Key Concepts: Chemical Formulas

A chemical formula is a representation of a substance using symbols for its constituent elements.
The formula provides two types of information: the elements present in a compound and the ratio of atoms of one element to another.
Subscripts in chemical formulas represent the number of atoms of an element in a compound. For instance, in H2O, there are 2 hydrogen atoms and 1 oxygen atom.
For molecules, molecular formulas show the actual number of atoms of each element present. E.g. for water, the molecular formula is H2O.
Empirical formulas represent the simplest ratio of atoms of each element in a compound. For instance, for hydrogen peroxide, which has a molecular formula of H2O2, the empirical formula would be HO.
Certain compounds can exist as discrete groups of atoms which are called moieties. For instance, sulphate ions (SO4 2-) or nitrate ions (NO3 -).
The structural formula of a compound shows the arrangement of atoms within the molecule. These are particularly useful when dealing with organic compounds. For example, the structual formula for methane is CH4.
Hydrates have water molecules associated with them. For example, copper sulfate pentahydrate (CuSO4.5H2O) consists of 1 molecule of copper sulfate associated with 5 water molecules.
Balancing chemical formulas in equations is key. To comply with the law of conservation of mass, each side of the equation must have the same number of atoms of each element.
Some elements, such as oxygen, nitrogen, and the halogens, exist as diatomic molecules under normal conditions. This means they exist in pairs in their natural state, represented as O2, N2, and so forth.
Ionic compounds are represented with their empirical formula. For example, the ionic compound sodium chloride is represented as NaCl, indicating a 1:1 ratio of sodium ions to chloride ions.
Polyatomic ions (ions containing multiple atoms) have their own chemical formulas and charges. For instance, the hydroxide ion (OH-) or the ammonia ion (NH4+).
In transition metals and their compounds, roman numerals indicate the charge of the ion. These numbers are usually included in the name, e.g. Iron (III) sulfate shows that the iron has a charge of +3.
Element symbols are always written with a capital first letter (and a lowercase second letter if present), following the international IUPAC system. E.g. Na, Cl, Mg, Fe.

Remember, learning the chemical formulas of common ions and compounds will facilitate understanding more complex chemical reactions and principles.