When an electron makes a transition between energy levels of a hydrogen atom, there are no restrictions on the initial and final values of the principal quantum number n. However, there is a quantum mechanical rule that restricts the initial and final values of the orbital angular momentum ℓ. This is the selection rule, which states that Δℓ = 61, that is, in a transition, the value of ℓ can only increase or decrease by one. According to this rule, which of the following transitions are allowed:
(a) 2s → 1s,
(b) 3p → 1s,
(c) 3d → 4f,
(d) 4d → 3s? In view of this selection rule, explain why it is possible to observe the various emission series shown in Figure 7.11.