UPSC Combined Geo-Scientist, Mains · 2017 · Mains Paper II
Q1(m). Atomic emission selection rules
Question
Which transitions are allowed in a normal atomic electronic emission spectrum? Give reasons: (i) 2s→1s; (ii) 2p→1s; (iii) 3d→2p; (iv) 3d→4s; (v) 5p→3s.
Answer in brief
Under electric-dipole orbital selection rules, (ii), (iii) and (v) are allowed; (i) and (iv) are forbidden.
Key points
- For an electric-dipole transition, Δl=±1 and parity must change. Magnetic sublevels obey Δmₗ=0,±1; spin is unchanged in the nonrelativistic approximation.
- 2s→1s has Δl=0 and is dipole-forbidden, although weak two-photon decay can occur.
- 2p→1s has Δl=-1 and 3d→2p has Δl=-1, so both satisfy the orbital rule.
- 3d→4s has Δl=-2 and is dipole-forbidden. In a hydrogenic atom it also increases energy, so it cannot be spontaneous emission.
- 5p→3s has Δl=-1 and is allowed when the upper level is populated. There is no electric-dipole restriction requiring Δn=±1.
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.
CoversAtomic emission selection rulesAtomic spectroscopy
All parts of Question 1
- 1(a) Van der Waals equation8 marks
- 1(b) Adiabatic expansion of helium6 marks
- 1(c) Arrhenius acids and bases6 marks
- 1(d) Carnot efficiency6 marks
- 1(e) Dimerization and temperature6 marks
- 1(f) Zero-order integrated law6 marks
- 1(g) Van't Hoff factor6 marks
- 1(h) Molar absorptivity6 marks
- 1(i) Units of magnetic moment6 marks
- 1(j) Surface tension of isomers5 marks
- 1(k) Water versus hydrogen sulfide5 marks
- 1(l) Momentum eigenfunction6 marks
- 1(m) Atomic emission selection rules8 marks