Electron configurations for Ni and Sc

[EvaLi_3J]

In the lecture, professor wrote the two electron configuration:
Ni: [Ar]3d8 4s2
Sc: [Ar]3d1 4s2

Can anyone please explain why 3d is filled with 8 and 1 electron in these two cases?

[Leyna Dang 2H]

On the periodic table, 3d starts from Scandium (Sc) and ends at Zinc (Zn). You can think of it as each element having a humber that corresponds with their position or order in that row. So Sc would be 1, Ti would be 2,..etc. Because Sc is first in the 3d row its configuration would be [Ar]3d1 4s2 and because Ni is 8th, its configuration would be [Ar]3d8 4s2. Hope this helped!