A molecule is a collection of atoms held together by covalent bonds. In our simple universe of hydrogen and helium, the only possible molecule is ; but the one-in-a-thousand heavier atoms are the basis for a vast array of more complex molecules. The champion of molecule-forming atoms is carbon, for reasons that will become clear as we learn more about atomic structure. The chemistry of carbon compounds is so varied that it is given a special name, organic chemistry.

The term "organic" is a reminder that carbon compounds are the basis for the most complex chemical phenomenon of all, life.

The molecular weight of any molecule is the sum of the atomic weights of all its atoms. Since the atomic weight of a hydrogen atom is 1.008 amu (relative to carbon-12 as exactly 12 amu), the molecular weight of the H2 molecule is twice this value, or 2.016 amu.