RUTHERFORD’S MODEL OF ATOM
Rutherford in 1911 performed an experiment which led to the downfall of Thomson’s model. Most of the experimental work was carried out by Geiger and Marsden, two of Rutherford’s students at Manchester University. Geiger and Marsden built the apparatus as shown in Fig. 5.7. The experiment involved the bombardment of a thin sheet of gold (thickness 4 x 10-5 cm) by a-particles. The source of a-particles was a piece of radium placed in a lead block. A narrow hole in the lead block allowed the a-particles to travel only in one direction through the evacuated vessel towards a fluorescent zinc sulphide screen. When an a-particle hits the fluorescent screen a tiny flash of light is produced which is observed with the help of a microscope. A thin gold foil was placed between the slit and the screen. The screen and the microscope were rotated to detect scattering of a-particles.
Ernest Rutherfo rd (1871-1937) was born in New Zealand. In 1908 he was awarded the Nobel prize for chemistry for his work on radioactivity.
In 1911, he discovered that a-particles could be deflected by thin metal foils.
Rutherford observed that:
1. Most of the a-particles (nearly 99%) passed through the gold foil undeflected.
2. Some of the a-particles were deflected by small angles.
Fig. 5.7. The apparatus used by Geiger and Marsden to investigate the scattering of a-particles by a thin metal foil.
3. Very few particles (1 in about 1 04) were either deflected by very large angles or were actually reflected back along their path.
These observations led to the downfall of Thomson model. According to Thomson model positive charge is uniformly distributed in the volume of the atom. Therefore, it is so diffused that the fast moving a-particles (He2+) should pass through this weak electric field largely undeflected or may suffer slight deflection.
In order to explain the observations of his scattering experiment, Rutherford assumed that the solid gold foil consists of layers of individual atoms which are touching each other so that there is hardly any empty space between them. As such the a-particles striking the gold foil must pass through the atoms. Rutherford explained his observations as follows:
- Since most of the α-particles pass through the foil undeflected, it indicates that the most of the space in an atom is empty (Fig. 5.8).
Fig. 5.8. Scattering of a-particles by gold atoms.
2. α-particles being positively charged and having considerable mass could be deflected only by some heavy, positively charged centre. The small angle of deflection of a-particles indicates the presence of a heavy positive centre in the atom. Rutherford named this positive centre as nucleus (Fig. 5.9).
3. a-particles which make head-on collision with heavy positive centre are deflected through large angles. Since the number of such a-particles is very small, the space occupied by the heavy positive centre must be very small.
From the data of scattering experiment, Rutherford was able to calculate the radius of the nucleus. Rutherford calculated that the nucleus of an atom would have radius of about 10-14 m. He showed that the radius of the nucleus is about 10-4 times the radius of the atom which is about 10-10 m.
Rutherford’s Nuclear Model of Atom
On the basis of scattering experiment, Rutherford put forward nuclear model of atom. Main points of this model are:
- Most of the mass and all the positive charge of an atom is concentrated in a very small region called nucleus. Size of the nucleus is extremely small as compared with the size of the atom. Radius of the nucleus is of the order of 10-14 m, whereas radius of atoms is of the order of 10-10 m.
- The nucleus is surrounded by electrons which are revolving around it at very high speeds. The electrostatic force of attraction between electrons and the nucleus is balanced by the centrifugal force acting on the revolving electrons.
- Total negative charge on the electrons is equal to the total positive charge on the nucleus so that atom on the whole is electrically neutral
Nuclear model of atom can be compared with the solar system. In an atom electrons revolve around the nucleus in just the same way as the planets revolve around the sun. Due to this comparison, revolving electrons are sometimes called planetary electrons.
Although Rutherford’s idea of planetary electrons has now been discarded, his idea of small positive nucleus still holds true.