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

The electrons are added to an atom following Aufbau Rule or n+l rule, Pauli's Exclusion principle and Hund's rule.

Accoding to Aufbau Principle electrons are filled first in orbitals having lower n+l value. We can also say that electrons are filled in order of their increasing energies.

If two orbitals have same n+l vaue, then the orbital with smaller n value will have the lower energy and will be filled first.

Case of 4s and 3d.

Case: 4a;

Here,

n = 4

l = 0

So,

n+l = 4+0 = 4

Case: 3d;

Here,

n = 3

l = 2

So,

n+l = 3+2 = 5

Conclusion:

As the n+l value of 4s is smaller than the n+l value of 3d so the 4s will be filled first.

The 4s subshell fills before the 3d subshell begins to fills as electrons are added because

They obey the Aufbau's principle which state that in the ground state of an atom or ion electrons fill atomic orbital of the lowest energy level before occupying higher level. 4s is of lower energy while 3d is of higher energy hence 4s is filled before 3d