{"id":1670,"date":"2014-10-20T20:52:30","date_gmt":"2014-10-20T20:52:30","guid":{"rendered":"https:\/\/courses.candelalearning.com\/apvccs\/?post_type=chapter&#038;p=1670"},"modified":"2017-06-14T16:42:22","modified_gmt":"2017-06-14T16:42:22","slug":"chemical-bonds","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/sjcd-austincc-ap1\/chapter\/chemical-bonds\/","title":{"raw":"Chemical Bonds","rendered":"Chemical Bonds"},"content":{"raw":"<div>\r\n<div class=\"bcc-box bcc-highlight\">\r\n<h3>Learning Objectives<\/h3>\r\n<div>\r\n<ul>\r\n \t<li>Explain the relationship between molecules and compounds<\/li>\r\n \t<li>Distinguish between ions, cations, and anions<\/li>\r\n \t<li>Identify the key difference between ionic and covalent bonds<\/li>\r\n \t<li>Distinguish between nonpolar and polar covalent bonds; describe the polarity of water<\/li>\r\n \t<li>Explain how water molecules link via hydrogen bonds<\/li>\r\n \t<li>Distinguish between hydrophilic &amp; hydrophobic substances and describe their interactions<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div>\r\n<ul>\r\n \t<li><a href=\"#m46000-fs-id2526643\">Ions and Ionic Bonds<\/a><\/li>\r\n \t<li><a href=\"#m46000-fs-id1616095\">Covalent Bonds<\/a>\r\n<ul>\r\n \t<li><a href=\"#m46000-fs-id2002703\">Nonpolar Covalent Bonds<\/a><\/li>\r\n \t<li><a href=\"#m46000-fs-id1648387\">Polar Covalent Bonds<\/a><\/li>\r\n<\/ul>\r\n<\/li>\r\n \t<li><a href=\"#m46000-fs-id2021878\">Hydrogen Bonds<\/a><\/li>\r\n \t<li><a href=\"#m46000-fs-id1616096\">Water and Hydrogen Bonding<\/a><\/li>\r\n \t<li><a href=\"#m46000-fs-id1616097\">Water Interactions:\u00a0Hydrophilic and Hydrophobic<\/a><\/li>\r\n<\/ul>\r\n<\/div>\r\nAtoms separated by a great distance cannot link; rather, they must come close enough for the electrons in their valence shells to interact. But do atoms ever actually touch one another? Most physicists would say no, because the negatively charged electrons in their valence shells repel one another. No force within the human body\u2014or anywhere in the natural world\u2014is strong enough to overcome this electrical repulsion. So when you read about atoms linking together or colliding, bear in mind that the atoms are not merging in a physical sense.\r\n\r\nInstead, atoms link by forming a chemical bond. A\u00a0<strong><em>bond<\/em><a id=\"id595577\"><\/a>\u00a0<\/strong>is a weak or strong electrical attraction that holds atoms in the same vicinity. The new grouping is typically more stable\u2014less likely to react again\u2014than its component atoms were when they were separate. A more or less stable grouping of two or more atoms held together by chemical bonds is called a\u00a0<strong><em>molecule<\/em><\/strong><a id=\"id595597\"><\/a>. The bonded atoms may be of the same element, as in the case of H<sub>2<\/sub>, which is called molecular hydrogen or hydrogen gas. When a molecule is made up of two or more atoms of different elements, it is called a chemical\u00a0<strong><em>compound<\/em><\/strong><a id=\"id595616\"><\/a>. Thus, a unit of water, or H<sub>2<\/sub>O, is a compound, as is a single molecule of the gas methane, or CH<sub>4<\/sub>.\r\n\r\nThree types of chemical bonds are important in human physiology, because they hold together substances that are used by the body for critical aspects of homeostasis, signaling, and energy production, to name just a few important processes. These are ionic bonds, covalent bonds, and hydrogen bonds.\r\n<div title=\"Ions and Ionic Bonds\">\r\n<div>\r\n<h3 id=\"m46000-fs-id2526643\">Ions and Ionic Bonds<\/h3>\r\n<\/div>\r\nRecall that an atom typically has the same number of positively charged protons and negatively charged electrons. As long as this situation remains, the atom is electrically neutral. But when an atom participates in a chemical reaction that results in its donating or accepting\u00a0of one or more electrons, the atom will then become positively or negatively charged. This happens frequently for most atoms in order to have a full valence shell, as described previously. This can happen either by gaining electrons to fill a shell that is more than half-full, or by giving away electrons to empty a shell than is less than half-full, thereby leaving the next smaller electron shell as the new, full, valence shell. An atom that has an electrical charge\u2014whether positive or negative\u2014is an\u00a0<strong><em>ion<\/em><\/strong><a id=\"id595673\"><\/a>.\r\n<div id=\"m46000-fs-id1285208\">\r\n<div>\r\n\r\nVisit this\u00a0<a href=\"http:\/\/openstaxcollege.org\/l\/electenergy\" target=\"_blank\" rel=\"noopener noreferrer\">website<\/a>\u00a0to learn about electrical energy and the attraction\/repulsion of charges. What happens to the charged electroscope when a conductor is moved between its plastic sheets, and why?\r\n\r\n<\/div>\r\n<\/div>\r\nPotassium (K), for instance, is an important element in all body cells. Its atomic number is 19. It has just one electron in its valence shell. This characteristic makes potassium highly likely to participate in chemical reactions in which it donates one electron. (It is easier for potassium to donate one electron than to gain seven electrons.) The loss will cause the positive charge of potassium\u2019s protons to be more influential than the negative charge of potassium\u2019s electrons. In other words, the resulting potassium ion will be slightly positive. A potassium ion is written K<sup>+<\/sup>, indicating that it has lost a single electron. A positively charged ion is known as a\u00a0<strong><em>cation<\/em><\/strong><a id=\"id595758\"><\/a>.\r\n\r\nNow consider fluorine (F), a component of bones and teeth. Its atomic number is nine, and it has seven electrons in its valence shell. Thus, it is highly likely to bond with other atoms in such a way that fluorine accepts one electron (it is easier for fluorine to gain one electron than to donate seven electrons). When it does, its electrons will outnumber its protons by one, and it will have an overall negative charge. The ionized form of fluorine is called fluoride, and is written as F<sup>\u2013<\/sup>. A negatively charged ion is known as an\u00a0<strong><em>anion<\/em><\/strong><a id=\"id595791\"><\/a>.\r\n\r\nAtoms that have more than one electron to donate or accept will end up with stronger positive or negative charges. A cation that has donated two electrons has a net charge of +2. Using magnesium (Mg) as an example, this can be written Mg<sup>++<\/sup>\u00a0or Mg<sup>2+<\/sup>. An anion that has accepted two electrons has a net charge of \u20132. The ionic form of selenium (Se), for example, is typically written Se<sup>2\u2013<\/sup>.\r\n\r\nThe opposite charges of cations and anions exert a moderately strong mutual attraction that keeps the atoms in close proximity forming an ionic bond. An\u00a0<strong><em>ionic bond<\/em><\/strong><a id=\"id595836\"><\/a>\u00a0is an ongoing, close association between ions of opposite charge. The table salt you sprinkle on your food owes its existence to ionic bonding. As shown in\u00a0<a title=\"Figure\u00a02.8.\u00a0Ionic Bonding\" href=\"#m46000-fig-ch02_02_01\">Figure\u00a02.8<\/a>, sodium commonly donates an electron to chlorine, becoming the cation Na<sup>+<\/sup>. When chlorine accepts the electron, it becomes the chloride anion, Cl<sup>\u2013<\/sup>. With their opposing charges, these two ions strongly attract each other.\r\n<div id=\"m46000-fig-ch02_02_01\" title=\"Figure\u00a02.8.\u00a0Ionic Bonding\">\r\n<div>\r\n<div><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180753\/207_Ionic_Bonding-01.jpg\" alt=\"The top panel of this figure shows the orbit model of a sodium atom and a chlorine atom and arrows pointing towards the transfer of electrons from sodium to chlorine to form sodium and chlorine ions. The bottom panel shows sodium and chloride ions in a crystal structure.\" width=\"420\" \/><\/div>\r\n<\/div>\r\n<address><strong>Figure\u00a02.8.\u00a0Ionic Bonding<\/strong><\/address><address>(a) Sodium readily donates the solitary electron in its valence shell to chlorine, which needs only one electron to have a full valence shell. (b) The opposite electrical charges of the resulting sodium cation and chloride anion result in the formation of a bond of attraction called an ionic bond. (c) The attraction of many sodium and chloride ions results in the formation of large groupings called crystals.<\/address><address>\u00a0<\/address><\/div>\r\nWater is an essential component of life because it is able to break the ionic bonds in salts to free the ions. In fact, in biological fluids, most individual atoms exist as ions. These dissolved ions produce electrical charges within the body. The behavior of these ions produces the tracings of heart and brain function observed as waves on an electrocardiogram (EKG or ECG) or an electroencephalogram (EEG). The electrical activity that derives from the interactions of the charged ions is why they are also called electrolytes.\r\n\r\n<\/div>\r\n<div title=\"Covalent Bonds\">\r\n<div>\r\n<h2 id=\"m46000-fs-id1616095\">Covalent Bonds<\/h2>\r\n<\/div>\r\nCovalent bonds are the type of bonds in which two atoms are connected to each other by the sharing of two or more electrons. So, unlike ionic bonds that are formed by the attraction between a cation\u2019s positive charge and an anion\u2019s negative charge, molecules formed by a\u00a0<em><strong>covalent<\/strong> <strong>bond<\/strong><\/em><strong><a id=\"id595934\"><\/a>\u00a0<\/strong>share electrons in a mutually stabilizing relationship. Like next-door neighbors whose kids hang out first at one home and then at the other, the atoms do not lose or gain electrons permanently. Instead, the electrons circling the atomic nucleus are shared, and they move back and forth between the elements. Because of the close sharing of pairs of electrons (one electron from each of two atoms), covalent bonds are <em>stronger<\/em> than ionic bonds. Covalent bonds found in inorganic molecules, such as H<sub>2<\/sub>O, O<sub>2<\/sub>, and CO<sub>2<\/sub>, as well as in, organic molecules, such as proteins and the DNA molecule.\r\n\r\nThe nature of the covalent bond is determined by the number of electrons shared and the nature of the two elements sharing the bond. Two or more atoms held together by covalent bonds in a specified arrangement is called a molecule. Covalent bonds are of two types: <strong>nonpolar<\/strong> and <strong>polar<\/strong>.<strong>\u00a0<\/strong>Nonpolar and polar covalent bonds differ on the basis of whether they share electrons equally or unequally. The relative attraction of an atom to an electron is known as its electronegativity. Atoms that attract an electron more strongly are considered to be more <em>electronegative<\/em>.\r\n<div title=\"Nonpolar Covalent Bonds\">\r\n<div>\r\n<h3 id=\"m46000-fs-id2002703\">Nonpolar Covalent Bonds<\/h3>\r\n<\/div>\r\nNonpolar covalent bonds form between atoms of the same or similar electronegativities, most often two nonmetals.\u00a0<a title=\"Figure\u00a02.9.\u00a0Covalent Bonding\" href=\"#m46000-fig-ch02_02_02\">Figure\u00a02.9<\/a>\u00a0shows several common types of covalent bonds. Notice that the two covalently bonded atoms typically share just one or two electron pairs, though larger sharings are possible. The important concept to take from this is that in covalent bonds, electrons in the outermost valence shell are shared to fill the valence shells of both atoms, ultimately stabilizing both of the atoms involved. In a single covalent bond, a single electron is shared between two atoms, while in a double covalent bond, two pairs of electrons are shared between two atoms. There even are triple covalent bonds, where three atoms are shared. The more covalent bonds between two atoms, the stronger the bond.\r\n<div id=\"m46000-fig-ch02_02_02\" title=\"Figure\u00a02.9.\u00a0Covalent Bonding\">\r\n<div>\r\n<div><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180755\/208_Covalent_Bonding-01.jpg\" alt=\"The top panel in this figure shows two hydrogen atoms sharing two electrons. The middle panel shows two oxygen atoms sharing four electrons, and the bottom panel shows two oxygen atoms and one carbon atom sharing 2 pairs of electrons each.\" width=\"550\" \/><\/div>\r\n<\/div>\r\n<address><strong>Figure\u00a02.9.\u00a0Covalent Bonding<\/strong><\/address><address>\u00a0<\/address><\/div>\r\nYou can see that the covalent bonds shown in\u00a0<a title=\"Figure\u00a02.9.\u00a0Covalent Bonding\" href=\"#m46000-fig-ch02_02_02\">Figure\u00a02.9<\/a>\u00a0are balanced. The sharing of the negative electrons is relatively equal, as is the electrical pull of the positive protons in the nucleus of the atoms involved. This is why covalently bonded molecules that are electrically balanced in this way are described as nonpolar; that is, no region of the molecule is either more positive or more negative than any other.\r\n\r\n<\/div>\r\n<div title=\"Polar Covalent Bonds\">\r\n<div>\r\n<h3 id=\"m46000-fs-id1648387\">Polar Covalent Bonds<\/h3>\r\n<\/div>\r\nGroups of legislators with completely opposite views on a particular issue are often described as \u201cpolarized\u201d by news writers. In chemistry, when electrons in a covalent bond are shared in an <em>unequal<\/em> manner it is termed a <em><strong>polar<\/strong><\/em>, or <strong><em>polar covalent, bond. <\/em><\/strong>Because of the unequal sharing and distribution of electrons between atoms, a slightly negative or a slightly positive charge develops,\u00a0and it results in the formation of a\u00a0<em>polar molecule<\/em><a id=\"id596049\"><\/a>\u00a0. A polar molecule is a molecule that contains regions that have opposite electrical charges.\r\n\r\nThe most familiar example of a polar molecule is water (<a title=\"Figure\u00a02.10.\u00a0Polar Covalent Bonds in a Water Molecule\" href=\"#m46000-fig-ch02_02_03\">Figure\u00a02.10<\/a>). The molecule has three parts: one atom of oxygen, the nucleus of which contains eight protons, and two hydrogen atoms, whose nuclei each contain only one proton. Because every proton exerts an identical positive charge, a nucleus that contains eight protons exerts a charge eight times greater than a nucleus that contains one proton. This means that the negatively charged electrons present in the water molecule are more strongly attracted to the oxygen nucleus than to the hydrogen nuclei. Each hydrogen atom\u2019s single negative electron therefore migrates toward the oxygen atom, making the oxygen end of their bond slightly more negative than the hydrogen end of their bond.\r\n<div id=\"m46000-fig-ch02_02_03\" title=\"Figure\u00a02.10.\u00a0Polar Covalent Bonds in a Water Molecule\">\r\n<div>\r\n<div><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180757\/209_Polar_Covalent_Bonds_in_a_Water_Molecule.jpg\" alt=\"This figure shows the structure of a water molecule. The top panel shows two oxygen atoms and one hydrogen atom with electrons in orbit and the shared electrons. The middle panel shows a three-dimensional model of a water molecule and the bottom panel shows the structural formula for water.\" width=\"380\" \/><\/div>\r\n<\/div>\r\n<address><strong>Figure\u00a02.10.\u00a0Polar Covalent Bonds in a Water Molecule<\/strong><\/address><address>\u00a0<\/address><\/div>\r\nWhat is true for the bonds is true for the water molecule as a whole; that is, the oxygen region has a slightly negative charge and the regions of the hydrogen atoms have a slightly positive charge. These charges are often referred to as \u201cpartial charges\u201d because the strength of the charge is less than one full electron, as would occur in an ionic bond. As shown in\u00a0<a title=\"Figure\u00a02.10.\u00a0Polar Covalent Bonds in a Water Molecule\" href=\"#m46000-fig-ch02_02_03\">Figure\u00a02.10<\/a>, regions of weak polarity are indicated with the Greek letter delta (\u2202) and a plus (+) or minus (\u2013) sign.\r\n\r\nEven though a single water molecule is unimaginably tiny, it has mass, and the opposing electrical charges on the molecule pull that mass in such a way that it creates a shape somewhat like a triangular tent (see\u00a0<a title=\"Figure\u00a02.10.\u00a0Polar Covalent Bonds in a Water Molecule\" href=\"#m46000-fig-ch02_02_03\">Figure\u00a02.10<\/a><strong>b<\/strong>). This <em>dipole<\/em>, with the positive charges at one end formed by the hydrogen atoms at the \u201cbottom\u201d of the tent and the negative charge at the opposite end (the oxygen atom at the \u201ctop\u201d of the tent) makes the charged regions highly likely to interact with charged regions of other polar molecules. For human physiology, the resulting bond is one of the most important formed by water\u2014the hydrogen bond.\r\n\r\n<\/div>\r\n<\/div>\r\n<div title=\"Hydrogen Bonds\">\r\n<div>\r\n<h2 id=\"m46000-fs-id2021878\">Hydrogen Bonds<\/h2>\r\n<\/div>\r\nA\u00a0<em>hydrogen bond<\/em><a id=\"id596194\"><\/a>\u00a0is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for example, the oxygen in the water molecule) is attracted to another electronegative atom from another molecule. In other words, hydrogen bonds always include hydrogen that is already part of a polar molecule.\r\n\r\nThe most common example of hydrogen bonding in the natural world occurs between molecules of water. It happens before your eyes whenever two raindrops merge into a larger bead, or a creek spills into a river. Hydrogen bonding occurs because the weakly negative oxygen atom in one water molecule is attracted to the weakly positive hydrogen atoms of two other water molecules (<a title=\"Figure\u00a02.11.\u00a0Hydrogen Bonds between Water Molecules\" href=\"#m46000-fig-ch02_02_04\">Figure\u00a02.11<\/a>).\r\n<div id=\"m46000-fig-ch02_02_04\" title=\"Figure\u00a02.11.\u00a0Hydrogen Bonds between Water Molecules\">\r\n<div>\r\n<div><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180800\/210_Hydrogen_Bonds_Between_Water_Molecules-01.jpg\" alt=\"This figure shows three water molecules and the hydrogen bonds between them.\" width=\"280\" \/><\/div>\r\n<\/div>\r\n<address><strong>Figure\u00a02.11.\u00a0Hydrogen Bonds between Water Molecules<\/strong><\/address><address>Notice that the bonds occur between the weakly positive charge on the hydrogen atoms and the weakly negative charge on the oxygen atoms. Hydrogen bonds are relatively weak, and therefore are indicated with a dotted (rather than a solid) line.<\/address><address>\u00a0<\/address><\/div>\r\nWater molecules also strongly attract other types of charged molecules as well as ions. This explains why \u201ctable salt,\u201d for example, actually is a molecule called a \u201csalt\u201d in chemistry, which consists of equal numbers of positively-charged sodium (Na<sup>+<\/sup>) and negatively-charged chloride (Cl<sup>\u2013<\/sup>), dissolves so readily in water, in this case forming dipole-ion bonds between the water and the electrically-charged ions (electrolytes). Water molecules also repel molecules with nonpolar covalent bonds, like fats, lipids, and oils. You can demonstrate this with a simple kitchen experiment: pour a teaspoon of vegetable oil, a compound formed by nonpolar covalent bonds, into a glass of water. Instead of instantly dissolving in the water, the oil forms a distinct bead because the polar water molecules repel the nonpolar oil.\r\n<h2 id=\"m46000-fs-id1616096\">Water and Hydrogen Bonding<\/h2>\r\nWater is the basis of life. Without it, life is not possible. Water accounts for up to 75 percent of the weight of the human body. Water provides a relatively stable medium in which chemical reactions can take place. Water\u2019s unique chemical properties (as a solvent with a high boiling point and high heat capacity) make it essential for homeostasis. The body\u2019s thermoregulation relies on water\u2019s high heat capacity to buffer it against swings in external temperature. Cooling of the body is carried out by evaporative water loss\u2014perspiration. Water is also vital as a transport medium. Oxygen is carried by red blood cells suspended in serum, which is mostly water. Nutritional substances are dissolved in water and transported to cells. Water is used to dissolve and dilute waste molecules. If the body is deprived of water for very long, death will result.\r\n\r\nWater has several properties that contribute to its suitability to support life as we know it. One of these properties is that water is a <strong>polar molecule<\/strong>. Oxygen is more electronegative than hydrogen and draws the electrons that it shares in the covalent bond towards itself. Because water is polar, the partial positive end of one water molecule will be attracted to the partial negative end of a neighboring water molecule. This attraction is called a hydrogen bond. Hydrogen bonding (defined as the attraction of an atom with high electronegativity for a hydrogen atom that is covalently bonded to another highly electronegative atom. This involves the attraction of a hydrogen atom with a partial positive charge to an atom with a partial negative charge. Only hydrogen atoms covalently bonded to a highly electronegative atom can participate in hydrogen bonding.) occurs between partially negatively charged atoms with high electronegativity\u2014oxygen, nitrogen or fluorine\u2014and partially positively charged hydrogen atoms that are bonded to oxygen, nitrogen or fluorine atoms.\r\n\r\n<strong>Hydrogen bonding<\/strong>, which is referred to as an intermolecular attraction, is a critical interaction within the cell. It is the principal force that holds the tertiary structure of proteins, carbohydrates and nucleic acids together and the overall stability of these molecules is due in part to the cumulative effect of the large number of hydrogen bonds found in the functional structures. Hydrogen bonds are found in and between a variety of molecules. For example, the enormous number of hydrogen bonds between strands of plant cellulose provide the strength and structure of the plant cell wall. As another example, wool (sheep hair) has lots of proteins with an enormous number of hydrogen bonds that provide the curly structure of individual wool fibers. These curly fibers trap air spaces which makes wool such a good insulator. When washed at high temperatures, these hydrogen bonds are broken and the wool fibers will lose their shape, probably damaging any wool clothing.\r\n\r\n<a href=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/198\/2014\/11\/20090439\/Screen-Shot-2014-11-20-at-2.45.25-PM.png\"><img class=\"alignnone size-medium wp-image-2695\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/198\/2014\/11\/20090439\/Screen-Shot-2014-11-20-at-2.45.25-PM-300x258.png\" alt=\"Screen Shot 2014-11-20 at 2.45.25 PM\" width=\"300\" height=\"258\" \/><\/a>\r\n\r\n<address class=\"caption\">The electronegative oxygen (red) draws electrons to it, creating the partial negative charge on oxygen and partial positive charge on hydrogen.<\/address><address class=\"caption\">\u00a0<\/address><iframe src=\"\/\/www.youtube.com\/embed\/NjERpZZw0A0\" width=\"640\" height=\"360\" frameborder=\"0\"><\/iframe>\r\n<h2 id=\"m46000-fs-id1616097\">Water Interactions<\/h2>\r\n<h3>Water and Ions<\/h3>\r\nThe attraction between oppositely charged ions in an ionic compound is strong. However, because of the polarity of water, when many ionic compounds are in aqueous solutions they become dissociated into ions. For instance, when an ionic compound such as table salt (NaCl) is dissolved in water, it separates into Na<sup>+<\/sup> and Cl<sup>-<\/sup> ions. The water molecules surround the ions to form polar interactions such that the positive ends of the water molecules are arranged around negative ions and the negative ends of the water molecules surround positive ions. Thus the ions become encapsulated by water spheres, which are called spheres of hydration. The biological world is very ionic, and spheres of hydration are important in a cell because they maintain the separation of the many ions of the cell from each other. The sphere of hydration must be broken in order for binding to take place with a specific binding partner.\r\n<h3>Hydrophilic Interaction<\/h3>\r\nThe nature of polar molecules is that they contain highly electronegative atoms. Consequently, many are capable of hydrogen bonding with aqueous or polar solvents. Because polar molecules are generally water soluble, they are referred to as being <em><strong>hydrophilic<\/strong><\/em>, or water-loving. The one-carbon alcohol, methanol, is an example of a polar molecule.\r\n<h3>Hydrophobic Interaction<\/h3>\r\nThe final type of interaction occurs between neutral <em><strong>hydrophobic<\/strong><\/em>, or water-fearing, molecules. These nonpolar molecules do not interact with water and are characterized by atoms with the same or nearly the same electronegativities. In aqueous solutions, the hydrophobic molecules are driven together to the exclusion of water. For example, shaking a bottle of oil and vinegar (acetic acid in water), such as in a salad dressing, results in the oil being dispersed as tiny droplets in the vinegar. As the mixture settles, the oil collects in larger and larger drops until it only exists as a layer, or phase, above the vinegar.\r\n\r\nA similar effect occurs in biological systems. As a protein folds to its final three-dimensional structure, the hydrophobic parts of the protein are forced together and away from the aqueous environment of the cell. Similarly, biological membranes are stabilized by the exclusion of water between layers of lipids as we will see later.\r\n\r\nAmphipathic molecules are molecules that have a distinct nonpolar, or hydrophobic, region, and a distinct polar region. These molecules do not form true solutions in water. Rather, the nonpolar parts are forced together into a nonpolar aggregate, leaving the polar part of the molecule to interact with the aqueous phase. Detergents and long-chain carboxylic acids are examples of amphipathic molecules.\r\n\r\n<\/div>","rendered":"<div>\n<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\n<div>\n<ul>\n<li>Explain the relationship between molecules and compounds<\/li>\n<li>Distinguish between ions, cations, and anions<\/li>\n<li>Identify the key difference between ionic and covalent bonds<\/li>\n<li>Distinguish between nonpolar and polar covalent bonds; describe the polarity of water<\/li>\n<li>Explain how water molecules link via hydrogen bonds<\/li>\n<li>Distinguish between hydrophilic &amp; hydrophobic substances and describe their interactions<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<div>\n<ul>\n<li><a href=\"#m46000-fs-id2526643\">Ions and Ionic Bonds<\/a><\/li>\n<li><a href=\"#m46000-fs-id1616095\">Covalent Bonds<\/a>\n<ul>\n<li><a href=\"#m46000-fs-id2002703\">Nonpolar Covalent Bonds<\/a><\/li>\n<li><a href=\"#m46000-fs-id1648387\">Polar Covalent Bonds<\/a><\/li>\n<\/ul>\n<\/li>\n<li><a href=\"#m46000-fs-id2021878\">Hydrogen Bonds<\/a><\/li>\n<li><a href=\"#m46000-fs-id1616096\">Water and Hydrogen Bonding<\/a><\/li>\n<li><a href=\"#m46000-fs-id1616097\">Water Interactions:\u00a0Hydrophilic and Hydrophobic<\/a><\/li>\n<\/ul>\n<\/div>\n<p>Atoms separated by a great distance cannot link; rather, they must come close enough for the electrons in their valence shells to interact. But do atoms ever actually touch one another? Most physicists would say no, because the negatively charged electrons in their valence shells repel one another. No force within the human body\u2014or anywhere in the natural world\u2014is strong enough to overcome this electrical repulsion. So when you read about atoms linking together or colliding, bear in mind that the atoms are not merging in a physical sense.<\/p>\n<p>Instead, atoms link by forming a chemical bond. A\u00a0<strong><em>bond<\/em><a id=\"id595577\"><\/a>\u00a0<\/strong>is a weak or strong electrical attraction that holds atoms in the same vicinity. The new grouping is typically more stable\u2014less likely to react again\u2014than its component atoms were when they were separate. A more or less stable grouping of two or more atoms held together by chemical bonds is called a\u00a0<strong><em>molecule<\/em><\/strong><a id=\"id595597\"><\/a>. The bonded atoms may be of the same element, as in the case of H<sub>2<\/sub>, which is called molecular hydrogen or hydrogen gas. When a molecule is made up of two or more atoms of different elements, it is called a chemical\u00a0<strong><em>compound<\/em><\/strong><a id=\"id595616\"><\/a>. Thus, a unit of water, or H<sub>2<\/sub>O, is a compound, as is a single molecule of the gas methane, or CH<sub>4<\/sub>.<\/p>\n<p>Three types of chemical bonds are important in human physiology, because they hold together substances that are used by the body for critical aspects of homeostasis, signaling, and energy production, to name just a few important processes. These are ionic bonds, covalent bonds, and hydrogen bonds.<\/p>\n<div title=\"Ions and Ionic Bonds\">\n<div>\n<h3 id=\"m46000-fs-id2526643\">Ions and Ionic Bonds<\/h3>\n<\/div>\n<p>Recall that an atom typically has the same number of positively charged protons and negatively charged electrons. As long as this situation remains, the atom is electrically neutral. But when an atom participates in a chemical reaction that results in its donating or accepting\u00a0of one or more electrons, the atom will then become positively or negatively charged. This happens frequently for most atoms in order to have a full valence shell, as described previously. This can happen either by gaining electrons to fill a shell that is more than half-full, or by giving away electrons to empty a shell than is less than half-full, thereby leaving the next smaller electron shell as the new, full, valence shell. An atom that has an electrical charge\u2014whether positive or negative\u2014is an\u00a0<strong><em>ion<\/em><\/strong><a id=\"id595673\"><\/a>.<\/p>\n<div id=\"m46000-fs-id1285208\">\n<div>\n<p>Visit this\u00a0<a href=\"http:\/\/openstaxcollege.org\/l\/electenergy\" target=\"_blank\" rel=\"noopener noreferrer\">website<\/a>\u00a0to learn about electrical energy and the attraction\/repulsion of charges. What happens to the charged electroscope when a conductor is moved between its plastic sheets, and why?<\/p>\n<\/div>\n<\/div>\n<p>Potassium (K), for instance, is an important element in all body cells. Its atomic number is 19. It has just one electron in its valence shell. This characteristic makes potassium highly likely to participate in chemical reactions in which it donates one electron. (It is easier for potassium to donate one electron than to gain seven electrons.) The loss will cause the positive charge of potassium\u2019s protons to be more influential than the negative charge of potassium\u2019s electrons. In other words, the resulting potassium ion will be slightly positive. A potassium ion is written K<sup>+<\/sup>, indicating that it has lost a single electron. A positively charged ion is known as a\u00a0<strong><em>cation<\/em><\/strong><a id=\"id595758\"><\/a>.<\/p>\n<p>Now consider fluorine (F), a component of bones and teeth. Its atomic number is nine, and it has seven electrons in its valence shell. Thus, it is highly likely to bond with other atoms in such a way that fluorine accepts one electron (it is easier for fluorine to gain one electron than to donate seven electrons). When it does, its electrons will outnumber its protons by one, and it will have an overall negative charge. The ionized form of fluorine is called fluoride, and is written as F<sup>\u2013<\/sup>. A negatively charged ion is known as an\u00a0<strong><em>anion<\/em><\/strong><a id=\"id595791\"><\/a>.<\/p>\n<p>Atoms that have more than one electron to donate or accept will end up with stronger positive or negative charges. A cation that has donated two electrons has a net charge of +2. Using magnesium (Mg) as an example, this can be written Mg<sup>++<\/sup>\u00a0or Mg<sup>2+<\/sup>. An anion that has accepted two electrons has a net charge of \u20132. The ionic form of selenium (Se), for example, is typically written Se<sup>2\u2013<\/sup>.<\/p>\n<p>The opposite charges of cations and anions exert a moderately strong mutual attraction that keeps the atoms in close proximity forming an ionic bond. An\u00a0<strong><em>ionic bond<\/em><\/strong><a id=\"id595836\"><\/a>\u00a0is an ongoing, close association between ions of opposite charge. The table salt you sprinkle on your food owes its existence to ionic bonding. As shown in\u00a0<a title=\"Figure\u00a02.8.\u00a0Ionic Bonding\" href=\"#m46000-fig-ch02_02_01\">Figure\u00a02.8<\/a>, sodium commonly donates an electron to chlorine, becoming the cation Na<sup>+<\/sup>. When chlorine accepts the electron, it becomes the chloride anion, Cl<sup>\u2013<\/sup>. With their opposing charges, these two ions strongly attract each other.<\/p>\n<div id=\"m46000-fig-ch02_02_01\" title=\"Figure\u00a02.8.\u00a0Ionic Bonding\">\n<div>\n<div><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180753\/207_Ionic_Bonding-01.jpg\" alt=\"The top panel of this figure shows the orbit model of a sodium atom and a chlorine atom and arrows pointing towards the transfer of electrons from sodium to chlorine to form sodium and chlorine ions. The bottom panel shows sodium and chloride ions in a crystal structure.\" width=\"420\" \/><\/div>\n<\/div>\n<address><strong>Figure\u00a02.8.\u00a0Ionic Bonding<\/strong><\/address>\n<address>(a) Sodium readily donates the solitary electron in its valence shell to chlorine, which needs only one electron to have a full valence shell. (b) The opposite electrical charges of the resulting sodium cation and chloride anion result in the formation of a bond of attraction called an ionic bond. (c) The attraction of many sodium and chloride ions results in the formation of large groupings called crystals.<\/address>\n<address>\u00a0<\/address>\n<\/div>\n<p>Water is an essential component of life because it is able to break the ionic bonds in salts to free the ions. In fact, in biological fluids, most individual atoms exist as ions. These dissolved ions produce electrical charges within the body. The behavior of these ions produces the tracings of heart and brain function observed as waves on an electrocardiogram (EKG or ECG) or an electroencephalogram (EEG). The electrical activity that derives from the interactions of the charged ions is why they are also called electrolytes.<\/p>\n<\/div>\n<div title=\"Covalent Bonds\">\n<div>\n<h2 id=\"m46000-fs-id1616095\">Covalent Bonds<\/h2>\n<\/div>\n<p>Covalent bonds are the type of bonds in which two atoms are connected to each other by the sharing of two or more electrons. So, unlike ionic bonds that are formed by the attraction between a cation\u2019s positive charge and an anion\u2019s negative charge, molecules formed by a\u00a0<em><strong>covalent<\/strong> <strong>bond<\/strong><\/em><strong><a id=\"id595934\"><\/a>\u00a0<\/strong>share electrons in a mutually stabilizing relationship. Like next-door neighbors whose kids hang out first at one home and then at the other, the atoms do not lose or gain electrons permanently. Instead, the electrons circling the atomic nucleus are shared, and they move back and forth between the elements. Because of the close sharing of pairs of electrons (one electron from each of two atoms), covalent bonds are <em>stronger<\/em> than ionic bonds. Covalent bonds found in inorganic molecules, such as H<sub>2<\/sub>O, O<sub>2<\/sub>, and CO<sub>2<\/sub>, as well as in, organic molecules, such as proteins and the DNA molecule.<\/p>\n<p>The nature of the covalent bond is determined by the number of electrons shared and the nature of the two elements sharing the bond. Two or more atoms held together by covalent bonds in a specified arrangement is called a molecule. Covalent bonds are of two types: <strong>nonpolar<\/strong> and <strong>polar<\/strong>.<strong>\u00a0<\/strong>Nonpolar and polar covalent bonds differ on the basis of whether they share electrons equally or unequally. The relative attraction of an atom to an electron is known as its electronegativity. Atoms that attract an electron more strongly are considered to be more <em>electronegative<\/em>.<\/p>\n<div title=\"Nonpolar Covalent Bonds\">\n<div>\n<h3 id=\"m46000-fs-id2002703\">Nonpolar Covalent Bonds<\/h3>\n<\/div>\n<p>Nonpolar covalent bonds form between atoms of the same or similar electronegativities, most often two nonmetals.\u00a0<a title=\"Figure\u00a02.9.\u00a0Covalent Bonding\" href=\"#m46000-fig-ch02_02_02\">Figure\u00a02.9<\/a>\u00a0shows several common types of covalent bonds. Notice that the two covalently bonded atoms typically share just one or two electron pairs, though larger sharings are possible. The important concept to take from this is that in covalent bonds, electrons in the outermost valence shell are shared to fill the valence shells of both atoms, ultimately stabilizing both of the atoms involved. In a single covalent bond, a single electron is shared between two atoms, while in a double covalent bond, two pairs of electrons are shared between two atoms. There even are triple covalent bonds, where three atoms are shared. The more covalent bonds between two atoms, the stronger the bond.<\/p>\n<div id=\"m46000-fig-ch02_02_02\" title=\"Figure\u00a02.9.\u00a0Covalent Bonding\">\n<div>\n<div><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180755\/208_Covalent_Bonding-01.jpg\" alt=\"The top panel in this figure shows two hydrogen atoms sharing two electrons. The middle panel shows two oxygen atoms sharing four electrons, and the bottom panel shows two oxygen atoms and one carbon atom sharing 2 pairs of electrons each.\" width=\"550\" \/><\/div>\n<\/div>\n<address><strong>Figure\u00a02.9.\u00a0Covalent Bonding<\/strong><\/address>\n<address>\u00a0<\/address>\n<\/div>\n<p>You can see that the covalent bonds shown in\u00a0<a title=\"Figure\u00a02.9.\u00a0Covalent Bonding\" href=\"#m46000-fig-ch02_02_02\">Figure\u00a02.9<\/a>\u00a0are balanced. The sharing of the negative electrons is relatively equal, as is the electrical pull of the positive protons in the nucleus of the atoms involved. This is why covalently bonded molecules that are electrically balanced in this way are described as nonpolar; that is, no region of the molecule is either more positive or more negative than any other.<\/p>\n<\/div>\n<div title=\"Polar Covalent Bonds\">\n<div>\n<h3 id=\"m46000-fs-id1648387\">Polar Covalent Bonds<\/h3>\n<\/div>\n<p>Groups of legislators with completely opposite views on a particular issue are often described as \u201cpolarized\u201d by news writers. In chemistry, when electrons in a covalent bond are shared in an <em>unequal<\/em> manner it is termed a <em><strong>polar<\/strong><\/em>, or <strong><em>polar covalent, bond. <\/em><\/strong>Because of the unequal sharing and distribution of electrons between atoms, a slightly negative or a slightly positive charge develops,\u00a0and it results in the formation of a\u00a0<em>polar molecule<\/em><a id=\"id596049\"><\/a>\u00a0. A polar molecule is a molecule that contains regions that have opposite electrical charges.<\/p>\n<p>The most familiar example of a polar molecule is water (<a title=\"Figure\u00a02.10.\u00a0Polar Covalent Bonds in a Water Molecule\" href=\"#m46000-fig-ch02_02_03\">Figure\u00a02.10<\/a>). The molecule has three parts: one atom of oxygen, the nucleus of which contains eight protons, and two hydrogen atoms, whose nuclei each contain only one proton. Because every proton exerts an identical positive charge, a nucleus that contains eight protons exerts a charge eight times greater than a nucleus that contains one proton. This means that the negatively charged electrons present in the water molecule are more strongly attracted to the oxygen nucleus than to the hydrogen nuclei. Each hydrogen atom\u2019s single negative electron therefore migrates toward the oxygen atom, making the oxygen end of their bond slightly more negative than the hydrogen end of their bond.<\/p>\n<div id=\"m46000-fig-ch02_02_03\" title=\"Figure\u00a02.10.\u00a0Polar Covalent Bonds in a Water Molecule\">\n<div>\n<div><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180757\/209_Polar_Covalent_Bonds_in_a_Water_Molecule.jpg\" alt=\"This figure shows the structure of a water molecule. The top panel shows two oxygen atoms and one hydrogen atom with electrons in orbit and the shared electrons. The middle panel shows a three-dimensional model of a water molecule and the bottom panel shows the structural formula for water.\" width=\"380\" \/><\/div>\n<\/div>\n<address><strong>Figure\u00a02.10.\u00a0Polar Covalent Bonds in a Water Molecule<\/strong><\/address>\n<address>\u00a0<\/address>\n<\/div>\n<p>What is true for the bonds is true for the water molecule as a whole; that is, the oxygen region has a slightly negative charge and the regions of the hydrogen atoms have a slightly positive charge. These charges are often referred to as \u201cpartial charges\u201d because the strength of the charge is less than one full electron, as would occur in an ionic bond. As shown in\u00a0<a title=\"Figure\u00a02.10.\u00a0Polar Covalent Bonds in a Water Molecule\" href=\"#m46000-fig-ch02_02_03\">Figure\u00a02.10<\/a>, regions of weak polarity are indicated with the Greek letter delta (\u2202) and a plus (+) or minus (\u2013) sign.<\/p>\n<p>Even though a single water molecule is unimaginably tiny, it has mass, and the opposing electrical charges on the molecule pull that mass in such a way that it creates a shape somewhat like a triangular tent (see\u00a0<a title=\"Figure\u00a02.10.\u00a0Polar Covalent Bonds in a Water Molecule\" href=\"#m46000-fig-ch02_02_03\">Figure\u00a02.10<\/a><strong>b<\/strong>). This <em>dipole<\/em>, with the positive charges at one end formed by the hydrogen atoms at the \u201cbottom\u201d of the tent and the negative charge at the opposite end (the oxygen atom at the \u201ctop\u201d of the tent) makes the charged regions highly likely to interact with charged regions of other polar molecules. For human physiology, the resulting bond is one of the most important formed by water\u2014the hydrogen bond.<\/p>\n<\/div>\n<\/div>\n<div title=\"Hydrogen Bonds\">\n<div>\n<h2 id=\"m46000-fs-id2021878\">Hydrogen Bonds<\/h2>\n<\/div>\n<p>A\u00a0<em>hydrogen bond<\/em><a id=\"id596194\"><\/a>\u00a0is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for example, the oxygen in the water molecule) is attracted to another electronegative atom from another molecule. In other words, hydrogen bonds always include hydrogen that is already part of a polar molecule.<\/p>\n<p>The most common example of hydrogen bonding in the natural world occurs between molecules of water. It happens before your eyes whenever two raindrops merge into a larger bead, or a creek spills into a river. Hydrogen bonding occurs because the weakly negative oxygen atom in one water molecule is attracted to the weakly positive hydrogen atoms of two other water molecules (<a title=\"Figure\u00a02.11.\u00a0Hydrogen Bonds between Water Molecules\" href=\"#m46000-fig-ch02_02_04\">Figure\u00a02.11<\/a>).<\/p>\n<div id=\"m46000-fig-ch02_02_04\" title=\"Figure\u00a02.11.\u00a0Hydrogen Bonds between Water Molecules\">\n<div>\n<div><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180800\/210_Hydrogen_Bonds_Between_Water_Molecules-01.jpg\" alt=\"This figure shows three water molecules and the hydrogen bonds between them.\" width=\"280\" \/><\/div>\n<\/div>\n<address><strong>Figure\u00a02.11.\u00a0Hydrogen Bonds between Water Molecules<\/strong><\/address>\n<address>Notice that the bonds occur between the weakly positive charge on the hydrogen atoms and the weakly negative charge on the oxygen atoms. Hydrogen bonds are relatively weak, and therefore are indicated with a dotted (rather than a solid) line.<\/address>\n<address>\u00a0<\/address>\n<\/div>\n<p>Water molecules also strongly attract other types of charged molecules as well as ions. This explains why \u201ctable salt,\u201d for example, actually is a molecule called a \u201csalt\u201d in chemistry, which consists of equal numbers of positively-charged sodium (Na<sup>+<\/sup>) and negatively-charged chloride (Cl<sup>\u2013<\/sup>), dissolves so readily in water, in this case forming dipole-ion bonds between the water and the electrically-charged ions (electrolytes). Water molecules also repel molecules with nonpolar covalent bonds, like fats, lipids, and oils. You can demonstrate this with a simple kitchen experiment: pour a teaspoon of vegetable oil, a compound formed by nonpolar covalent bonds, into a glass of water. Instead of instantly dissolving in the water, the oil forms a distinct bead because the polar water molecules repel the nonpolar oil.<\/p>\n<h2 id=\"m46000-fs-id1616096\">Water and Hydrogen Bonding<\/h2>\n<p>Water is the basis of life. Without it, life is not possible. Water accounts for up to 75 percent of the weight of the human body. Water provides a relatively stable medium in which chemical reactions can take place. Water\u2019s unique chemical properties (as a solvent with a high boiling point and high heat capacity) make it essential for homeostasis. The body\u2019s thermoregulation relies on water\u2019s high heat capacity to buffer it against swings in external temperature. Cooling of the body is carried out by evaporative water loss\u2014perspiration. Water is also vital as a transport medium. Oxygen is carried by red blood cells suspended in serum, which is mostly water. Nutritional substances are dissolved in water and transported to cells. Water is used to dissolve and dilute waste molecules. If the body is deprived of water for very long, death will result.<\/p>\n<p>Water has several properties that contribute to its suitability to support life as we know it. One of these properties is that water is a <strong>polar molecule<\/strong>. Oxygen is more electronegative than hydrogen and draws the electrons that it shares in the covalent bond towards itself. Because water is polar, the partial positive end of one water molecule will be attracted to the partial negative end of a neighboring water molecule. This attraction is called a hydrogen bond. Hydrogen bonding (defined as the attraction of an atom with high electronegativity for a hydrogen atom that is covalently bonded to another highly electronegative atom. This involves the attraction of a hydrogen atom with a partial positive charge to an atom with a partial negative charge. Only hydrogen atoms covalently bonded to a highly electronegative atom can participate in hydrogen bonding.) occurs between partially negatively charged atoms with high electronegativity\u2014oxygen, nitrogen or fluorine\u2014and partially positively charged hydrogen atoms that are bonded to oxygen, nitrogen or fluorine atoms.<\/p>\n<p><strong>Hydrogen bonding<\/strong>, which is referred to as an intermolecular attraction, is a critical interaction within the cell. It is the principal force that holds the tertiary structure of proteins, carbohydrates and nucleic acids together and the overall stability of these molecules is due in part to the cumulative effect of the large number of hydrogen bonds found in the functional structures. Hydrogen bonds are found in and between a variety of molecules. For example, the enormous number of hydrogen bonds between strands of plant cellulose provide the strength and structure of the plant cell wall. As another example, wool (sheep hair) has lots of proteins with an enormous number of hydrogen bonds that provide the curly structure of individual wool fibers. These curly fibers trap air spaces which makes wool such a good insulator. When washed at high temperatures, these hydrogen bonds are broken and the wool fibers will lose their shape, probably damaging any wool clothing.<\/p>\n<p><a href=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/198\/2014\/11\/20090439\/Screen-Shot-2014-11-20-at-2.45.25-PM.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-medium wp-image-2695\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/198\/2014\/11\/20090439\/Screen-Shot-2014-11-20-at-2.45.25-PM-300x258.png\" alt=\"Screen Shot 2014-11-20 at 2.45.25 PM\" width=\"300\" height=\"258\" \/><\/a><\/p>\n<address class=\"caption\">The electronegative oxygen (red) draws electrons to it, creating the partial negative charge on oxygen and partial positive charge on hydrogen.<\/address>\n<address class=\"caption\">\u00a0<\/address>\n<p><iframe loading=\"lazy\" src=\"\/\/www.youtube.com\/embed\/NjERpZZw0A0\" width=\"640\" height=\"360\" frameborder=\"0\"><\/iframe><\/p>\n<h2 id=\"m46000-fs-id1616097\">Water Interactions<\/h2>\n<h3>Water and Ions<\/h3>\n<p>The attraction between oppositely charged ions in an ionic compound is strong. However, because of the polarity of water, when many ionic compounds are in aqueous solutions they become dissociated into ions. For instance, when an ionic compound such as table salt (NaCl) is dissolved in water, it separates into Na<sup>+<\/sup> and Cl<sup>&#8211;<\/sup> ions. The water molecules surround the ions to form polar interactions such that the positive ends of the water molecules are arranged around negative ions and the negative ends of the water molecules surround positive ions. Thus the ions become encapsulated by water spheres, which are called spheres of hydration. The biological world is very ionic, and spheres of hydration are important in a cell because they maintain the separation of the many ions of the cell from each other. The sphere of hydration must be broken in order for binding to take place with a specific binding partner.<\/p>\n<h3>Hydrophilic Interaction<\/h3>\n<p>The nature of polar molecules is that they contain highly electronegative atoms. Consequently, many are capable of hydrogen bonding with aqueous or polar solvents. Because polar molecules are generally water soluble, they are referred to as being <em><strong>hydrophilic<\/strong><\/em>, or water-loving. The one-carbon alcohol, methanol, is an example of a polar molecule.<\/p>\n<h3>Hydrophobic Interaction<\/h3>\n<p>The final type of interaction occurs between neutral <em><strong>hydrophobic<\/strong><\/em>, or water-fearing, molecules. These nonpolar molecules do not interact with water and are characterized by atoms with the same or nearly the same electronegativities. In aqueous solutions, the hydrophobic molecules are driven together to the exclusion of water. For example, shaking a bottle of oil and vinegar (acetic acid in water), such as in a salad dressing, results in the oil being dispersed as tiny droplets in the vinegar. As the mixture settles, the oil collects in larger and larger drops until it only exists as a layer, or phase, above the vinegar.<\/p>\n<p>A similar effect occurs in biological systems. As a protein folds to its final three-dimensional structure, the hydrophobic parts of the protein are forced together and away from the aqueous environment of the cell. Similarly, biological membranes are stabilized by the exclusion of water between layers of lipids as we will see later.<\/p>\n<p>Amphipathic molecules are molecules that have a distinct nonpolar, or hydrophobic, region, and a distinct polar region. These molecules do not form true solutions in water. Rather, the nonpolar parts are forced together into a nonpolar aggregate, leaving the polar part of the molecule to interact with the aqueous phase. Detergents and long-chain carboxylic acids are examples of amphipathic molecules.<\/p>\n<\/div>\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-1670\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Original<\/div><ul class=\"citation-list\"><li>Revision and Adaptation. <strong>Authored by<\/strong>: AThomas. <strong>Provided by<\/strong>: Austin Community College. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em><\/li><\/ul><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Chapter 2. <strong>Authored by<\/strong>: OpenStax College. <strong>Provided by<\/strong>: Rice University. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@7.1@7.1.\">http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@7.1@7.1.<\/a>. <strong>Project<\/strong>: Anatomy &amp; Physiology. <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>: This content is available for free at http:\/\/cnx.org\/content\/col11496\/1.6<\/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":74,"menu_order":4,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Chapter 2\",\"author\":\"OpenStax College\",\"organization\":\"Rice University\",\"url\":\"http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@7.1@7.1.\",\"project\":\"Anatomy & Physiology\",\"license\":\"cc-by\",\"license_terms\":\"This content is available for free at http:\/\/cnx.org\/content\/col11496\/1.6\"},{\"type\":\"original\",\"description\":\"Revision and Adaptation\",\"author\":\"AThomas\",\"organization\":\"Austin Community College\",\"url\":\"\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1670","chapter","type-chapter","status-publish","hentry"],"part":1635,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/sjcd-austincc-ap1\/wp-json\/pressbooks\/v2\/chapters\/1670","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/sjcd-austincc-ap1\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/sjcd-austincc-ap1\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/sjcd-austincc-ap1\/wp-json\/wp\/v2\/users\/74"}],"version-history":[{"count":19,"href":"https:\/\/courses.lumenlearning.com\/sjcd-austincc-ap1\/wp-json\/pressbooks\/v2\/chapters\/1670\/revisions"}],"predecessor-version":[{"id":3311,"href":"https:\/\/courses.lumenlearning.com\/sjcd-austincc-ap1\/wp-json\/pressbooks\/v2\/chapters\/1670\/revisions\/3311"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/sjcd-austincc-ap1\/wp-json\/pressbooks\/v2\/parts\/1635"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/sjcd-austincc-ap1\/wp-json\/pressbooks\/v2\/chapters\/1670\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/sjcd-austincc-ap1\/wp-json\/wp\/v2\/media?parent=1670"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/sjcd-austincc-ap1\/wp-json\/pressbooks\/v2\/chapter-type?post=1670"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/sjcd-austincc-ap1\/wp-json\/wp\/v2\/contributor?post=1670"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/sjcd-austincc-ap1\/wp-json\/wp\/v2\/license?post=1670"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}