Molecules and formulae

Molecules of elements

Oxygen is an element. All its atoms are oxygen atoms. However, they do not exist separately. Instead, they pair up to form oxygen molecules (O2). Hydrogen also has molecules with two atoms (H2). We show this in their chemical formulae.

Oxygen atoms depicted as pairs of circlesOxygen
Hydrogen atoms depicted as pairs of circlesHydrogen

Molecules of compounds

Compounds are made from two or more atoms chemically joined together. For example, water is a compound of hydrogen and oxygen. Each water molecule contains two hydrogen atoms and one oxygen atom. We show this in its chemical formula, H2O.

Water: two hydrogen atoms and one oxygen atom joined together.Water
Carbon dioxode: two oxygen atoms and one carbon atom joined togetherCarbon dioxide
Alcohol: five hydrogen atoms attached to two carbon atoms, and one oxygen atom with one hydrogen atomEthanol
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