Molecular Formula

A molecule is two or more atoms that have been chemically combined. A molecular formula is a chemical formula of a molecular compound that shows the kinds and numbers of atoms present in a molecule of the compound. Ammonia is a compound of nitrogen and hydrogen as shown below:

Note from the example that there are some standard rules to follow in writing molecular formulas. The arrangements of the elements depend on the particular structure, so we will not concern ourselves with that point right now. The number of atoms of each kind is indicated by a subscript following the atom. If there is only one atom, no number is written. If there is more than one atom of a specific kind, the number is written as a subscript following the atom. We would not write N₃H for ammonia, because that would mean that there are three nitrogen atoms and one hydrogen atom in the molecule, which is incorrect.

The molecular formula does not tell us anything about the shape of the molecule or where the different atoms are. The molecular formula for sucrose (table sugar) is C₁₂H₂₂O₁₁. This simply tells us the number of carbon, hydrogen, and oxygen atoms in the molecule. There is nothing said about where the individual atoms are located. We need a much more complicated formula (shown below) to communicate that information.

Empirical Formula

Discovering that a new compound exists is the start of a long research project. In order to make this new compound in the lab, we need to know a lot about its structure. Often, the place to start is to determine the elements in the material. Then we can find out the relative amounts of each element to continue our evaluation of this new material.

An empirical formula is a formula that shows the elements in a compound in their lowest whole-number ratio. Glucose is an important simple sugar that cells use as their primary source of energy. Its molecular formula is C₆H₁₂O₆. Since each of the subscripts is divisible by 6, the empirical formula for glucose is CH₂O. When chemists analyze an unknown compound, often the first step is to determine its empirical formula. There are a great many compounds whose molecular and empirical formulas are the same. If the molecular formula cannot be simplified into a smaller whole-number ratio, as in the case of H₂O or P₂O₅, then the empirical formula is also the molecular formula.

How would we determine an empirical formula for a compound? Let’s take a compound composed of carbon, hydrogen, and oxygen. We can analyze the relative amounts of each element in the compound. When we get a percent figure for each element, we now know how many grams of each are in 100 grams of the original material. This allows us to determine the number of moles for each element. The ratios can then be reduced to small whole numbers to give the empirical formula. If we wanted a molecular formula, we would need to determine the molecular weight of the compound.

Cations

Many of the elements we know about do not exist in their native form. They are so reactive that they are found only in compounds. These non-elemental forms are known as ions. Their properties are very different from those of the elements they come from. The term comes from a Greek word meaning “move” and was first coined by Michael Faraday, who studied the movement of materials in an electrical field.

Some elements lose one or more electrons in forming ions. These ions are known as “cations” because they are positively charged and migrate toward the negative electrode (cathode) in an electrical field. Looking at the periodic table below, we know that the group 1 elements are all characterized by having one s electron in the outer orbit and group 2 elements have two s electrons in the outer orbit. These electrons are loosely attached to the atom and can easily be removed, leaving more protons in the atom that there are electrons, so the resulting ion has a positive charge. Cations can also be formed from electron loss to many of the transition elements.

The cations are designated by the symbol for the parent element and a plus charge as a superscript after the element symbol: the potassium cation would be indicated as K⁺. Note that the charge is placed after the symbol and not before it. The potassium ion is monovalent, meaning that it has lost one electron and has a +1 charge. The symbol for the magnesium cation would be Mg²⁺ or Mg⁺⁺ to indicate that it has lost two electrons and has a +2 charge, so the magnesium cation would be referred to as a divalent cation.

The cations are simply named as the parent element. The sodium cation is still called “sodium.” Often, the charge would be attached for clarity, so the sodium cation might be referred to as “sodium one plus.”

Applications of Cations

Cations play important roles in our daily lives. Sodium, potassium, and magnesium ions are essential for such processes as blood pressure regulation and muscle contraction. Calcium ion is an important part of bone structure. Sodium ions can used in water softeners to remove other harmful elements. We put sodium chloride (table salt) on our food and use it as a preservative.

Anions

When a metal loses an electron, energy is needed to remove that electron. The other part of this process involves the addition of the electron to another element. The electron adds to the outer shell of the new element. Just as the loss of the electron from the metal produces a full shell, when the electron or electrons are added to the new element, it also results in a full shell.

Anions are negative ions that are formed when a nonmetal atom gains one or more electrons. Anions are so named because they are attracted to the anode (positive field) in an electric field. Atoms typically gain electrons so that they will have the electron configuration of a noble gas. All the elements in Group 17 have seven valence electrons due to the outer ns ² np ⁵ configuration. Therefore, each of these elements would gain one electron and become an anion with a 1− charge. Likewise, Group 16 elements form ions with a 2− charge, and the Group 15 nonmetals form ions with a 3− charge.

Naming anions is slightly different than naming cations. The ending of the element’s name is dropped and replaced with the – ide suffix. For example, F ^– is the fluoride ion, while O ^2- is the oxide ion. As is the case with cations, the charge on the anion is indicated by a superscript following the symbol. Common anions are listed in the Table below :

Anion Name	Symbol and Charge
fluoride	F –
chloride	Cl –
bromide	Br –
iodide	I –
oxide	O 2-
sulfide	S 2-
nitride	N 3-

Uses for Anions

Fluoride ion is widely used in water supplies to help prevent tooth decay. Chloride is an important component in ion balance in blood. Iodide ion is needed by the thyroid gland to make the hormone thyroxine.

Transition Metal Ions

The group 1 and 2 elements form cations through a simple process that involves the loss of one or more outer shell electrons. These electrons come from the s orbital and are removed very readily.

Most transition metals differ from the metals of Groups 1, 2, and 13 in that they are capable of forming more than one cation with different ionic charges. As an example, iron commonly forms two different ions. It can sometimes lose two electrons to form the Fe²⁺ ion, while at other times it loses three electrons to form the Fe³⁺ ion. Tin and lead, though members of the p block rather than the d block, also are capable of forming multiple ions.

Ionic formation for transition metals is complicated by the fact that these elements have unfilled inner d shells. Although the next higher s orbitals are actually at a lower energy level than the d level, these s electrons are the ones that are removed during ionization.

The Table below lists the names and formulas of some of the common transition metal ions:

Common Transition Metal Ions

1+	2+	3+	4+
copper(I), Cu +	cadmium, Cd 2+	chromium(III), Cr 3+	lead(IV), Pb 4+
gold(I), Au +	chromium(II), Cr 2+	cobalt(III), Co 3+	tin(IV), Sn 4+
mercury(I), Hg 2 2+	cobalt(II), Co 2+	gold(III), Au 3+
silver, Ag +	copper(II), Cu 2+	iron(III), Fe 3+
	iron(II), Fe 2+
	lead(II), Pb 2+
	manganese(II), Mn 2+
	mercury(II), Hg 2+
	nickel(II), Ni 2+
	platinum(II), Pt 2+
	tin(II), Sn 2+
	zinc, Zn 2+

Uses for Transition Metals

Because there are so many metals in this group, there are a wide variety of uses. Many of the metals are used in electronics, while others (such as gold and silver) are used in monetary systems. Iron is a versatile structural material. Cobalt, nickel, platinum, and other metals are employed as catalysts in a number of chemical reactions. Zinc is a significant component of batteries.

Naming Binary Ionic Compounds

A binary ionic compound is a compound composed of a monatomic metal cation and a monatomic nonmetal anion .

When examining the formula of a compound in order to name it, you must first decide what kind of compound it is. For a binary ionic compound, a metal will always be the first element in the formula, while a nonmetal will always be the second. The metal cation is named first, followed by the nonmetal anion. Subscripts in the formula do not affect the name. The Table below shows three examples:

Naming Binary Ionic Compounds

Formula	Name
KF	potassium fluoride
Na 3 N	sodium nitride
Ca 3 P 2	calcium phosphide

Notice that in each of the formulas above, the overall charge of the compound is zero. Potassium ion is K⁺, while fluoride ion is F⁻. Since the magnitude of the charges is equal, the formula contains one of each ion. This would also be the case for a compound such as MgS, in which the ions are Mg²⁺ and S²⁻. For sodium nitride, the sodium ion is Na⁺, while the nitride ion is N³⁻. In order to make a neutral compound, three of the 1+ sodium ions are required in order to balance out the single 3− nitride ion. So the Na is given a subscript of 3. For calcium phosphide, the calcium ion is Ca²⁺, while the phosphide ion is P³⁻. The least common multiple of 2 and 3 is 6. To make the compound neutral, three calcium ions have a total charge of 6+, while two phosphide ions have a total charge of 6−. The Ca is given a subscript of 3, while the P is given a subscript of 2.

Stock System Naming

Transition metals have more than one possibility for ion formation. In order to name these compounds correctly, we need to be able to indicate which ion is involved in any given compound.

Naming Compounds Using the Stock System

Naming compounds that involve transition metal cations necessitates the use of the Stock system. Consider the binary ionic compound FeCl₃. To simply name this compound iron chloride would be incomplete because iron is capable of forming two ions with different charges. The name of any iron-containing compound must reflect which iron ion is in the compound. In this case, the subscript in the formula indicates that there are three chloride ions, each with a 1− charge. Therefore, the charge of the single iron ion must be 3+. The correct name of FeCl₃ is iron(III) chloride, with the cation charge written as the Roman numeral. Here are several other examples.

Formula	Name
Cu2O	copper(I) oxide
CuO	copper(II) oxide
SnO2	tin(IV) oxide

The first two examples are both oxides of copper (shown in Figure 3). The ratio of copper ions to oxide ions determines the name. Since the oxide ion is O^₂₋, the charges of the copper ion must be 1+ in the first formula and 2+ in the second formula. In the third formula, there is one tin ion for every two oxide ions. This means that the tin must carry a 4+ charge, making the name tin(IV) oxide.

Copper(I) oxide, a red solid, and copper(II) oxide, a black solid, are different compounds because of the charge of the copper ion.

Formulas for Binary Ionic Compounds

Different professions also use a type of shorthand in communication to save time. Chemists use chemical symbols in combination to indicate specific compounds. There are two advantages to this approach:

1. The compound under discussion is clearly described so there can be no confusion about its identity.
2. Chemical symbols represent a universal language that all chemists can understand, no matter what their native language is.

Writing Formulas for Binary Ionic Compounds

If you know the name of a binary ionic compound, you can write its chemical formula . Start by writing the metal ion with its charge, followed by the nonmetal ion with its charge. Because the overall compound must be electrically neutral, decide how many of each ion is needed in order for the positive and negative charge to cancel each other out. Consider the compound aluminum nitride. The ions are:

Since the ions have charges that are equal in magnitude, one of each will be the lowest ratio of ions in the formula. The formula of aluminum nitride is AlN.

The ions for the compound lithium oxide are:

In this case, two lithium ions are required to balance out the charge of one oxide ion. The formula of lithium oxide is Li ₂ O.

An alternative way to writing a correct formula for an ionic compound is to use the crisscross method. In this method, the numerical value of each of the ion charges is crossed over to become the subscript of the other ion. Signs of the charges are dropped. Shown below is the crisscross method for aluminum oxide.

The red arrows indicate that the 3 from the 3+ charge will cross over to become the subscript of the O. The 2 from the 2− charge will cross over to become the subscript of the Al. The formula for aluminum oxide is Al ₂ O ₃ .

Be aware that ionic compounds are empirical formulas and so must be written as the lowest ratio of the ions. In the case of aluminum nitride, the crisscross method would yield a formula of Al ₃ N ₃ , which is not correct. It must be reduced to AlN. Following the crisscross method to write the formula for lead(IV) oxide would involve the following steps:

The crisscross first yields Pb ₂ O ₄ for the formula, but that must be reduced to the lower ratio and PbO ₂ is the correct formula.

Polyatomic Ions

Many materials exist as simple binary compounds composed of a metal cation and a non-metal anion, with each ion consisting of only one type of atom. Other combinations of atoms also exist, either larger ionic complexes or complete molecules. Some of the most useful materials we work with contain polyatomic ions.

A polyatomic ion is an ion composed of more than one atom. The ammonium ion consists of one nitrogen atom and four oxygen atoms. Together, they comprise a single ion with a 1+ charge and a formula of NH₄⁺. The carbonate ion consists of one carbon atom and three oxygen atoms and carries an overall charge of 2−. The formula of the carbonate ion is CO₃²⁻. The atoms of a polyatomic ion are tightly bonded together and so the entire ion behaves as a single unit. The figures below show several models.

A. The ammonium ion (NH4+) is a nitrogen atom (blue) bonded to four hydrogen atoms (white).	B. The hydroxide ion (OH−) is an oxygen atom (red) bonded to a hydrogen atom.	C. The carbonate ion (CO32−) is a carbon atom (black) bonded to three oxygen atoms.

The Table below lists a number of polyatomic ions by name and by structure. The heading for each column indicates the charge on the polyatomic ions in that group. Note that the vast majority of the ions listed are anions – there are very few polyatomic cations.

Common Polyatomic Ions
1−	2−	3−	1+	2+
acetate, CH3COO−	carbonate, CO32−	arsenate, AsO33−	ammonium, NH4+	dimercury, Hg22+
bromate, BrO3−	chromate, CrO42−	phosphite, PO33−
chlorate, ClO3−	dichromate, Cr2O72−	phosphate, PO43−
chlorite, ClO2−	hydrogen phosphate, HPO42−
cyanide, CN−	oxalate, C2O42−
dihydrogen phosphate, H2PO4−	peroxide, O22−
hydrogen carbonate, HCO3−	silicate, SiO32−
hydrogen sulfate, HSO4−	sulfate, SO42−
hydrogen sulfide, HS−	sulfite, SO32−
hydroxide, OH−
hypochlorite, ClO−
nitrate, NO3−
nitrite, NO2−
perchlorate, ClO4−
permanganate, MnO4−

The vast majority of polyatomic ions are anions, many of which end in –ate or –ite. Notice that in some cases such as nitrate (NO₃⁻) and nitrite (NO₂⁻), there are multiple anions that consist of the same two elements. In these cases, the difference between the ions is in the number of oxygen atoms present, while the overall charge is the same. As a class, these are called oxoanions. When there are two oxoanions for a particular element, the one with the greater number of oxygen atoms gets the –ate suffix, while the one with the fewer number of oxygen atoms gets the –ite suffix. The four oxoanions of chlorine are shown below.

• ClO⁻, hypochlorite
• ClO₂⁻, chlorite
• ClO₃⁻, chlorate
• ClO₄⁻, perchlorate

In cases such as this, the ion with one more oxygen atom than the –ate anion is given a per- prefix. The ion with one fewer oxygen atom than the –ite anion is given a hypo- prefix.

Names and Formulas of Ternary Ionic Compounds

Not all ionic compounds are composed of only monatomic ions. A ternary ionic compound is an ionic compound composed of three or more elements. In a typical ternary ionic compound, there is still one type of cation and one type of anion involved. The cation, the anion, or both, is a polyatomic ion.

Naming Ternary Ionic Compounds

The process of naming ternary ionic compounds is the same as naming binary ionic compounds. The cation is named first, followed by the anion. Some examples are shown in the Table below:

Examples of Ternary Ionic Compounds

Formula	Name
NaNO3	sodium nitrate
NH4Cl	ammonium chloride
Fe(OH)3	iron(III) hydroxide

When more than one polyatomic ion is present in a compound, the formula of the ion is placed in parentheses with a subscript outside of the parentheses that indicates how many of those ions are in the compound. In the last example above, there is one Fe³⁺ cation and three OH⁻ anions.

Writing Formulas for Ternary Ionic Compounds

Writing a formula for a ternary ionic compound also involves the same steps as for a binary ionic compound. Write the symbol and charge of the cation followed by the symbol and charge of the anion. Use the crisscross method to ensure that the final formula is neutral. Calcium nitrate is composed of a calcium cation and a nitrate anion.

The charge is balanced by the presence of two nitrate ions and one calcium ion. Parentheses are used around the nitrate ion because more than one of the polyatomic ion is needed. If only one polyatomic ion is in a formula, parentheses are not used. As an example, the formula for calcium carbonate is CaCO₃. The carbonate ion carries a 2− charge and so exactly balances the 2+ charge of the calcium ion.

There are two polyatomic ions that produce unusual formulas. The Hg₂²⁺ ion is called either the dimercury ion or, preferably, the mercury(I) ion. When bonded with an anion with a 1− charge, such as chloride, the formula is Hg₂Cl₂. Because the cation consists of two Hg atoms bonded together, this formula is not reduced to HgCl. Likewise, the peroxide ion, O₂²⁻, is also a unit that must stay together in its formulas. For example, the formula for potassium peroxide is K₂O₂.

Naming Binary Molecular Compounds

Inorganic chemical compounds can be broadly classified into two groups: ionic compounds and molecular compounds. The structure of all ionic compounds is an extended three-dimensional array of alternating positive and negative ions. Since ionic compounds do not take the form of individual molecules, they are represented by empirical formulas. Now we will begin to examine the formulas and nomenclature of molecular compounds.

Molecular Compounds

Molecular compounds are inorganic compounds that take the form of discrete molecules. Examples include such familiar substance as water (H₂O) and carbon dioxide (CO₂). These compounds are very different from ionic compounds like sodium chloride (NaCl). Ionic compounds are formed when metal atoms lose one or more of their electrons to nonmetal atoms. The resulting cations and anions are electrostatically attracted to each other.

So what holds the atoms of a molecule together? Rather than forming ions, the atoms of a molecule share their valence electrons in such a way that a bond forms between pairs of atoms. In a carbon dioxide molecule, there are two of these bonds, each occurring between the carbon atom and one of the two oxygen atoms.

Larger molecules can have many, many bonds that serve to keep the molecule together. In a large sample of a given molecular compound, all of the individual molecules are identical.

Naming Binary Molecular Compounds

Recall that a molecular formula shows the number of atoms of each element that a molecule contains. A molecule of water contains two hydrogen atoms and one oxygen atom, so its formula is H₂O. A molecule of octane, which is a component of gasoline, contains 8 atoms of carbon and 18 atoms of hydrogen. The molecular formula of octane is C₈H₁₈.

Figure 5 shows nitrogen dioxide (NO₂), which is a reddish-brown toxic gas that is a prominent air pollutant produced by internal combustion engines.

A binary molecular compound is a molecular compound that is composed of two elements. The elements that combine to form binary molecular compounds are both nonmetal atoms. This contrasts with ionic compounds, which were formed from a metal ion and a nonmetal ion. Therefore, binary molecular compounds are different because ionic charges cannot be used to name them or to write their formulas. Another difference is that two nonmetal atoms will frequently combine with one another in a variety of ratios. Consider the elements nitrogen and oxygen. They combine to make several compounds including NO, NO₂, and N₂O. They all can’t be called nitrogen oxide. How would someone know which one you were talking about? Each of the three compounds has very different properties and reactivity. A system to distinguish between compounds such as these is necessary.

Prefixes are used in the names of binary molecular compounds to identify the number of atoms of each element. The Table below shows the prefixes up to ten.

Numerical Prefixes

Number of Atoms	Prefix
1	mono-
2	di-
3	tri-
4	tetra-
5	penta-
6	hexa-
7	hepta-
8	octa-
9	nona-
10	deca-

The rules for using the prefix system of nomenclature of binary molecular compounds can be summarized as follows.

1. Generally, the less-electronegative element is written first in the formula, though there are a few exceptions. Carbon is always first in a formula and hydrogen is after nitrogen in a formula such as NH₃. The order of common nonmetals in binary compound formulas is C, P, N, H, S, I, Br, Cl, O, F.
2. When naming, the appropriate prefix is used only if there are more than one atom of that element in the formula.
3. The second element is named after the first, but with the ending of the element’s name changed to – ide. The appropriate prefix is always used for the second element.
4. The a or o at the end of a prefix is usually dropped from the name when the name of the element begins with a vowel. As an example, four oxygen atoms is tetroxide instead of tetraoxide.

Some examples of molecular compounds are listed in the Table below .

Formula	Name
NO	nitrogen monoxide
N2O	dinitrogen monoxide
S2Cl2	disulfur dichloride
Cl2O7	dichlorine heptoxide

Notice that the mono- prefix is not used with the nitrogen in the first compound, but is used with the oxygen in both of the first two examples. The S₂Cl₂ emphasizes that the formulas for molecular compounds are not reduced to their lowest ratios. The o of mono- and the a of hepta- are dropped from the name when paired with oxide.

Naming Acids

Acids

An acid can be defined in several ways. The most straightforward definition is that an acid is a molecular compound that contains one or more hydrogen atoms and produces hydrogen ions (H⁺) when dissolved in water.

This is a different type of compound than the others we have seen so far. Acids are molecular, which means that in their pure state they are individual molecules and do not adopt the extended three-dimensional structures of ionic compounds like NaCl. However, when these molecules are dissolved in water, the chemical bond between the hydrogen atom and the rest of the molecule breaks, leaving a positively-charged hydrogen ion and an anion. This can be symbolized in a chemical equation:

HCl → H⁺ + Cl⁻

Since acids produce H⁺ cations upon dissolving in water, the H of an acid is written first in the formula of an inorganic acid. The remainder of the acid (other than the H) is the anion after the acid dissolves. Organic acids are also an important class of compounds, but will not be discussed here. A binary acid is an acid that consists of hydrogen and one other element. The most common binary acids contain a halogen. An oxoacid is an acid that consists of hydrogen, oxygen, and a third element. The third element is usually a nonmetal.

Naming Acids

Since all acids contain hydrogen, the name of an acid is based on the anion that goes with it. These anions can either be monatomic or polyatomic. The name of all monatomic ions ends in – ide. The majority of polyatomic ions end in either – ate or – ite , though there are a few exceptions such as the cyanide ion (CN⁻). It is this suffix of the anion that determines how the acid is named as displayed in the rules and Table below.

Naming System for Acids

Anion Suffix	Example	Name of Acid	Example
– ide	chloride (Cl−)	hydro_____ic acid	hydrochloric acid (HCl)
– ate	sulfate (SO42−)	_____ic acid	sulfuric acid (H2SO4)
– ite	nitrite (NO2−)	_____ous acid	nitrous acid (HNO2)

The three different suffixes that are possible for the anions lead to the three rules below.

1. When the anion ends in – ide, the acid name begins with the prefix hydro-. The root of the anion name goes in the blank ( chlor for chloride), followed by the suffix –ic . HCl is hydrochloric acid because Cl⁻ is the chloride ion. HCN is hydrocyanic acid because CN⁻ is the cyanide ion.
2. When the anion ends in – ate, the name of the acid is the root of the anion followed by the suffix –ic . There is no prefix. H₂SO₄ is sulfuric acid (not sulfic) because SO₄²⁻ is the sulfate ion.
3. When the anion ends in – ite, the name of the acid is the root of the anion followed by the suffix –ous . Again, there is no prefix. HNO₂ is nitrous acid because NO₂⁻ is the nitrite ion.

Note how the root for a sulfur-containing oxoacid is sulfur- instead of just sulf-. The same is true for a phosphorus-containing oxoacid. The root is phosphor- instead of simply phosph-.

Many foods and beverages contain citric acid. Vinegar is a dilute solution of acetic acid. Car batteries contain sulfuric acid that helps in the release of electrons to create electricity.

Writing Formulas for Acids

Like other compounds that we have studied, acids are electrically neutral. Therefore, the charge of the anion part of the formula must be exactly balanced out by the H⁺ ions. Since H⁺ ions carry a single negative charge, the number of H⁺ ions in the formula is equal to the quantity of the negative charge on the anion. Two examples from the table above illustrate this point. The chloride ion carries a 1− charge, so only one H is needed in the formula of the acid (HCl). The sulfate ion carries a 2− charge, so two H’s are needed in the formula of the acid (H₂SO₄). Another way to think about writing the correct formula is to utilize the crisscross method, shown below for sulfuric acid.

Names and Formulas of Bases

Bases

The simplest way to define a base is an ionic compound that produces hydroxide ions when dissolved in water. One of the most commonly used bases is sodium hydroxide, illustrated below.

Names and Formulas of Bases

There is no special system for naming bases. Since they all contain the OH⁻ anion, names of bases end in hydroxide. The cation is simply named first. Some examples of names and formulas for bases are shown in the Table below.

Formula	Name
NaOH	sodium hydroxide
Ca(OH)2	calcium hydroxide
NH4OH	ammonium hydroxide

Notice that because bases are ionic compounds, the number of hydroxides in the formula does not affect the name. The compound must be neutral, so the charges of the ions are balanced just as for other ionic compounds. Sodium ion (Na⁺) requires one OH⁻ ion to balance the charge, so the formula is NaOH. Calcium ion (Ca²⁺) requires two OH⁻ ions to balance the charge, so the formula is Ca(OH)₂. Hydroxide ion is a polyatomic ion and must be put in parentheses when there are more than on in a formula.