Molecule is the smallest amount of a chemical substance that can exist.
Molecules are made up of atoms that are stuck together in a particular shape.
Not all combinations of atoms are possible; atoms like certain shapes, but not others.
For example, oxygen atoms always have two bonds with other atoms, carbon atoms always have four bonds with other atoms, and nitrogen atoms always have three bonds with other atoms.
Molecules are formed with two or more atoms.
For example. H = atom, while H +H → H2 is a molecule.