Gaseous metal ion `M^(2+)` has `5` unpaired electron. What is the atomic number?

Options

A. `26`
B. `25`
C. `27`
D. `24`

Answer

B

Explanation

Let us write the electronic configuraiton of `Z = 24`:
`1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)3d^(5)4s^(1)`
or simply `[Ar] 3d^(5)4s^(1)`
`(Z = 25):[Ar] 3d^(5)4s^(2)`
`(Z = 26):[Ar]3d^(6)4s^(2)`
`(Z = 27):[Ar]3d^(7)4s^(2)`
If we take `2e^(-)s` from each of the above configurations, we notice that only `(Z =25)` leads to the formation of `M^(2+)` with `5` unpaired electrons.
Shortcut method:
The presence of `5` unpaired electrons can be justified by`d^(5)` subshell only. Thus, the electronic configuration of `M^(2+)`ion will be
`1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)3d^(5)`
accounting for `23e^(-)s`. Thus, neutral `M` atom should have `23 +2 = 25e^(-)s`, i.e., the atomic number should ber `25`.