We know that as per Bohr Model.

  • Electrons move around Nucleus of an atom in discrete orbits .
  • These are also called energy levels or shells .

What is the number of Electrons in a Shell?

  • The distribution of the electrons in the energy shells is known as electronic configuration .

 

  • This is based on the Bohr-Bury scheme according to which, the maximum number of electrons that can be present in a particular energy shell of an atom is given by 2n 2 , where “n” is the number of energy shells.

 

  • The energy shells K, L, M, N are the first four energy shells given by Bohr .

 

  • So, the maximum number of electrons which can be accommodate in each shell is given as:

Electron Shell

Maximum Capacity

K shell

2 electron

L shell

8 electron

M shell

18 electron

N shell

32 electron

 

14. orbits-Teachoo-01.png

Note: - The order of energy of these energy shells starting form nucleus is K < L < M < N.

 

How is Maximum Number of Electrons in a shell Calculated

It is calculated by using formula, 2n 2

Example: - 

  • For K shell, n = 1

 

So number of Electrons = 2n 2 = 2 x 1 2 = 2

  • For L shell, n = 2

 

So number of Electrons = 2n 2 = 2 x 2 2 = 2 x 4 = 8

  • For M shell, n = 3

 

So number of Electrons = 2n 2 = 2 x 3 2 = 2 x 9 = 18

  • For N shell, n = 4

 

So number of Electrons = 2n 2 = 2 x 4 2 = 2 x 16 = 32




Note:   Electrons are first filled in the innermost shell first.

Then, it is filled in the outer shell.



Example:

 

  1. Helium has 2 Electrons
    All these are in K Shell (as K Shell can hold max 2 electrons)
    15. helium-Teachoo-01.png
  2. Lithium has 3 Electrons
    2 of these are held in K Shell (as K Shell can hold max 2 electrons)
    Remaining 1 are held in L Shell
    16. lithium-Teachoo-01.png
  3. Sodium has 11 Electrons
    2 of this are held in K Shell (as K Shell can hold max 2 electrons)
    8 of this are held in L Shell (as L Shell can hold max 8 electrons)
    Remaining 1 is held in M Shell

17. sodium-Teachoo-01.png

Examples -

In text Question - Page 50 Q1, Q2

NCERT Back Exercise - Q6, Q9, Q12

  1. Class 9
  2. Chapter 4 Class 9 - Structure of Atom (Term 2)

About the Author

CA Maninder Singh's photo - Founder at Teachoo
CA Maninder Singh
CA Maninder Singh is a Chartered Accountant for the past 11 years and a teacher from the past 11 years. He teaches Science, Accounts and English at Teachoo