UPSC Combined Geo-Scientist, Mains · 2016 · Mains Paper I
Q1(i). Ground term and angular momenta
Question
Identify the ground term symbol of set of terms ¹G, ³F, ³P, ¹F. Specify the spin (S) and angular momenta (L) of ground state and also the possible total angular momentum (J) value.
Answer in brief
The lowest term is ³F: S=1, L=3, with J=2,3,4.
Key points
- Hund's first rule favours maximum multiplicity. The triplets ³F and ³P therefore lie below the singlets in the usual LS-coupling comparison.
- Among the triplets, Hund's second rule favours the larger L: F means L=3, while P means L=1.
- From 2S+1=3, S=1. Angular momentum addition gives J=|L-S|,…,L+S=2,3,4.
- A particular lowest J cannot be selected without knowing whether the shell is less or more than half full: less than half gives J=2, more…
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.
CoversGround termangular momentaAtomic term symbols
All parts of Question 1
- 1(a) Occurrence of lithium5 marks
- 1(b) Bond order and ionisation5 marks
- 1(c) Solvent levelling5 marks
- 1(d) Disproportionation and its reverse5 marks
- 1(e) Interpreting the redox data5 marks
- 1(f) Existence of hydrogen and helium dimers5 marks
- 1(g) Packing fraction5 marks
- 1(h) Square-planar geometrical isomers5 marks
- 1(i) Ground term and angular momenta5 marks
- 1(j) Effective atomic number5 marks