Class 9 science Solutions English Medium

Chapter 4. Structure of The Atom

Thomson's Model of An Atom: 

Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons, in a sphere of positive charge, were like currants (dry fruits) in a spherical Christmas pudding.

You can take watermelon as example.

The positive charge in the atom is spread all over like the red edible part of
the watermelon, while the electrons are studded in the positively charged sphere, like the seeds in the watermelon.

        Thomson's Model of An Atom

Thomson prposed that: 

(i) An atom consists of a positively charged sphere and the electrons are
embedded in it.
(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

The Limitation of Thomson's Model of An Atom: 

(i) The results of experiments carried out by other scientists could not be explained by this model.

The Discovery of Nucleus: 

Rutherford discovered the nucleus of the atom, Rutherford designed an experiment for this. In this experiment, fast moving alpha (α)-particles were made to fall on a thin gold foil. The results showed that the atom is empty most of the α -particles passed directly from the gold foil without getting deflected but some alpha particles were deflected by a very small angle. If these particles were deflected, in fact, that was nucleus of the atom or centroid. Thus Rutherford discovered the nucleus.

Results of Alpha Scattering Experiment of E. Rutherford:

The α-particle scattering experiment gave totally unexpected results. The following observations were made:
(i) Most of the fast moving α-particles passed straight through the gold foil.
(ii) Some of the α-particles were deflected by the foil by small angles.
(iii) Surprisingly one out of every 12000 particles appeared to rebound.

The Features of Atomic Nucleus Model of Rutherford : 

On the basis of his experiment, Rutherford put forward the nuclear model of
an atom, which had the following features:
(i) There is a positively charged centre in an atom called the nucleus.

(ii) All the mass of an atom resides in the nucleus.
(iii) The electrons revolve around the nucleus in well-defined orbits.
(iv) The size of the nucleus is very small as compared to the size of the atom.

(v) The Number of electrons are equal to the number of protons in an atom. 

Rutherford's Atomic Model:

Rutherford concluded from the α-particle scattering experiment that–

(i) Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.

(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.

(iii) A very small fraction of α-particles were deflected by 1800, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

The Limitations of Atomic Model: 

Acording to Rutherford Atomic model : 

The electrons revolve around the nucleus. The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. In this case the atom should be highly unstable and hence matter would not exist in the form. But this is not fact we know atom is stable. 

Neils Bohr's Model of Atom:

Neils Bohr put forward the following postulates about the model of an atom:
(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy.