A chemical formula states which elements are present in a chemical species and how many atoms of each element belong to one molecular or formula unit.

A subscript applies to the element symbol immediately before it. Thus

$$\mathrm{H_2O}$$

contains two hydrogen atoms and one oxygen atom per molecule. An omitted subscript means one.

Parentheses group atoms that repeat together. In

$$\mathrm{Ca(NO_3)_2},$$

the outer subscript multiplies everything inside the parentheses, so one formula unit contains

• $1$ Ca atom,
• $2$ N atoms,
• $6$ O atoms.

For a discrete covalent molecule, the formula can describe the atom count in one molecule. For an extended ionic solid, there may be no separate molecule; a formula such as $\mathrm{NaCl}$ instead gives the simplest electrically neutral ratio of ions in a formula unit.

A superscript charge belongs to the whole ion rather than changing atom counts. For example,

$$\mathrm{SO_4^{2-}}$$

contains one sulfur atom and four oxygen atoms and carries net charge $-2$.

Subscripts are part of a species' identity. Changing $\mathrm{H_2O}$ to $\mathrm{H_2O_2}$ changes the compound itself; subscripts must therefore not be altered merely to make a later reaction equation balance.