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Why do the sixth and seventh periods each have room for 32 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 sixth and seventh periods of the periodic table each contain 32 elements because they correspond to the complete filling of a specific set of electron subshells.1

According to the principles of quantum mechanics and the Pauli exclusion principle, electrons occupy orbitals within shells and subshells that have defined capacities. The capacity of 32 electrons in these periods is the result of filling the following subshells:1

f-subshell: 14 electrons (corresponding to the lanthanoids in the sixth period and the actinoids in the seventh).12
d-subshell: 10 electrons (corresponding to the transition metals).1
s and p-subshells: 8 electrons (forming the outer octet).1

Summing these capacities ( $14 + 10 + 8$ ) accounts for the 32 elements found in each of these "very long" periods. While these periods are often condensed into 18 columns in standard table layouts to maintain chemical grouping, the underlying electronic structure necessitates the inclusion of these 32 elements to complete the respective shells.13