Question

Iron has 14 electrons in M shell and 2 electrons in N shell. Why can't it be 15 electrons in M shell and 1 electron in N shell?

Answer 1

As we all know that iron belongs to d-block.
Generally in d – block elements the last electrons are enter into (n-1)d orbital.

In Iron the outermost shell is N shell contains  ns orbitals. Whereas M shell contains (n-1)d Orbital.

In the d-block elements the ns – orbital is first filled and then the electrons enter in (n-1)d shell, so the electronic configuration of Fe is written as 1s2 2s2 2p6 3s2 3p6 3d6 4s2

Here, the electons enter into 3d orbital after filling the 4s orbital. Hence the total number of electons in M is 14 inspite of 15 or 16.