UPSC Civil Services (Mains), Chemistry optional · 2019 · Paper I

Q1(a)(i). Covalent character of NaBr and MgBr₂

Question

In which of the following compounds do the bonds have greater covalent character:
(a) NaBr or (b) MgBr₂ ?
Give reasons to support your answer.

MgBr₂. The small, doubly charged Mg²+ ion polarises the large Br⁻ ion much more than Na⁺ does (Fajans' rules).

the comparison rests only on the cation's polarising power, which grows with charge and falls with size.
Na⁺ 102 pm, charge +1; Mg²+ 72 pm, charge +2 (six-coordinate radii). The charge density z/r² of Mg²+ is about 4 times that of Na⁺.
the large, soft Br⁻ cloud is drawn towards Mg²+; electron density builds up between the nuclei. This sharing of electron density is covalent character.
Δχ (Pauling) Na–Br = 2.96 − 0.93 = 2.03; Mg–Br = 2.96 − 1.31 = 1.65. A smaller difference means a less ionic bond.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

Same anion: the comparison rests only on the cation's polarising power, which grows with charge and falls with size.
Size and charge: Na⁺ 102 pm, charge +1; Mg²+ 72 pm, charge +2 (six-coordinate radii). The charge density z/r² of Mg²+ is about 4 times that of Na⁺.
Polarisation: the large, soft Br⁻ cloud is drawn towards Mg²+; electron density builds up between the nuclei. This sharing of electron density is covalent character.
Electronegativity: Δχ (Pauling) Na–Br = 2.96 − 0.93 = 2.03; Mg–Br = 2.96 − 1.31 = 1.65. A smaller difference means a less ionic bond.
Evidence: NaBr has the rock-salt structure; MgBr₂ has a layer structure and dissolves in ether and alcohols, typical of partly covalent halides.

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