3. Anomalous properties of lithium

ANOMALOUS BEHAVIOUR OF LITHIUM

Lithium shows anomalous behaviour due to the following reasons:

1. It has the smallest size in its group.

2. It has very high ionization enthalpy and highest electronegativity in the group.

3. Absence of d-orbitals inits valence shell.

As a result, it differs from the other alkali metals in the following properties :

• Lithium is harder than other alkali metals, due to strong metallic bond.
• Lithium combines with O2 to form lithium monoxide, Li₂O whereas other alkali metals form peroxides (M₂O₂) and superoxides (MO₂).
• Lithium, unlike the other alkali metals, reacts with nitrogen to form the nitride.

           6Li + N₂ → 2Li₃N

                         Lithium nitride

• Li₂CO₃ ,LiF and lithium phosphate are insoluble in water while the corresponding salts of other alkali metals are soluble in water.
• Li₂CO₃ decomposes on heating to evolve CO₂ whereas other alkali metal carbonates do not.
• Lithium nitrate on heating evolves O₂ and NO₂ and forms Li₂O while other alkali metal nitrates on heating form their respective nitrites.

Diagonal Relationship

Lithium shows diagonal resemblance with magnesium [the element of group 2] and this resemblance is due to similar polarising power, i.e.,
[ionic charge / (ionic radius)²] of both these elements.

Lithium resembles magnesium in the following respects :

1. The atomic radius of lithium is 1.31 Å while that of magnesium is 1.34 Å.
2. The ionic radius of Li⁺ⁱ on is 0.60 Å, which is very close to that of Mg²⁺ ion (0.65 Å).
3. Lithium (1.0) and magnesium (1.2) have almost similar electronegativities.
4. Both Li and Mg are hard metals.
5. LiF is partially soluble in water like MgF₂.
6. Both combine with O₂ to form monoxides, e.g., Li₂O and MgO.
7. Both LiOH and Mg(OH)₂ are weak bases.
8. Both LiCI and MgCl₂ are predominantly covalent.
9. Both Li and Mg combine with N₂ to form their respective nitrides, Li₃N and Mg₃N₂.

Both lithium and magnesium nitrates on heating evolve NO₂ and O₂ leaving behind their oxides.