A basic skill in chemistry is the ability to write and understand chemical formulas. The formula for a chemical compound describes the number and type of atoms within a molecule. The formula identifies a very precise compound, distinguishable from other compounds. Chemical formulas are often written using the name of the compound although the ultimate source of information for determining both the name and formula of a compound are the results of experiments. An understanding of the arrangement of elements on the periodic table as well as the information the table provides will greatly expedite the writing of chemical formulas.
Differentiating Ionic and Molecular Compounds
Know the location of metals and nonmetals on the periodic table. Metals are located toward the left and middle of the table, and nonmetals are located toward the right side of the table.
Identify the compound as ionic by the presence of ions in the molecule. Metals are ions, so the presence of a metal in the compound indicates that the compound is ionic. Binary ionic compounds are easily identifiable as having two (hence, binary) elements, one being a metal positive ion (cation) and the other a nonmetal negative ion (anion). Ionic compounds may have more than two elements by incorporating a polyatomic ion (i.e., an ion containing more than one atom). Note that hydrogen can act as an anion with other nonmetals.
Identify molecular compounds by the absence of ions and the presence of only nonmetal atoms.
Formula for Ionic Compounds
Write the atomic symbol for each element in the molecule. Atomic symbols are single or double letter shorthand descriptions for each element. For example, C is the atomic symbol for carbon, Ne is the atomic symbol for Neon, and Fe is the atomic symbol for Iron. Atomic symbols are prominently displayed on the periodic table of the elements.
Determine the charge of each ion. For monoatomic (one element) ions that are not transition metals, this can be accomplished using the periodic table. With the exception of transition metals located in the middle of the table, the charge is fixed for the elements in each group (column) on the periodic table. Group 1A elements (called the alkali metals and consisting of lithium, sodium, etc.) have a +1 charge. Group 7A elements (called the halogens and consisting of chlorine, fluorine, etc.) have a -1 charge.
Write the charge of each ion above the symbol for the ion. In the ionic compound sodium chloride, the sodium is +1 and the chlorine is -1. A +1 is written above the symbol for sodium, Na, and a -1 is written above the symbol for chlorine (Cl).
Determine the number of ions in the compound by increasing the number of ions until the net charge is zero. For example, in potassium oxide the potassium ion has a +1 charge, but the oxygen ion has a -2 charge. Adding another potassium ion creates a +2 charge, which when subtracted from the -2 charge of oxygen produces a net charge of zero. Therefore, in potassium oxide there are two atoms of potassium for every one atom of oxygen.
Write a numeral (usually presented in subscript) after the ion if more than one ion is present. Polyatomic ions are enclosed in parentheses to indicate that the number represents the number of polyatomic ion “units” rather than additional atoms within the polyatomic ion. Potassium oxide in the previous example is written as K2O. Calcium hydroxide is written as Ca(OH)2, since there are two hydroxide ions (a polyatomic ion composed of hydrogen and oxygen with a charge of -1) for every one calcium ion (charge of +2).
Formula for Ionic Compounds with Transition Metals
Determine whether the compound has a transition metal for the ion. Transition metals are located in the center of the periodic table and have a variable oxidation number (charge). The charge of transition metals is written in the compound name by the inclusion of a Roman numeral surrounded by parentheses after the anion. Copper (I) sulfate has a +1 charge on the copper ion indicated by the Roman numeral I enclosed in parentheses. Copper (II) sulfate has a +2 charge on the copper ion indicated by the Roman numeral II.
Calculate the number of atoms for each element in the compound. Iron (II) oxide is chemically distinct from iron (III) oxide. In this case the Roman numeral depicts the oxidation number for the metal iron and is used for transition metals, which may have a variable oxidation number resulting in a variety of compounds.
Write the symbol for each ion in the compound and the charge of each ion above it.
Calculate the lowest common denominator for both oxidation values. In the example of iron (II) oxide, the iron has an oxidation number of +2, and oxygen (always) has an oxidation number of -2. The lowest common denominator for the absolute value of both numbers is 2. In the case of iron (III) oxide, iron has an oxidation number of +3, and oxygen has (again, always) an oxidation number of -2. The lowest common denominator for both numbers is 6.
Divide the lowest common denominator by the oxidation number to determine the number of atoms for each element in the compound. Dividing 2 into 2 yields a quotient of 1; therefore, in iron (II) oxide there is one atom of iron and one atom of oxygen. For iron (III) oxide, however, dividing the lowest common denominator into the oxidation number for each element yields a quotient of 2 for iron and 3 for oxygen. Thus, the chemical formula for iron (II) oxide is FeO whereas that of iron (III) oxide is Fe2O3.
Formula for Molecular Compounds
Write the symbol for each element in the chemical name.
Write the number of atoms after each symbol. Molecular compounds include the number of atoms in the name using a prefix. No prefix or the prefix “mono-” indicates one atom. The prefix “di-“ is used for two atoms, “tri-“ for three, “tetra-“ for four, and so on. Carbon tetrachloride is written as CCl4 since there is one atom of carbon and four atoms of chlorine.
Verify your formula by writing the chemical name that would be derived from the formula you wrote.