Elements combine to form chemical compounds that are often divided into two categories.

Metals often react with nonmetals to form ionic compounds. These compounds are composed of positive and negative ions formed by adding or subtracting electrons from neutral atoms and molecules.

Nonmetals combine with each other to form covalent compounds, which exist as neutral molecules.

The shorthand notation for a compound describes the number of atoms of each element, which is indicated by a subscript written after the symbol for the element. By convention, no subscript is written when a molecule contains only one atom of an element. Thus, water is H₂O and carbon dioxide is CO₂.

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