Anomalous Behaviour of Lithium and its Diagonal Relationship with Magnesium
Lithium shows anomalous behaviour and departs from the general group trend. This is mainly because of its small size. The polarizing power [ionic charge/(ionic radius)2]of lithium ion is the highest of all the alkali metal ions and is close to that of the magnesium ion. Thus, it exhibits similarities to diagonally placed magnesium (diagonal relationship).
Both lithium and magnesium form monoxides when heated in air or oxygen while other alkali metals form peroxides or super-oxides.
4Li + O2 2Li2O
2Mg + O2 2MgO
Both lithium and magnesium form nitrides when heated in nitrogen.
6Li + N2 2Li3N
3Mg + N2 Mg3N2
Carbonates, hydroxides and nitrates of lithium and magnesium decompose on heating to give oxides.
Li2CO3 Li2O + CO2
MgCO3 MgO + CO2
2LiOH Li2O + H2O
Mg(OH)2 MgO + H2O
4LiNO3 2Li2O + 4NO2 + O2
2Mg(NO3)2 2MgO + 4NO2 + O2
The other alkali metal carbonates and hydroxides remain unchanged on heating. The nitrates decompose to nitrites.
2NaNO3 2NaNO2 + O2
Both LiCl and MgCl2 are soluble in alcohol. The other alkali metal chlorides remain insoluble.