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Degeneracy in rotational levels for H-atom along z-axis for l = 0, 1, 2, 3 are :

(Given : \(\dfrac{\hbar^{2}}{2I}\) = 1.2 × 10−21 J)

  • A.1, 2, 3, 4
  • B.1, 3, 5, 7
  • C.2, 4, 6, 8
  • D.1, 4, 9, 16

Answer: B

The degeneracies are 1, 3, 5, 7.

For angular momentum quantum number l, the projection quantum number ml takes the values −l, ..., 0, ..., +l, so each level is (2l + 1)-fold degenerate:

\[ \begin{aligned} l=0 &: 1,\quad l=1: 3 \\ l=2 &: 5,\quad l=3: 7 \end{aligned} \]

The value of ℏ2/2I fixes the energies, E = l(l + 1)ℏ2/2I, but not the degeneracy. 1, 2, 3, 4 counts l + 1; 2, 4, 6, 8 doubles that; and 1, 4, 9, 16 is (l + 1)2, the degeneracy of the hydrogen shells n = 1 to 4, not of a single l level.

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