Incorrect Answers - Core 3

Question 1

Explain why, in the absence of an externally applied field, the degeneracy associated with the total orbital angular momentum in an atom is partially lifted in a diatomic molecule.

Answer 1

The two nuclei in a diatomic molecule generate an axially symmetric electric field with the result that the orbital angular momentum couples to the field and each ML pair assumes its own energy level (Stark effect)

Question 2

Suggest a reason why the angular momentum selection rules might break down in the case of bromine.

Answer 2

As bromine contains nuclei of relatively large Z, spin-orbit coupling will be strong and the S quantum number is poorly defined. Consequently, the delta S = 0 selection rule breaks down - we can go from singlets to triplets!