Only certain electron transitions are allowed from one energy level to another. In one-electron species, the change in the quantum number l of an allowed transition must be ±1. For example, a 3p electron can drop directly to a 2s orbital but not to a 2p. Thus, in the UV series, where nfinal = 1, allowed electron transitions can start in a p orbital (l = 1) of n = 2 or higher, not in an s (l = 0) or d (l = 2) orbital of n = 2 or higher. From what orbital do each of the allowed electron transitions start for the first four emission lines in the visible series (nfinal = 2)?

Now for the first orbit , the angular quantum number L (i.e the sub shells) can have a value of only '0'.Considering the orbital n = 2, (L) can have a value '0' and '1' .This mean that the possible transitions from upper levels with [tex]n_{final}[/tex] = 2 will be

n = 6 → n = 2, n = 5 → n= 2, n = 4 → n= 2, n= 3 → n= 2

Generally the second orbit contains, both s(L = 0) and p(L = 1) orbitals. This means that according to the selection rule which states that the change in the quantum number (L) of an allowed transition must be ± 1, Hence the allowed transition in the second orbit are

From n = 3 to n = 2

Generally there are three orbitals that exist in the third orbit, this means that the possible transitions of an electron from n = 3 to n = 2 are

3p → 2s, 3d → 2p , 3s → 2p

Also the possible transition from n = 4 to n = 2 are

4p → 2s , 4d → 2p , 4s → 2p

This then mean that four spectral lines are gotten from these transition as stated below

3p → 2s, 3d → 2p, 3s → 2p, 4s → 2p