The anomalous behaviour of lithium is due to the : (i) exceptionally small size of its atom and ion, and (ii) high polarising power (i.e., charge/ radius ratio).
As a result, there is increased covalent character of lithium compounds which is responsible for their solubility in organic solvents.
Further, lithium shows diagonal relationship to magnesium which has been discussed subsequently.
10.3.1 Points of Difference between Lithium and other Alkali Metals
(i)Lithium is much harder. Its m.p. and b.p. are higher than the other alkali metals.
(ii) Lithium is least reactive but the strongest reducing agent among all the alkali metals. On combustion in air it forms mainly monoxide, Li2O and the nitride, Li3N unlike other alkali metals.
(iii) LiCl is deliquescent and crystallises as a hydrate, LiCl.2H2O whereas other alkali metal chlorides do not form hydrates.
(iv) Lithium hydrogencarbonate is not obtained in the solid form while all other elements form solid hydrogencarbonates.
(v) Lithium unlike other alkali metals forms no ethynide on reaction with ethyne.
(vi) Lithium nitrate when heated gives lithium oxide, Li2O, whereas other alkali metal nitrates decompose to give the corresponding nitrite.
4LiNO3 → 2Li2O + 4NO2 + O2
2NaNO3 → 2NaNO2 + O2
(vii) LiF and Li2O are comparatively much less soluble in water than the corresponding compounds of other alkali metals.
10.3.2 Points of Similarities between Lithium and Magnesium
The similarity between lithium and magnesium is particularly striking and arises because of their similar sizes : atomic radii, Li = 152 pm, Mg = 160 pm; ionic radii : Li+ = 76 pm, Mg2+= 72 pm. The main points of similarity are:
(i) Both lithium and magnesium are harder and lighter than other elements in the respective groups.
(ii) Lithium and magnesium react slowly with water. Their oxides and hydroxides are much less soluble and their hydroxides decompose on heating. Both form a nitride, Li3N and Mg3N2, by direct combination with nitrogen.
(iii) The oxides, Li2O and MgO do not combine with excess oxygen to give any superoxide.
(iv) The carbonates of lithium and magnesium decompose easily on heating to form the oxides and CO2. Solid hydrogencarbonates are not formed by lithium and magnesium.
(v) Both LiCl and MgCl2 are soluble in ethanol. (vi) Both LiCl and MgCl2 are deliquescent and crystallise from aqueous solution as hydrates, LiCl·2H2O and MgCl2·8H2O.