Question

give 2 points on Thomson model.features of Rutherfords model. fully explain bhor's model

Answer 1

Thomson Model
a) An atom consists of a positively charged sphere and the electrons are embedded in it.
b) The negative and positive charges are equal in magnitude and so an atom as a whole is
electrically neutral.


 The salient features of this model are as follows:

(i) The atom contains a central part called nucleus which is surrounded by electrons.
(ii) The nucleus of an atom is positively charged.
(iii) The size of the nucleus is very small as compared to the atomic size.
(iv) The mass of an atom is mainly concentrated within its nucleus.
(v) Atom as a whole is electrically neutral, i.e., number of protons inside the nucleus of an atom and the number of electrons surrounding it are equal.
(vi) Nucleus is dense and hard and most of the atom is hollow.
(vii) To explain that the electrons do not fall into the nucleus because of attraction, Rutherford said that electrons were not stationary but were revolving around the nucleus in certain circular orbits. The force of attraction is balanced by the centrifugal force which comes into play due to revolution.
Rutherford’s nuclear model is similar to our solar system where the nucleus is like the sun and the electrons are like the planets.

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 – K2nd orbit is denoted by – L3rd 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 and so onn..