3.5 c&d

[Imelda Mena 3I]

Can someone explain the answers to these two question please. (c) is asking for ground state electron configuration for Ga3+ and (d) is asking for Tl3+. Why the electron configuration for the ion Tl3+ is [Xe]4f^14 5d^10? I thought it was [Xe]4f^14 5d^8 6s^2? Same thing with Ga3+, I thought it was [Ar]3d^8 4s^2 but the answer is [Ar]3d^10. I'm confused, why is the d-orbital full with 10 electrons? Why don't we include electrons in the s-orbital?

Thanks in advance!

[Xiangkun Shi 3E]

When electrons start filling in the orbitals, they fill 4s first and then fill 3d because 4s has lower energy state than 3d at first. But once 4s is filled, its energy became higher than 3d. When losing electrons, electrons in higher energy levels are given out first since they are less stable.So electrons in 4s are given out first, not electrons in 3d.

[Imelda Mena 3I]

Okay this makes a bit more sense now! So these are the electron configuration for the ions now what if we were ask for the electron configuration of Nickel (Ni)? Would it be [Ar]3d^8 4s ^2?