Britannica Sources used in this answer

Why do the second and third periods each contain eight elements?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

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 second and third periods of the periodic table each contain eight elements because they correspond to the filling of specific electron subshells within an atom.12

According to the Pauli exclusion principle, each orbital can hold a maximum of two electrons with opposite spins. In the second period, the 2s and 2p subshells are filled, while the third period involves the filling of the 3s and 3p subshells.13

Specifically, the s subshell can accommodate two electrons, and the p subshell can accommodate six electrons, totaling eight electrons. As these subshells are filled, the atoms reach a stable configuration, which is why each of these periods terminates at a noble gas with a complete outer shell of eight electrons, often referred to as an octet.12