Atomic structure of small- and large-period elements

Elements of small periods stand in one row. In period 1 (H, He) there is one shell: hydrogen 1s¹, helium 1s² — the shell is complete with 2 electrons. In period 2 (Li to Ne) there are two shells and the second takes from 1 to 8 electrons: Li 1s² 2s¹, Be 1s² 2s², B 1s² 2s² 2p¹ ... Ne 1s² 2s² 2p⁶. In period 3 (Na to Ar) there are three shells: Na 2, 8, 1; Mg 2, 8, 2; Al 2, 8, 3 ... Ar 2, 8, 8. Since the third shell is the outer one it holds no more than 8 electrons, and the 3d sublevel is not filled in this period. The period number equals the number of shells, and the number of outer electrons equals the group number. H, He, Li, Be, Na, Mg are s-elements; B to Ne and Al to Ar are p-elements. Ne and Ar, with complete outer shells, are chemically inert.