CBSE Class 9 Answered

However, according to the rules distribution of electrons, the next shell cannot be filled unless the shell before is completely filled but it also says that the valence shell can only have 8 electrons. This is because; the maximum capacity of each shell to hold electrons is 8.

If the atoms have 8 electrons in their outer most electron shell, atoms are more stable, and except first electron shell is more stable with two electrons. 

When the M shell is filled, the next electron goes in the N shell even though the shell before is not filled completely.

Keep in mind that an electron shell need not completely filled before electron begun to fill the next level of electron shell (higher level). In this case, atom can have more than one valency, which is termed as variable valency.

Variable valency can be defined as,

Sometimes, same element may exhibit one valency in one compound and another valency in other compound. This property is called as variable valency.

An atom of an element can sometimes lose more electrons than are present in its valence shell i.e. loss from the penultimate shell and hence exhibit more than 1 or variable valency.