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Class Nine 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.

Therefore,

  • 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 Orbit or Shell:

The distribution of electrons in an orbit is obtained by 2n 2, where ‘n’ is number of orbit.

Therefore,

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

Here n = 1

Therefore,

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

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