Question

Why is $3\mathrm{d}$ orbital filled after $4\mathrm{s}$?

Answer 1

Aufbau principle:

The Aufbau principle dictates the manner in which electrons are filled in the atomic orbitals of an atom in its ground state.

Since, the energy of $3\mathrm{d}$ orbital is more than $4\mathrm{s}$ orbital, on the basis of the Aufbau principle, it is filled after $4\mathrm{s}$ orbital.

But during ionization, electrons in the outermost orbital are lost.

Since, $4\mathrm{s}$ will be the outermost orbital, in this case, electrons from this orbital will be ionized first.