Valence and the anomalous second period

Valence of representative elements usually equals the number of outer electrons or eight minus it. Groups 1, 2, 13, 14, 15, 16, 17, 18 have 1 to 8 valence electrons and valences 1, 2, 3, 4, 3 or 5, 2 or 6, 1 or 7, 0 or 8.

Oxidation state is the charge an atom takes on when shared electrons are assigned by electronegativity. In OF₂ (F > O) each F is −1, so O is +2; in Na₂O, O takes an electron from each Na and is −2, and each Na is +1.

Predicting formulas: Si (group 14, valence 4) with Br (valence 1) gives SiBr₄; Al (group 13, valence 3) with S (group 16, valence 2) gives Al₂S₃.

Hydrides and oxides follow the valence: LiH, CaH₂, B₂H₆, CH₄, NH₃, H₂O, HF; Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₃, Cl₂O₇. Transition elements and actinoids commonly show variable valence.

The first member of groups 1, 2 and 13-17 (Li, Be, B to F) differs from the rest of its group. Li and Be form compounds with clear covalent character, while their heavier relatives are mainly ionic.

Diagonal relationship: Li resembles Mg, and Be resembles Al, the second element of the next group. Metallic radii (pm): Li 152, Mg 160; Be 111, Al 143.

Causes: small size, large charge/radius ratio and high electronegativity; also, a second-period atom has only four valence orbitals (2s, 2p) against nine (3s, 3p, 3d) for the next member. So its maximum covalency is 4 (B forms BF₄⁻) while Al can form AlF₆³⁻.

Second-period p-block atoms form pπ–pπ multiple bonds readily, with themselves (C=C, C≡C, N=N, N≡N) and with other second-period atoms (C=O, C=N, C≡N, N=O).

Oxidation state and covalency differ: in [AlCl(H₂O)₅]²⁺, Al has oxidation state +3 but covalency 6.