UPSC Civil Services (Mains), Chemistry optional · 2022 · Paper I
Q1(b). Non-bonding interactions between orbitals of A and B
Question
Consider the following atomic orbitals of A and B atoms in a heterodiatomic AB molecule: A — 2s, 2pₓ; B — 2py, 2pz [orbital pairing given in the paper]. Depict the nonbonding interactions among these atomic orbitals with reason.
Answer in brief
With z as the internuclear axis, three of the four pairs are non-bonding (overlap integral S = 0): 2s–2py, 2pₓ–2pz and 2pₓ–2py; only 2s(A) with 2pz(B) has a net overlap.
Key points
- Two atomic orbitals combine only if they are close in energy, overlap well and have the same symmetry about the internuclear axis.
- 2s(A) with 2py(B): 2s has one sign everywhere while 2py has equal + and − lobes on either side of the axis, so the overlaps cancel.
- 2pₓ(A) with 2pz(B): the same cancellation with the roles exchanged.
- 2pₓ(A) with 2py(B): both π type but in perpendicular planes; ψAψB is positive in two quadrants and negative in two, so S = 0.
- Because the cancellation comes from symmetry it is exact at every A–B distance; with S = 0 the energy does not change, so the pair is non-bonding.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coversoverlap integralnon-bonding orbitalssymmetry of orbitalsheteronuclear diatomic MOsigma and pi overlap
All parts of Question 1
- 1(a) Average and most probable distance of the 1s electron10 marks
- 1(b) Non-bonding interactions between orbitals of A and B10 marks
- 1(c) F-centres: one colour by every route; effect on density10 marks
- 1(d) ΔG for the formation of ozone in smog10 marks
- 1(e) ΔG° and K for the zinc–copper cell reaction10 marks