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In the transition metals, $4s$ has a higher energy level than $3d$, because in ionization, the electrons from the $4s$ energy level are lost first.
I was thinking that this might be the same for much of the p-block from elements Ga-Rn because:
Ga has a electron configuration of: $1s^22s^22p^63s^22p^63s^23p^63d^{10}4s^24p^1$
Ga$^{3+}$ has an electron configuration of: $1s^22s^22p^63s^23p^63d^{10}$
Well the only exception in the orbitals is for $s$ and $d$ orbitals. Meaning you will first remove electrons from $(n+1)s$ and then onto $nd$ orbital. It is only because the $(n+1)s$ sublevel is only lower in energy if there are no electrons in the d sublevel. As per the Aufbau principle, the the s orbital is therefore filled before. Then as the $d$ becomes filled with electrons, then the energy starts to flunctuate relative to one another and the s orbital ends up higher in energy. That is why when electrons are removed, they are first removed from s and then $d$. Luckily this only applies to $s$ and $d$ orbitals. When are you working with $p$ orbitals, well then the $np$ orbital will always be higher than the $(n-1)d$ orbital. Therefore for making ions, you will have to first removed electrons from $p$ then $s$ and then $d$ and so on.
