9th science

Structure of The Atom

Bohr’s Model of Atom

Neils Bohr, a Danish physicist, in 1913 proposed model of atom which rectified the problems left by Rutherford’s Model. He proposed that

(a) Electrons revolve round the nucleus in a fixed orbit.

(b) He called these orbits as ‘stationary orbit’.

(c) Each stationary orbit is associated with fixed amount of energy, thus electrons do not radiate energy as long as they keep on revolving around the nucleus in fixed orbit.

The circular path around the nucleus is called orbit, energy level or shell. Energy level are represented by letter – K, L, M, N, …. and so on.


  • 1st orbit is denoted by – K
  • 2nd orbit is denoted by – L
  • 3rd orbit is denoted by – M, and so on.

The orbits are denoted by 1, 2, 3, …. and so on.

Distribution of Electrons in a Orbit or Shell:

The distribution of electrons in an orbit can be obtained by using formulae 2n 2

where ‘n’ is number of orbit.

Number of electrons in K-shell i.e. in 1st orbit.

Here n = 1


2n2 = 2 x 12 = 2

Thus, maximum number of electrons in K-shell i.e. 1st shell = 2

Number of electrons in L-shell, i.e. in 2ndorbit

Here n = 2, therefore,

2n2 = 2 x 22 = 8

Thus, maximum number of electrons in L-shell = 8

Number of electrons in M-shell, i.e. in 3rd orbit

Here n = 3, therefore,

2n2 = 2 x 32 = 18

Thus, maximum number of electrons in M-shell = 18

Number of electrons in N-shell, i.e. in 4th shell

Here n = 4, therefore,

2n2 = 2 x 42 = 32

Thus, maximum number of electrons in N-shell = 32

Number of electrons in O-shell, i.e. in 5th shell

Here n = 5, therefore,

2n2 = 2 x 52 = 50

Thus, maximum number of electrons in O-shell = 50

Number of electrons in P-shell, i.e. in 6th shell

Here n = 6, therefore,

2n2 = 2 x 62 = 72

Thus, maximum number of electrons in P-shell = 72

In similar way maximum number of electrons in any shell can be calculated.

  • A higher number of n means the orbit is farther from the nucleus. The orbit which is farther from the nuclues has higher level of energy. The orbit which is nearer the nucleus has lower level of energy.
  • When an electron continues to move in a definite orbite, it does not lose energy.
  • When an atom gets energy from any external source then an electron jumps to a higher energy level.
  • When an electron loses some energy, it jumps down to a lower energy level.

Thomson Model

Bohr Model

Atomic Number

Electronic Configuration

Elec Config N to Mg

Elec Config Al to Ca


In Text Solution

NCERT Solution