6th edition, 3.19b

[Tyra Nguyen 4H]

The electron configuration of Ni²⁺ is [Ar]3d⁸, so why is the number of unpaired electrons 2?

[Ahmet_Dikyurt_3L]

That is 3.19e but to answer your question, we need to look at Hund's Rule. We need to fill each orbital one by one, so:
In d orbital, there are 5 orbitals, so the first 5 electrons go to each orbital, then the remaining 3 goes to the first 3 orbitals. The last two orbitals are left with a single electron, and they are not paired.