Chemistry help needed, AGAIn!!?

Question

1- What's the maximum number of electrons that can occupy the third principal energy level?


2- Why do electrons fill the 4s orbital before they start to occupy the 3d orbital?

Answer 1

Violet12.. is right on both questions. Now, about the second question:

For the 4s orbital: n + l = 4 + 0 = 4.
For the 3d orbital: n + l = 3 + 2 = 5

4 < 5, so the 4s orbital has less energy than 3d, so we put electrons first in 4s orbital and then in 3d.

Answer 2

1. 18 electrons
s=2, p=6, d=10
2+6+10 = 18

2. The 4s orbital is filled first because it is lower in energy than 3d. Why it is lower in energy, I do not know.