What is a chemical formula?

A chemical formula is a way of representing molecules without drawing them. The chemical formula shows the ratio of atoms that form any given molecule. Covalent and ionic compounds can be represented as chemical formulas. The following are the guidelines to writing and interpreting chemical formulas, including some examples of common ionic compounds and their formulas.

1. Always use the symbols from the periodic table; for example: "Mg".

2. The subscript numbers on each element represent the relative numbers of particles of each type in the molecule. These numbers represent the ratio of atoms within a molecule; for example: MgCl₂ has one magnesium atom (Mg) for every two chlorine atoms (Cl2).
3. If the element in a molecule is a metal, its symbol goes first and on the left; for example: NaCl, where Na (sodium) is the metal, and chlorine (Cl) is the nonmetal.

Here are some common ionic compounds found on Earth: NaCl, MgCl₂, HCl, NaH, NaF, LiF, CaCl₂. There are thousands more.