How many electrons can each energy level hold? This is the last link needed to complete the chain leading to the periodic table. Each orbital, or quantum state, identified by a particular combination of n, l, and m values, can hold a pair of electrons with opposite spins. The 1s orbital has room for two electrons, the 2s orbital can hold two more, the three 2p orbitals can hold a total of six, and so on. The five 3d orbitals have room for ten electrons, and the seven 4f orbitals can accommodate fourteen electrons. When several orbitals have the same energy but different orientations, as the , and orbitals, each one adds a single electron before any of them becomes filled with two. This permits the electrons to remain as far from one another as possible, and minimizes the repulsions between their negative charges.

If the ten-electron d orbitals suggest the ten transition metals to you, and the fourteen-electron f orbitals suggest the inner transition metals, you are on the right track. Filling the energy levels depicted at the left from the bottom up, with two electrons per orbital, creates the periodic table as it was shown on page 12. The connection is easier to see with the aid of a "filling diagram" showing which orbitals are being filled in the various regions of the table (also available on page 12).