Question

What is the maximum number of electrons present in $ K,L{\text{ and }}M $ shells?
(A) $ 2,4,6 $
(B) $ 2,6,8 $
(C) $ 2,8,18 $
(D) $ 2,8,16 $

Answer 1

Hint :The electrons in shells are filled one by one and no higher shell is filled before the shells below it are already filled with electrons. Thus each shell has a fixed number of electrons that it can accommodate and these numbers are also indicated in the electronic configuration of atoms.

Complete Step By Step Answer:
The structure of an atom is such that it contains a dense positively charged nucleus that is known to contain the entire mass of an atom concentrated in a small spherical volume.
The electrons are negatively charged and the mass contributed by them is almost negligible in comparison to other subatomic particles like protons and neutrons.
The electrons are not accumulated in a small volume like protons but are placed in fixed energy levels where they revolve around the nucleus. These energy levels are known as shells that increase in radius with every level. Each shell contains a fixed number of revolving electrons in it and once the shell is filled, the incoming electron automatically occupies a position in the next shell.
The formula for the maximum number of electrons accommodated by a shell is given as follows:
 $ {\text{maximum electrons}} = 2 \times {n^2} $
Where, $ n $ is the order of the shell.
 $ {\text{maximum electrons in First shell}} = 2 \times {1^2} = 2 $
The first energy level or shell is termed the $ K $ shell and can only accommodate two electrons. Thus each atom can only have two electrons in its first shell.
 $ {\text{maximum electrons in second shell}} = 2 \times {2^2} = 8 $
The second energy state or shell is termed the $ L $ shell and contains a total of eight electrons in it.
 $ {\text{maximum electrons in third shell}} = 2 \times {3^2} = 18 $
The third energy state or shell is termed the $ M $ shell and also contains a total of eighteen electrons in it.
Hence, the correct option is (c) $ 2,8,18 $ i.e. the maximum electrons present in $ K,L{\text{ and }}M $ shells are $ 2,8,18 $ .

Note :
The octet rule is based upon filling up of the outermost shell i.e. valence shell to obtain a noble gas electronic configuration. Since shells like $ L $ can accommodate only eight electrons in it, this forms the basis of octet rule for the stabilization of a bond. But this rule is not applicable in all cases.