Writing Chemical Formulas
A chemical formula is a scientific abbreviation used by chemists to represent a chemical compound. A chemical formula shows which elements are present in the compound and how many atoms of each element are present in each molecule. Each element is represented by its chemical symbol, which is followed by a subscript indicating the number of atoms of that element in a molecule of the compound. When the element is present as one atom, no subscript is shown.
For example, the chemical formula of water is H2O. The subscript 2 indicates the number of hydrogen atoms in each molecule. Oxygen is not followed by any number, meaning there is only one oxygen atom in each molecule
A chemical formula also indicates the ratio of atoms of each element in a compound. The ratio of hydrogen atoms to oxygen atoms in water is 2 to 1. Similarly, the chemical formula Fe2O3 indicates that the ratio of iron to oxygen atoms is 2 to 3.
Although ions are charged, the net charge on an ionic compound is zero; the charges of the cation and anion cancel each other. Knowing the charges of the ions making up the compound, you can predict the chemical formula for an ionic compound composed of two elements. One way to determine the formula of an ionic compound is the crisscross method: use the charge on each ion as a subscript of the other ion. For example, magnesium is a group 2 metal that forms a 2+ cation. Nitrogen is in group 5 and forms a 3− anion called nitride.
For instance, the crisscross method is applied as shown below to determine the chemical formula of magnesium nitride.
If the formula is correct, the charges will cancel, showing the compound has zero charge. To check the formula, multiply the subscript of each ion by its charge and find their sum: [3 × (+2)] + [2 × (−3)] = 0.
Like all ionic compounds, compounds containing polyatomic ions must be neutral. The charge indicated for a polyatomic ion applies to the ion as a whole. Therefore, when you construct chemical formulas, treat a polyatomic ion the same way you treat an ion consisting of one element. For example, the chemical formula of a compound formed of Na+ and NO3− can be written by considering the charges on the two ions and then using the crisscross method as follows:
If multiples of a polyatomic ion are present in an ionic compound, put parentheses around the formula of the polyatomic ion, and put the subscript to the right of the parentheses. For example, the chemical formula of a compound formed of Ba2+ and OH− can be determined using the crisscross method as shown below.
