Molecule

In chemistry, the molecule is the smallest indivisible portion of a pure
compound that retains a set of unique chemical and physical properties. A
molecule consists of two or more atoms bonded together.

Most molecules are much too small to be seen with the naked eye, but there
are exceptions. A grain of salt, or the diamond on an engagement ring, are
giant crystal lattices, repetitive molecules with atomic bonding (usually
ionic bonding) connecting the entire structure.

Molecules with the same atoms in different arrangements are called isomers.