While filling up of electrons in the atomic orbitals, the $4 s$ orbital is filled before the $3 d$ orbital but reverse happens during the ionisation of the atom. Explain why?

In an atom, electrons are filled according to $(\mathrm{n}+l)$ rule of increasing energy order.

$3 d \Rightarrow \mathrm{n}=3, l=2 \Rightarrow(\mathrm{n}+l)=5$

$4 s \Rightarrow \mathrm{n}=4, l=0 \Rightarrow(\mathrm{n}+l)=4$

$4 s$ is lower in energy than $3 d$ and so, electrons first enters in $4 s$ orbitals.

During ionization, the electrons are first removed from $4 s$-orbitals because electrons in $4 \mathrm{~s}$ are loosely held by the nucleus.