[P7.1, P8.1]
is: the smallest freely existing part of a chemical element or chemical compound that retains the chemical identity of that chemical element or chemical compound. [P7.1, P8.1]
therefore is usually: a group of atoms bound together. For compounds composed of identical molecules, the type and relative number of each sort of atom present in each molecule is indicated by the chemical formula of that substance. For example, a molecule of water contains one atom of oxygen and two atoms of hydrogen, and is represented by [P7.1, P8.1]
exceptionally: some molecules consist of single atoms (e.g. noble gases).
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