Li differs from other Group 1 metals: (1) Hardest alkali metal (highest m.p. = 181 degrees C). (2) Highest IE in Group 1 $\frac{520 kJ}{mol}$. (3) Forms normal oxide Li2O only (not peroxide or superoxide). (4) LiOH is weak base (others are strong). (5) LiF, Li2CO3, Li3PO4 are insoluble (unlike Na, K salts). (6) LiCl is covalent, hygroscopic, soluble in organic solvents (Fajans' rules: small $Li^+$ polarises $Cl^-$). (7) Li2CO3 decomposes on heating (others except Na2CO3 do too, but Li decomposes most easily). (8) Li reacts directly with N2 to form Li3N (no other alkali metal forms stable nitride). (9) Diagonal relationship with Mg: both form nitrides, both carbonates decompose, both halides are covalent.