a basic question

electron enters the energy shells according to their energies... i.e lower energy subshells get filled first then higher ones. So, as energy of 4s is less than 3d (n+l rule) that's why electron enters 4s first...

As for leaving, 4s subshell is farther away from the nucleus than 3d.. as n=4 > n=3 and radius is proportional to n². So, it is easier to remove the electrons farther away. Hence electron leaves from 4s first then 3d

as 4s energy is less it means it s more stable n thus more near to nucleus so it must be difficult to remove e frm there

the electron is always removed from the valence shell.
And the 3d orbital belongs to pen ultimate shell.
Also since the ionisation energy depends on how far the orbital is from the nucleus.
Larger the distance less is the ionisation energy.
Hence electron is removed from 4s orbital rather than 3d.

ok:)