Electrons in atoms surround the nucleus in a series of shells, with similar energies within one shell and different energies from one shell to the next. The innermost shell can hold two electrons and the next, eight. A completely filled shell is a particularly stable arrangement for an atom.

Helium atoms will not combine with one another, for each already has the two electrons necessary to fill its inner electron shell. Hydrogen atoms lack one electron of having a completely filled shell, and two H atoms can share a pair of electrons to form an molecule. In this way each of the atoms in the molecule has two electrons in its immediate vicinity, and thereby attains a full-shell structure. The bond in the H-H molecule can be thought of as the prototype of the electron-pair or covalent bond in larger molecules.

An amount of any compound in grams, numerically equal to its atomic or molecular weight in amu, is one mole of that substance. The mole concept allows one to measure equal numbers of atoms or molecules of various material, even without a knowledge of how many molecules there are. The actual number of molecules in one mole, Avogadro's number, has been measured as N = 6.022 X 10 .

From the way in which the mole is defined, this value is also the conversion factor between amu and grams as units of mass: 1 g = 6.022 X 10 amu. One mole of molecules weighs 2.016 grams, and one mole of He atoms, 4.003 g.