When an electron makes a transition between energy levels of an atom, there are
ID: 592400 • Letter: W
Question
When an electron makes a transition between energy levels of an atom, there are no restrictions on the initial and final values of the principal quantum number n. According to quantum mechanics, however, there is a rule that restricts the initial and final values of the orbital quantum number 1, This rule is called a selection rule and states that Ala1 An other words, when an electron makes a transition between energy levels, the value of can only increase or decrease by one. The value of ( may not remain the same nor may it increase or decrease by more than one. According to this rule, which of the following energy level transitions are allowed? a) 2s 1s b) 2p 1s d) 4s 2p e) 3d 3sExplanation / Answer
As the question states, there is no restriction on the principal quantum number or 'n'. Thus, we can ignore that and focus solely on the orbital quantum number 'l'. In order of know which transitions are allowed and which are restricted, one should know the l values for each orbital.
l value for 's' = 0
l value for 'p' = 1
l value for 'd' = 2
l value for 'f' = 3
There are other orbitals after f like 'g', 'h', 'i' etc. but we won't need them for this question.
a.] the transition is from s to s i.e. from 0 to 0 (Look at the l values above). delta l = 0. The selection rule states that the delta l value must be +/- 1. Hence, this transition is forbidden.
b.] p to s i.e. 1 to 0. delta l = +1. Hence, this transition is allowed.
c.] p to p i.e. 1 to 1. Thus, delta l = 0. Hence, this transition is forbidden.
d.] s to p i.e. 0 to 1. Thus, delta l = -1. Hence, this transition is allowed.
e.] d to s i.e. 2 to 0. Thus delta l = +2. This violates the selection rule and so this transition is forbidden.
The negative or positive signs of delta l don't matter. What really matters is that the value of delta l musn't increase or decrease more than unit 1.
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