Each energy level (shell) has more subshells available to it: The first shell only has the s subshell ⟹ 2 electrons. The second shell has the s and p subshells ⟹ 2 + 6 = 8 electrons. The third shell has the s, p, and d subshells ⟹ 2 + 6 + 10 = 18 electrons.
