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: 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

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

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