Electronic configuration of atoms?

The principle according to which electrons are filled within the orbits of an atom  is known as the Aufbau Principle. According to this principle, each shell consists of n number of sub-shells, where n is the sub-shell number. The electrons are filled within these sub-shells according to their energy levels. The order of filling is shown in the diagram. 

The capacity of the various sub-shells is as follows:

s(sharp) sub-shell - 2 electrons

p(principal) sub-shell - 6 electrons

d(diffuse) sub-shell - 10 electrons

f(fundamental) sub-shell - 14 electrons

These names are derived from the characteristics of their spectroscopic lines.

Therefore, the first orbit can accommodate a maximum of 2 electrons, the second orbit can accommodate 2 + 6 = 8 electrons, the third 18 and so on.

(Electrons do not go to the sub-shells shown in green under normal conditions.)

From this principle, we can understand why electrons come to the N shell before 18 electrons are filled in the M shell,  2 electrons are filled in the O shell before the ninth electron comes in the N shell etc.