a0<strong>Composition of Ions<\/strong><\/h3>\n<p>An ion found in some compounds used as antiperspirants contains 13 protons and 10 electrons. What is its symbol?<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q386703\">Show Answer<\/span><\/p>\n<div id=\"q386703\" class=\"hidden-answer\" style=\"display: none\">\n<p>Because the number of protons remains unchanged when an atom forms an ion, the atomic number of the element must be 13. Knowing this lets us use the periodic table to identify the element as Al (aluminum). The Al atom has lost three electrons and thus has three more positive charges (13) than it has electrons (10). This is the aluminum cation, Al<sup>3+<\/sup>.<\/p>\n<\/div>\n<\/div>\n<h4><strong>Check Your Learning<\/strong><\/h4>\n<p>Give the symbol and name for the ion with 34 protons and 36 electrons.<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q644629\">Show Answer<\/span><\/p>\n<div id=\"q644629\" class=\"hidden-answer\" style=\"display: none\">Se<sup>2\u2212<\/sup>, the selenide ion<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox examples\">\n<h3>Example 2:\u00a0<strong>Formation of Ions<\/strong><\/h3>\n<p>Magnesium and nitrogen react to form an ionic compound. Predict which forms an anion, which forms a cation, and the charges of each ion. Write the symbol for each ion and name them.<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q31643\">Show Answer<\/span><\/p>\n<div id=\"q31643\" class=\"hidden-answer\" style=\"display: none\">\n<p>Magnesium\u2019s position in the periodic table (group 2) tells us that it is a metal. Metals form positive ions (cations). A magnesium atom must lose two electrons to have the same number electrons as an atom of the previous noble gas, neon. Thus, a magnesium atom will form a cation with two fewer electrons than protons and a charge of 2+. The symbol for the ion is Mg<sup>2+<\/sup>, and it is called a magnesium ion.<\/p>\n<p>Nitrogen\u2019s position in the periodic table (group 15) reveals that it is a nonmetal. Nonmetals form negative ions (anions). A nitrogen atom must gain three electrons to have the same number of electrons as an atom of the following noble gas, neon. Thus, a nitrogen atom will form an anion with three more electrons than protons and a charge of 3\u2212. The symbol for the ion is N<sup>3\u2212<\/sup>, and it is called a nitride ion.<\/p>\n<\/div>\n<\/div>\n<h4><strong>Check Your Learning<\/strong><\/h4>\n<p>Aluminum and carbon react to form an ionic compound. Predict which forms an anion, which forms a cation, and the charges of each ion. Write the symbol for each ion and name them.<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q219108\">Show Answer<\/span><\/p>\n<div id=\"q219108\" class=\"hidden-answer\" style=\"display: none\">Al will form a cation with a charge of 3+: Al<sup>3+<\/sup>, an aluminum ion. Carbon will form an anion with a charge of 4\u2212: C<sup>4\u2212<\/sup>, a carbide ion.<\/div>\n<\/div>\n<\/div>\n<p>The ions that we have discussed so far are called <strong>monatomic ions<\/strong>, that is, they are ions formed from only one atom. We also find many <strong>polyatomic ions<\/strong>. These ions, which act as discrete units, are electrically charged molecules (a group of bonded atoms with an overall charge). Some of the more important polyatomic ions are listed in Table 1. <strong>Oxyanions<\/strong> are polyatomic ions that contain one or more oxygen atoms. At this point in your study of chemistry, you should memorize the names, formulas, and charges of the most common polyatomic ions. Because you will use them repeatedly, they will soon become familiar.<\/p>\n<table id=\"fs-idp278664880\" class=\"span-all\" summary=\"This table has three columns labeled \u201ccharge,\u201d \u201cname,\u201d and \u201cformula.\u201d Ammonium has a charge of positive 1 and the formula N H subscript 4 superscript plus sign. Acetate has a charge of negative 1 and the formula C subscript 2 H subscript 3 O subscript 2 superscript negative sign. Cyanide has a charge of negative 1, and the formula C N superscript negative sign. Hydroxide has a charge of negative 1 and the formula O H superscript negative sign. Nitrate has a charge of negative 1 and the formula N O subscript 3 superscript negative sign. Nitrite has a charge of negative 1 and the formula N O subscript 2 superscript negative sign. Perchlorate has a charge of negative 1 and the formula C l O subscript 4 superscript negative sign. Chlorate has a charge of negative 1 and the formula C l O subscript 3 superscript negative sign. Chlorite has a charge of negative 1 and the formula C l O subscript 2 superscript negative sign. Hypochlorite has a charge of negative 1 and the formula C l O superscript negative sign. Permanganate has a charge of negative 1 and the formula M n O subscript 4 superscript negative sign. Hydrogen carbonate, or bicarbonate has a charge of negative 1 and the formula H C O subscript 3 superscript negative sign. Carbonate has a charge of negative 2 and the formula C O subscript 3 superscript 2 negative sign. Peroxide has a charge of negative 2 and the formula O subscript 2 superscript 2 negative sign. Hydrogen sulfate, or bisulfate, has a charge of negative 1 and the formula H S O subscript 4 superscript negative sign. Sulfate has a charge of negative 2 and the formula S O subscript 4 superscript 2 negative sign. Sulfite has a charge of negative 2 and the formula S O subscript 3 superscript 2 negative sign. Dihydrogen phosphate has a charge of negative 1 and the formula H subscript 2 P O subscript 4 superscript negative sign. Hydrogen phosphate has a charge of negative 2 and the formula H P O subscript 4 superscript 2 negative sign. Phosphate has a charge of negative 3 and the formula P O subscript 4 superscript 3 negative sign.\">\n<thead>\n<tr>\n<th colspan=\"7\">Table 1. Common Polyatomic Ions<\/th>\n<\/tr>\n<tr>\n<th style=\"width: 10%;\">Charge<\/th>\n<th style=\"width: 20%;\">Name<\/th>\n<th style=\"width: 10%;\">Formula<\/th>\n<th style=\"width: 10%;\"><\/th>\n<th style=\"width: 10%;\">Charge<\/th>\n<th style=\"width: 30%;\">Name<\/th>\n<th style=\"width: 10%;\">Formula<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>1+<\/td>\n<td>ammonium<\/td>\n<td>[latex]{\\text{NH}}_{4}{}^{+}[\/latex]<\/td>\n<td><\/td>\n<td>1\u2212<\/td>\n<td>permanganate<\/td>\n<td>[latex]{\\text{MnO}}_{4}{}^{-}[\/latex]<\/td>\n<\/tr>\n<tr>\n<td>1\u2212<\/td>\n<td>acetate<\/td>\n<td>[latex]{\\text{C}}_{2}{\\text{H}}_{3}{\\text{O}}_{2}{}^{-}[\/latex]<\/td>\n<td><\/td>\n<td>1\u2212<\/td>\n<td>hydrogen carbonate, or bicarbonate<\/td>\n<td>[latex]{\\text{HCO}}_{3}{}^{-}[\/latex]<\/td>\n<\/tr>\n<tr>\n<td>1\u2212<\/td>\n<td>cyanide<\/td>\n<td>[latex]\\text{CN}^-[\/latex]<\/td>\n<td><\/td>\n<td>2\u2212<\/td>\n<td>carbonate<\/td>\n<td>[latex]{\\text{CO}}_{3}{}^{2-}[\/latex]<\/td>\n<\/tr>\n<tr>\n<td>1\u2212<\/td>\n<td>hydroxide<\/td>\n<td>[latex]\\text{OH}^-[\/latex]<\/td>\n<td><\/td>\n<td>2\u2212<\/td>\n<td>peroxide<\/td>\n<td>[latex]{\\text{O}}_{2}{}^{2-}[\/latex]<\/td>\n<\/tr>\n<tr>\n<td>1\u2212<\/td>\n<td>nitrate<\/td>\n<td>[latex]{\\text{NO}}_{3}{}^{-}[\/latex]<\/td>\n<td><\/td>\n<td>1\u2212<\/td>\n<td>hydrogen sulfate, or bisulfate<\/td>\n<td>[latex]{\\text{HSO}}_{4}{}^{-}[\/latex]<\/td>\n<\/tr>\n<tr>\n<td>1\u2212<\/td>\n<td>nitrite<\/td>\n<td>[latex]{\\text{NO}}_{2}{}^{-}[\/latex]<\/td>\n<td><\/td>\n<td>2\u2212<\/td>\n<td>sulfate<\/td>\n<td>[latex]{\\text{SO}}_{4}{}^{2-}[\/latex]<\/td>\n<\/tr>\n<tr>\n<td>1\u2212<\/td>\n<td>perchlorate<\/td>\n<td>[latex]{\\text{ClO}}_{4}{}^{-}[\/latex]<\/td>\n<td><\/td>\n<td>2\u2212<\/td>\n<td>sulfite<\/td>\n<td>[latex]{\\text{SO}}_{3}{}^{2-}[\/latex]<\/td>\n<\/tr>\n<tr>\n<td>1\u2212<\/td>\n<td>chlorate<\/td>\n<td>[latex]{\\text{ClO}}_{3}{}^{-}[\/latex]<\/td>\n<td><\/td>\n<td>1\u2212<\/td>\n<td>dihydrogen phosphate<\/td>\n<td>[latex]{\\text{H}}_{2}{\\text{PO}}_{4}{}^{-}[\/latex]<\/td>\n<\/tr>\n<tr>\n<td>1\u2212<\/td>\n<td>chlorite<\/td>\n<td>[latex]{\\text{ClO}}_{2}{}^{-}[\/latex]<\/td>\n<td><\/td>\n<td>2\u2212<\/td>\n<td>hydrogen phosphate<\/td>\n<td>[latex]{\\text{HPO}}_{4}{}^{2-}[\/latex]<\/td>\n<\/tr>\n<tr>\n<td>1\u2212<\/td>\n<td>hypochlorite<\/td>\n<td>[latex]\\text{ClO}^-[\/latex]<\/td>\n<td><\/td>\n<td>3\u2212<\/td>\n<td>phosphate<\/td>\n<td>[latex]{\\text{PO}}_{4}{}^{3-}[\/latex]<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Note that there is a system for naming some polyatomic ions; <em>-ate<\/em> and <em>-ite<\/em> are suffixes designating polyatomic ions containing more or fewer oxygen atoms. <em>Per-<\/em> (short for \u201chyper\u201d) and <em>hypo-<\/em> (meaning \u201cunder\u201d) are prefixes meaning more oxygen atoms than <em>-ate<\/em> and fewer oxygen atoms than <em>-ite<\/em>, respectively. For example, perchlorate is [latex]{\\text{ClO}}_{4}{}^{-}[\/latex], chlorate is [latex]{\\text{ClO}}_{3}{}^{-}[\/latex], chlorite is [latex]{\\text{ClO}}_{2}{}^{-}[\/latex] and hypochlorite is ClO<sup>&#8211;<\/sup>. Unfortunately, the number of oxygen atoms corresponding to a given suffix or prefix is not consistent; for example, nitrate is [latex]{\\text{NO}}_{3}{}^{-}[\/latex] while sulfate is [latex]{\\text{SO}}_{4}{}^{2-}.[\/latex] This will be covered in more detail in the next module on nomenclature.<\/p>\n<p>The nature of the attractive forces that hold atoms or ions together within a compound is the basis for classifying chemical bonding. When electrons are transferred and ions form, <strong>ionic bonds<\/strong> result. Ionic bonds are electrostatic forces of attraction, that is, the attractive forces experienced between objects of opposite electrical charge (in this case, cations and anions). When electrons are \u201cshared\u201d and molecules form, <strong>covalent bonds<\/strong> result. Covalent bonds are the attractive forces between the positively charged nuclei of the bonded atoms and one or more pairs of electrons that are located between the atoms. Compounds are classified as ionic or molecular (covalent) on the basis of the bonds present in them.<\/p>\n<h2>Ionic Compounds<\/h2>\n<p>When an element composed of atoms that readily lose electrons (a metal) reacts with an element composed of atoms that readily gain electrons (a nonmetal), a transfer of electrons usually occurs, producing ions. The compound formed by this transfer is stabilized by the electrostatic attractions (ionic bonds) between the ions of opposite charge present in the compound. For example, when each sodium atom in a sample of sodium metal (group 1) gives up one electron to form a sodium cation, Na<sup>+<\/sup>, and each chlorine atom in a sample of chlorine gas (group 17) accepts one electron to form a chloride anion, Cl<sup>\u2212<\/sup>, the resulting compound, NaCl, is composed of sodium ions and chloride ions in the ratio of one Na<sup>+<\/sup> ion for each Cl<sup>\u2212<\/sup> ion. Similarly, each calcium atom (group 2) can give up two electrons and transfer one to each of two chlorine atoms to form CaCl<sub>2<\/sub>, which is composed of Ca<sup>2+<\/sup> and Cl<sup>\u2212<\/sup> ions in the ratio of one Ca<sup>2+<\/sup> ion to two Cl<sup>\u2212<\/sup> ions.<\/p>\n<p>A compound that contains ions and is held together by ionic bonds is called an <strong>ionic compound<\/strong>. The periodic table can help us recognize many of the compounds that are ionic: When a metal is combined with one or more nonmetals, the compound is usually ionic. This guideline works well for predicting ionic compound formation for most of the compounds typically encountered in an introductory chemistry course. However, it is not always true (for example, aluminum chloride, AlCl<sub>3<\/sub>, is not ionic).<\/p>\n<p>You can often recognize ionic compounds because of their properties. Ionic compounds are solids that typically melt at high temperatures and boil at even higher temperatures. For example, sodium chloride melts at 801 \u00b0C and boils at 1413 \u00b0C. (As a comparison, the molecular compound water melts at 0 \u00b0C and boils at 100 \u00b0C.) In solid form, an ionic compound is not electrically conductive because its ions are unable to flow (\u201celectricity\u201d is the flow of charged particles). When molten, however, it can conduct electricity because its ions are able to move freely through the liquid (Figure\u00a03).<\/p>\n<div style=\"width: 891px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/887\/2015\/04\/23211104\/CNX_Chem_02_06_NaClMolten1.jpg\" alt=\"This figure shows three photos connected by right-facing arrows. The first shows a light bulb as part of a complex lab equipment setup. The light bulb is not lit. The second photo shows a substances being heated or set on fire. The third shows the light bulb again which is lit.\" width=\"881\" height=\"373\" data-media-type=\"image\/jpeg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a03. Sodium chloride melts at 801 \u00b0C and conducts electricity when molten. (credit: modification of work by Mark Blaser and Matt Evans)<\/p>\n<\/div>\n<div class=\"textbox\">\n<p>Watch this video to see a mixture of salts melt and conduct electricity.<\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-1\" title=\"Conductivity molten salt\" width=\"500\" height=\"375\" src=\"https:\/\/www.youtube.com\/embed\/ePzEVPDyJV8?feature=oembed&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<\/div>\n<p>In every ionic compound, the total number of positive charges of the cations equals the total number of negative charges of the anions. Thus, ionic compounds are electrically neutral overall, even though they contain positive and negative ions. We can use this observation to help us write the formula of an ionic compound. The formula of an ionic compound must have a ratio of ions such that the numbers of positive and negative charges are equal.<\/p>\n<div class=\"textbox examples\">\n<h3>Example 3:\u00a0<strong>Predicting the Formula of an Ionic Compound<\/strong><\/h3>\n<p>The gemstone sapphire (Figure\u00a04) is mostly a compound of aluminum and oxygen that contains aluminum cations, Al<sup>3+<\/sup>, and oxygen anions, O<sup>2\u2212<\/sup>. What is the formula of this compound?<\/p>\n<div style=\"width: 335px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/887\/2015\/04\/23211106\/CNX_Chem_02_06_Sapphire1.jpg\" alt=\"This is a photograph of a ring with a sapphire set in it.\" width=\"325\" height=\"248\" data-media-type=\"image\/jpeg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a04. Although pure aluminum oxide is colorless, trace amounts of iron and titanium give blue sapphire its characteristic color. (credit: modification of work by Stanislav Doronenko)<\/p>\n<\/div>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q960063\">Show Answer<\/span><\/p>\n<div id=\"q960063\" class=\"hidden-answer\" style=\"display: none\">\n<p>Because the ionic compound must be electrically neutral, it must have the same number of positive and negative charges. Two aluminum ions, each with a charge of 3+, would give us six positive charges, and three oxide ions, each with a charge of 2\u2212, would give us six negative charges. The formula would be Al<sub>2<\/sub>O<sub>3<\/sub>.<\/p>\n<\/div>\n<\/div>\n<h4><strong>Check Your Learning<\/strong><\/h4>\n<p>Predict the formula of the ionic compound formed between the sodium cation, Na<sup>+<\/sup>, and the sulfide anion, S<sup>2\u2212<\/sup>.<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q974651\">Show Answer<\/span><\/p>\n<div id=\"q974651\" class=\"hidden-answer\" style=\"display: none\">Na<sub>2<\/sub>S<\/div>\n<\/div>\n<\/div>\n<p>Many ionic compounds contain polyatomic ions (Table 1) as the cation, the anion, or both. As with simple ionic compounds, these compounds must also be electrically neutral, so their formulas can be predicted by treating the polyatomic ions as discrete units. We use parentheses in a formula to indicate a group of atoms that behave as a unit. For example, the formula for calcium phosphate, one of the minerals in our bones, is Ca<sub>3<\/sub>(PO<sub>4<\/sub>)<sub>2<\/sub>. This formula indicates that there are three calcium ions (Ca<sup>2+<\/sup>) for every two phosphate [latex]\\left({\\text{PO}}_{4}{}^{3-}\\right)[\/latex] groups. The [latex]{\\text{PO}}_{4}{}^{3-}[\/latex] groups are discrete units, each consisting of one phosphorus atom and four oxygen atoms, and having an overall charge of 3-. The compound is electrically neutral, and its formula shows a total count of three Ca, two P, and eight O atoms.<\/p>\n<div class=\"textbox examples\">\n<h3>Example 4:\u00a0<strong>Predicting the Formula of a Compound with a Polyatomic Anion<\/strong><\/h3>\n<p>Baking powder contains calcium dihydrogen phosphate, an ionic compound composed of the ions Ca<sup>2+<\/sup> and [latex]{\\text{H}}_{2}{\\text{PO}}_{4}{}^{-}[\/latex]. What is the formula of this compound?<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q807500\">Show Answer<\/span><\/p>\n<div id=\"q807500\" class=\"hidden-answer\" style=\"display: none\">\n<p>The positive and negative charges must balance, and this ionic compound must be electrically neutral. Thus, we must have two negative charges to balance the 2+ charge of the calcium ion. This requires a ratio of one Ca<sup>2+<\/sup> ion to two [latex]{\\text{H}}_{2}{\\text{PO}}_{4}{}^{-}[\/latex] ions. We designate this by enclosing the formula for the dihydrogen phosphate ion in parentheses and adding a subscript 2. The formula is Ca(H<sub>2<\/sub>PO<sub>4<\/sub>)<sub>2<\/sub>.<\/p>\n<\/div>\n<\/div>\n<h4><strong>Check Your Learning<\/strong><\/h4>\n<p>Predict the formula of the ionic compound formed between the lithium ion and the peroxide ion, [latex]{\\text{O}}_{2}{}^{2-}[\/latex] (Hint: Use the periodic table to predict the sign and the charge on the lithium ion.)<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q571395\">Show Answer<\/span><\/p>\n<div id=\"q571395\" class=\"hidden-answer\" style=\"display: none\">Li<sub>2<\/sub>O<sub>2<\/sub><\/div>\n<\/div>\n<\/div>\n<p>Because an ionic compound is not made up of single, discrete molecules, it may not be properly symbolized using a <em>molecular<\/em> formula. Instead, ionic compounds must be symbolized by a formula indicating the <em>relative numbers<\/em> of its constituent cations. For compounds containing only monatomic ions (such as NaCl) and for many compounds containing polyatomic ions (such as CaSO<sub>4<\/sub>), these formulas are just the empirical formulas introduced earlier in this chapter. However, the formulas for some ionic compounds containing polyatomic ions are not empirical formulas. For example, the ionic compound sodium oxalate is comprised of Na<sup>+<\/sup> and [latex]{\\text{C}}_{2}{\\text{O}}_{4}{}^{2-}[\/latex] ions combined in a 2:1 ratio, and its formula is written as Na<sub>2<\/sub>C<sub>2<\/sub>O<sub>4<\/sub>. The subscripts in this formula are not the smallest-possible whole numbers, as each can be divided by 2 to yield the empirical formula, NaCO<sub>2<\/sub>. This is not the accepted formula for sodium oxalate, however, as it does not accurately represent the compound\u2019s polyatomic anion, [latex]{\\text{C}}_{2}{\\text{O}}_{4}{}^{2-}[\/latex].<\/p>\n<h2>Molecular Compounds<\/h2>\n<p>Many compounds do not contain ions but instead consist solely of discrete, neutral molecules. These <strong>molecular compounds<\/strong> (covalent compounds) result when atoms share, rather than transfer (gain or lose), electrons. Covalent bonding is an important and extensive concept in chemistry, and it will be treated in considerable detail in a later chapter of this text. We can often identify molecular compounds on the basis of their physical properties. Under normal conditions, molecular compounds often exist as gases, low-boiling liquids, and low-melting solids, although many important exceptions exist.<\/p>\n<p>Whereas ionic compounds are usually formed when a metal and a nonmetal combine, covalent compounds are usually formed by a combination of nonmetals. Thus, the periodic table can help us recognize many of the compounds that are covalent. While we can use the positions of a compound\u2019s elements in the periodic table to predict whether it is ionic or covalent at this point in our study of chemistry, you should be aware that this is a very simplistic approach that does not account for a number of interesting exceptions. Shades of gray exist between ionic and molecular compounds, and you\u2019ll learn more about those later.<\/p>\n<div class=\"textbox examples\">\n<h3>Example 5:\u00a0<strong>Predicting the Type of Bonding in Compounds<\/strong><\/h3>\n<p>Predict whether the following compounds are ionic or molecular:<\/p>\n<ol>\n<li>KI, the compound used as a source of iodine in table salt<\/li>\n<li>H<sub>2<\/sub>O<sub>2<\/sub>, the bleach and disinfectant hydrogen peroxide<\/li>\n<li>CHCl<sub>3<\/sub>, the anesthetic chloroform<\/li>\n<li>Li<sub>2<\/sub>CO<sub>3<\/sub>, a source of lithium in antidepressants<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q262423\">Show Answer<\/span><\/p>\n<div id=\"q262423\" class=\"hidden-answer\" style=\"display: none\">\n<ol>\n<li>Potassium (group 1) is a metal, and iodine (group 17) is a nonmetal; KI is predicted to be ionic.<\/li>\n<li>Hydrogen (group 1) is a nonmetal, and oxygen (group 16) is a nonmetal; H<sub>2<\/sub>O<sub>2<\/sub> is predicted to be molecular.<\/li>\n<li>Carbon (group 14) is a nonmetal, hydrogen (group 1) is a nonmetal, and chlorine (group 17) is a nonmetal; CHCl<sub>3<\/sub> is predicted to be molecular.<\/li>\n<li>Lithium (group 1A) is a metal, and carbonate is a polyatomic ion; Li<sub>2<\/sub>CO<sub>3<\/sub> is predicted to be ionic.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<h4><strong>Check Your Learning<\/strong><\/h4>\n<p>Using the periodic table, predict whether the following compounds are ionic or covalent:<\/p>\n<ol>\n<li>SO<sub>2<\/sub><\/li>\n<li>CaF<sub>2<\/sub><\/li>\n<li>N<sub>2<\/sub>H<sub>4<\/sub><\/li>\n<li>Al<sub>2<\/sub>(SO<sub>4<\/sub>)<sub>3<\/sub><\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q341117\">Show Answer<\/span><\/p>\n<div id=\"q341117\" class=\"hidden-answer\" style=\"display: none\">\n<ol>\n<li>molecular<\/li>\n<li>ionic<\/li>\n<li>molecular<\/li>\n<li>ionic<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Key Concepts and Summary<\/h3>\n<p>Metals (particularly those in groups 1 and 2) tend to lose the number of electrons that would leave them with the same number of electrons as in the preceding noble gas in the periodic table. By this means, a positively charged ion is formed. Similarly, nonmetals (especially those in groups 16 and 17, and, to a lesser extent, those in Group 15) can gain the number of electrons needed to provide atoms with the same number of electrons as in the next noble gas in the periodic table. Thus, nonmetals tend to form negative ions. Positively charged ions are called cations, and negatively charge ions are called anions. Ions can be either monatomic (containing only one atom) or polyatomic (containing more than one atom).<\/p>\n<p>Compounds that contain ions are called ionic compounds. Ionic compounds generally form from metals and nonmetals. Compounds that do not contain ions, but instead consist of atoms bonded tightly together in molecules (uncharged groups of atoms that behave as a single unit), are called covalent compounds. Covalent compounds usually form from two nonmetals.<\/p>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Exercises<\/h3>\n<ol>\n<li>Using the periodic table, predict whether the following chlorides are ionic or covalent: KCl, NCl<sub>3<\/sub>, ICl, MgCl<sub>2<\/sub>, PCl<sub>5<\/sub>, and CCl<sub>4<\/sub>.<\/li>\n<li>Using the periodic table, predict whether the following chlorides are ionic or covalent: SiCl<sub>4<\/sub>, PCl<sub>3<\/sub>, CaCl<sub>2<\/sub>, CsCl, CuCl<sub>2<\/sub>, and CrCl<sub>3<\/sub>.<\/li>\n<li>For each of the following compounds, state whether it is ionic or covalent. If it is ionic, write the symbols for the ions involved:\n<ol style=\"list-style-type: lower-alpha;\">\n<li>NF<sub>3<\/sub><\/li>\n<li>BaO,<\/li>\n<li>(NH<sub>4<\/sub>)<sub>2<\/sub>CO<sub>3\u00a0<\/sub><\/li>\n<li>Sr(H<sub>2<\/sub>PO<sub>4<\/sub>)<sub>2<\/sub><\/li>\n<li>IBr<\/li>\n<li>Na<sub>2<\/sub>O<\/li>\n<\/ol>\n<\/li>\n<li>For each of the following compounds, state whether it is ionic or covalent, and if it is ionic, write the symbols for the ions involved:\n<ol style=\"list-style-type: lower-alpha;\">\n<li>KClO<sub>4 \u00a0<\/sub><\/li>\n<li>MgC<sub>2<\/sub>H<sub>3<\/sub>O<sub>2 \u00a0<\/sub><\/li>\n<li>H<sub>2<\/sub>S<\/li>\n<li>Ag<sub>2<\/sub>S<\/li>\n<li>N<sub>2<\/sub>Cl<sub>4 \u00a0<\/sub><\/li>\n<li>Co(NO<sub>3<\/sub>)<sub>2<\/sub><\/li>\n<\/ol>\n<\/li>\n<li>For each of the following pairs of ions, write the symbol for the formula of the compound they will form:\n<ol style=\"list-style-type: lower-alpha;\">\n<li>Ca<sup>2+<\/sup>, S<sup>2- \u00a0<\/sup><\/li>\n<li>[latex]{\\text{NH}}_{4}{}^{+}[\/latex], [latex]{\\text{SO}}_{4}{}^{2-}[\/latex]<\/li>\n<li>Al<sup>3+<\/sup>, Br<sup>&#8211; \u00a0<\/sup>(d) Na<sup>+<\/sup>, [latex]{\\text{HPO}}_{4}{}^{2-}[\/latex] \u00a0(e) Mg<sup>2+<\/sup>, [latex]{\\text{PO}}_{4}{}^{3-}[\/latex]<\/li>\n<\/ol>\n<\/li>\n<li>For each of the following pairs of ions, write the symbol for the formula of the compound they will form:\n<ol style=\"list-style-type: lower-alpha;\">\n<li>K<sup>+<\/sup>, O<sup>2-<\/sup><\/li>\n<li>[latex]{\\text{NH}}_{4}{}^{+}[\/latex], [latex]{\\text{PO}}_{4}{}^{3-}[\/latex]<\/li>\n<li>Al<sup>3+<\/sup>, O<sup>2-<\/sup><\/li>\n<li>Na<sup>+<\/sup>, [latex]{\\text{CO}}_{3}{}^{2-}[\/latex]<\/li>\n<li>Ba<sup>2+<\/sup>, [latex]{\\text{PO}}_{4}{}^{3-}[\/latex]<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q265027\">Show Selected Answers<\/span><\/p>\n<div id=\"q265027\" class=\"hidden-answer\" style=\"display: none\">\n<p>1.\u00a0Ionic: KCl, MgCl<sub>2<\/sub>; Covalent: NCl<sub>3<\/sub>, ICl, PCl<sub>5<\/sub>, CCl<sub>4<\/sub><\/p>\n<p>3.\u00a0(a) covalent; (b) ionic, Ba<sup>2+<\/sup>, O<sup>2-<\/sup>; (c) ionic, [latex]{\\text{NH}}_{4}{}^{+}[\/latex], [latex]{\\text{CO}}_{3}{}^{2-};[\/latex] (d) ionic, Sr<sup>2+<\/sup>, [latex]{\\text{H}}_{2}{\\text{PO}}_{4}{}^{-};[\/latex] (e) covalent; (f) ionic, Na<sup>+<\/sup>, O<sup>2-<\/sup><\/p>\n<p>5.\u00a0(a) CaS; (b) (NH<sub>4<\/sub>)<sub>2<\/sub>CO<sub>3<\/sub>; (c) AlBr<sub>3<\/sub>; (d) Na<sub>2<\/sub>HPO<sub>4<\/sub>; (e) Mg<sub>3<\/sub> (PO<sub>4<\/sub>)<sub>2<\/sub><\/p>\n<\/div>\n<\/div>\n<\/div>\n<h2>Glossary<\/h2>\n<p><strong>covalent bond:\u00a0<\/strong>attractive force between the nuclei of a molecule\u2019s atoms and pairs of electrons between the atoms<\/p>\n<p><strong>covalent compound:\u00a0<\/strong>(also, molecular compound) composed of molecules formed by atoms of two or more different elements<\/p>\n<p><strong>ionic bond:\u00a0<\/strong>electrostatic forces of attraction between the oppositely charged ions of an ionic compound<\/p>\n<p><strong>ionic compound:\u00a0<\/strong>compound composed of cations and anions combined in ratios, yielding an electrically neutral substance<\/p>\n<p><strong>molecular compound:\u00a0<\/strong>(also, covalent compound) composed of molecules formed by atoms of two or more different elements<\/p>\n<p><strong>monatomic ion:\u00a0<\/strong>ion composed of a single atom<\/p>\n<p><strong>oxyanion:\u00a0<\/strong>polyatomic anion composed of a central atom bonded to oxygen atoms<\/p>\n<p><strong>polyatomic ion:\u00a0<\/strong>ion composed of more than one atom<\/p>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-1606\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Chemistry. <strong>Provided by<\/strong>: OpenStax College. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/openstaxcollege.org\">http:\/\/openstaxcollege.org<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at https:\/\/openstaxcollege.org\/textbooks\/chemistry\/get<\/li><\/ul><div class=\"license-attribution-dropdown-subheading\">All rights reserved content<\/div><ul class=\"citation-list\"><li>Conductivity molten salt. <strong>Authored by<\/strong>: Guy Ashkenazi. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/youtu.be\/ePzEVPDyJV8\">https:\/\/youtu.be\/ePzEVPDyJV8<\/a>. <strong>License<\/strong>: <em>All Rights Reserved<\/em>. <strong>License Terms<\/strong>: Standard YouTube License<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":11,"template":"","meta":{"_candela_citation":"[{\"type\":\"copyrighted_video\",\"description\":\"Conductivity molten salt\",\"author\":\"Guy Ashkenazi\",\"organization\":\"\",\"url\":\"https:\/\/youtu.be\/ePzEVPDyJV8\",\"project\":\"\",\"license\":\"arr\",\"license_terms\":\"Standard YouTube License\"},{\"type\":\"cc\",\"description\":\"Chemistry\",\"author\":\"\",\"organization\":\"OpenStax College\",\"url\":\"http:\/\/openstaxcollege.org\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at https:\/\/openstaxcollege.org\/textbooks\/chemistry\/get\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1606","chapter","type-chapter","status-publish","hentry"],"part":3014,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-chem-atoms-first\/wp-json\/pressbooks\/v2\/chapters\/1606","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-chem-atoms-first\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-chem-atoms-first\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-chem-atoms-first\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":8,"href":"https:\/\/courses.lumenlearning.com\/suny-chem-atoms-first\/wp-json\/pressbooks\/v2\/chapters\/1606\/revisions"}],"predecessor-version":[{"id":5613,"href":"https:\/\/courses.lumenlearning.com\/suny-chem-atoms-first\/wp-json\/pressbooks\/v2\/chapters\/1606\/revisions\/5613"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-chem-atoms-first\/wp-json\/pressbooks\/v2\/parts\/3014"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-chem-atoms-first\/wp-json\/pressbooks\/v2\/chapters\/1606\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-chem-atoms-first\/wp-json\/wp\/v2\/media?parent=1606"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-chem-atoms-first\/wp-json\/pressbooks\/v2\/chapter-type?post=1606"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-chem-atoms-first\/wp-json\/wp\/v2\/contributor?post=1606"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-chem-atoms-first\/wp-json\/wp\/v2\/license?post=1606"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}