anomalous chemistry of lithium

Lithium sits at the top of the alkali metals, but it is the black sheep of the family. In a dozen small ways it breaks ranks: it forms a simple oxide where its cousins form peroxides and superoxides, its carbonate and nitrate decompose on heating where sodium's are stable, its fluoride and carbonate are only sparingly soluble, and it even reacts directly with nitrogen to form a nitride, Li3N, which no other Group 1 metal does. Why is the smallest member so different?

The root cause is that the lithium ion Li+ is tiny — by far the smallest cation in the group — so it carries a very high charge density (charge packed into a small volume). A small, dense positive ion pulls hard on the electron clouds of nearby anions, polarizing them and giving its bonds significant covalent character (this is what Fajans' rules describe). That extra covalency and the compact size shift lithium's chemistry away from the purely ionic behaviour of sodium and potassium. The same compact Li+ also matches the small oxide ion, explaining why it makes Li2O rather than a peroxide, and its small, hard nature drives the low solubilities and the easy thermal decomposition of its salts.

This anomaly matters because lithium's high charge density makes its size and polarizing power closely resemble those of magnesium, Mg2+, one row down and one group across — the famous diagonal relationship. Lithium and magnesium share traits the rest of Group 1 lacks: both burn in nitrogen to nitrides, both form covalent organometallic compounds widely used in synthesis (organolithiums, Grignard reagents), and both have many sparingly soluble salts. The honest framing is that lithium is not breaking the rules — it is obeying the deeper rule that charge-to-size ratio, not group number alone, governs an ion's chemistry.