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Why do the fourth and fifth periods each contain 18 elements?

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Periodic table of the elements with color-coded categories and labeled element groups, showing atomic numbers, symbols, and basic information for each chemical element. — periodic table Modern version of the periodic table of the elements. To learn an element's name, atomic number, electron configuration, atomic weight, and more, select one from the table.

The fourth and fifth periods of the periodic table each contain 18 elements because they correspond to the filling of specific electron subshells that accommodate a total of 18 electrons.1

In these periods, the electronic structure involves the completion of an outer octet (the s and p subshells) and the filling of an inner d subshell. Specifically:1

The fourth period (potassium to krypton) begins with the introduction of electrons into the 4s orbital. It then involves the filling of the five 3d orbitals—which accommodate 10 electrons—and the completion of the 4p subshell.1
The fifth period (rubidium to xenon) follows a similar pattern, representing the completion of an outer octet and the next inner subshell of ten 4d electrons.1

The capacity of these subshells—2 electrons for s, 6 for p, and 10 for d—sums to 18, which dictates the number of elements in these long periods.1