Question

Quantum numbers of the fourth electron of beryllium

Answer 1

In the building up process, we go one electron at a time. Therefore, we will use the three from lithium and add one more.

Fourth Electron

n = 2
ℓ = 0
mℓ = 0
ms = -½

Notice the same n, ℓ, and m values as the third electron, but ms for the fourth electron has shifted from positive ½ to negative ½.