Question

In which species would 2 unpaired electrons be present in the ground state: `"a. sodium atom;"` `"b. cobalt atom;"` `"c. iron dication,"` `Fe^(2+)`; `"d. iron atom?"`

Answer 1

`"Option (d)"`.

Explanation:

We can immediately eliminate `Na`, `Z=11`, and `Co`, `Z=27`, because these give rise to an odd number of electrons in the ground state, the which must necessarily be UNPAIRED.

As regards, `b.`, i.e. `Fe`, we have `Z=26`, however, Hunds' rule dictates a configuration of `1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(2)3d^6`. In the isolated ATOM, we must fit 6 electrons into the 5 valence `"d-orbitals"`. At least 2 of the electrons must be unpaired.