Each p level on the energy-level diagram on page 14 is labeled with the symbol of the noble gas that results when that level is filled. Both the noble gases and the natural breaks between rows of the periodic table are dictated by these larger energy spacings between p and s levels.

The interruptions for insertion of ten transition metals in rows four and five of the table arise because the 3d energy level lies between the 4s and 4p levels (see page 14), and the 4d level lies between the 5s and 5p levels. In rows six and seven, the inner transition metals result from the filling of a set of seven f orbitals with fourteen electrons. The order of filling of levels in row six is 6s, 4f, 5d, 6p, and in row seven of the table the order is 7s, 5f, 6d, 7p.

The closely related chemical properties of the transition metals, and the near-identity of the inner transition metals, can be explained in terms of orbital structure. The chemical behavior of an atom is dictated primarily by its outermost electrons, since these are what a neighboring atom "sees" and may react with. Even though the 3d orbitals have slightly higher energy than the 4s orbital, and hence come later in the filling sequence, they do not extend as far out from the nucleus as the 4s orbitals do. From the vantage point of a neighboring atom, there is less difference between iron and cobalt (filling of 3d) than between potassium and calcium (filling of 4s), because the orbitals in which the changes are occurring are less exposed. Hence less difference in chemical properties is seen between two horizontally adjacent transition metals than between neighboring representative elements.