Anomalous Properties of the First Elements

Across every group of the p-block, the first member differs noticeably from the rest of its own family -- a pattern traceable to three compounding factors: the first member's exceptionally small size, its high ionisation enthalpy and high electronegativity, and the complete absence of d orbitals in its valence shell (which the heavier members of the group do have available, even if unused in the ground state).

Boron (group 13) is a metalloid while every other member of its group is a reactive metal; it additionally shows a diagonal relationship with silicon (group 14) -- the oxides of both elements are acidic, both form covalent hydrides that hydrolyse readily, and (with the single exception of boron trifluoride) the halides of both elements are readily hydrolysed too.

Carbon (group 14) is strictly a non-metal, while the rest of its group ranges from metalloid (silicon, germanium) to metal (tin, lead). Uniquely among group 14 elements, carbon readily forms multiple bonds (C=C, C=O, etc.) and shows by far the greatest tendency toward catenation -- chaining with itself or with other atoms -- a property that falls away sharply on descending the group (C >> Si > Ge ≈ Sn > Pb).

Nitrogen (group 15), like carbon, can form multiple bonds (N=N, C=N, N=O, etc.) and, unusually for its group, exists as a diatomic gas; oxygen (group 16) likewise exists as a diatomic gas unlike the rest of its group, and its high electronegativity lets it form hydrogen bonds. …