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

The energy of the 4-s subshell is lower than the energy of 3-d subshell.

Explanation:

During the filling of electrons in subshells, the lower energy levels are filled before the higher energy levels. Also known as Aufbau principle.

Energy of the sublevel = (n + l)

where : n = Principal quantum number

             l = Azimuthal quantum number(s=0,p=1,d=2,f=3)

Energy of 4-s subshell= (4+0) = 4

Energy of 3-d subshell=(3+2) = 5

Energy of 4-s subshell is lower than the energy of 3-d subshell, that is why 4s orbital is filled before the 3-d subshell.