Err a 2 questions on chemistry?

Question

why does the 4s sublevel fills before the 3d sublevel begins to fill as electrons are added.

and

what is the electron configuration for a neutral atom of chlorine?

Answer 1

First question: very perceptive. In a hydrogen atom, the energy depends only on n, the principal quantum number, but in a many electron atom, for rather complicated reasons, energy increases with l (letter el) as well as with n.

In neutral atoms, 4s is slightly lower in energy than 3d, but in all transition metal compounds, 3d is lower than 4s. So an iron atom with eight valence electrons is 4s2 3d6, but a Ni2+ cation with the same number of electrons is 3d8.

Second question: look up the atomic number of chlorine, and feed in that many electrons from the lowest level up.