Lord Rutherford (1911) and his coworkers performed an experiment. They bombarded a very thin, gold foil with alpha particles from a radioactive source. They selected the gold foil because they wanted as thin a layer as possible. The gold foil was about 1000 atoms thick.
It was expected that –
(a) all alpha particles would be deflected by the sub-atomic particles scattered in the gold atoms as same charges repel each other. So, a positively charged body of an atom will repel another positively charged alpha particles.
(b) Since the alpha particles were much heavier than the protons, they would be deflected with little deflection (no large deflection).
But, the Rutherford’s Alpha Particles scattering experiment gave completely unexpected results.
It was found that –
- Most alpha particles passed straight without showing any deflection from their original path through the gold foil.
- Some of the alpha particles were deviated at small angles and a few were scattered at large angles. A very few (1 out of 12000) alpha particles completely bounced back towards the source.
Rutherford explained these unexpected observations through following conclusions –
(i) The fact that majority of the particles went through the foil undeflected [opposite to (a)] shows that most of the space occupied by an atom is empty.
(ii) The deflection of a few particles shows that there is a ‘Centre of positive charge’ and it occupies a very little space. When alpha particles strike with heavy body having positive charge, they deflect, as similar charges repel each other.
(iii) The deflection of a few particles over a wide angle of 1500 to 1800 [opposite to (b)] indicates that all the positive charge and mass of the gold atom are concentrated in a very small volume within the atom. This heavy, tiny, dense, positively charged central part of the atom is called nucleus.
On the basis of the following conclusions, Rutherford put forward a nuclear model of an atom, which had the following features–
1. Most of the space occupied by an atom is empty. There is a heavy, tiny, dense, positively charged central part of the atom, called nucleus.
2. Nearly all of the mass of atom is concentrated in the nucleus.
3. The size of the nucleus is very small (1/10000) as compared with the size of atom.
4. The light, negatively charged electrons revolve around the positively charged nucleus in circular paths. These circular paths of the electrons are called orbits or energy levels or shells.
Read More –
Rutherford model of an atom resembles our solar system. It has following defects:
- It failed to explain the stability of the atom. If an object (or a particle e.g. electron) moves in a circular path (or orbit), the motion of object is called accelerated motion. It means, the negatively charged
electrons revolve around the positively charged nucleus in circular paths in accelerated motion. According to classical electromagnetic theory, when electron being charged body undergoes accelerated motion, it will emit (or lose) energy continuously in the form of radiations. Thus, the energy and consequently the speed of electron will decrease gradually. The electron will then be attracted more strongly towards positively charged nucleus. Further, the orbit of the revolving electron would become smaller and smaller until it would fall into the nucleus by taking a spiral path and atomic structure would collapse. But, it does not happen. Electrons never fall into the nucleus and atomic structure never collapse on their own.
- If revolving electron emits energy continuously then there should be a continuous spectrum but a line spectrum is obtained.
More to know –
Rutherford (1911) discovered the nucleus and provided the basis for the modern atomic structure through his alpha particle scattering experiment. According to Rutherford, the atoms is made of two parts: the nucleus and the extra-nuclear part. His experiments proved that the atom is largely empty and has a heavy positively-charged body at the center called the nucleus. The central nucleus is positively-charged and the negatively-charged electrons revolve around the nucleus.
James Chadwick (1932) discovered neutrons. According to Chadwick, atoms contain neutral particles called neutrons in their nucleus along with the subatomic particle protons.
Bohr (Nobel prize winner) provided the modern concept of the atomic model. According to Bohr, the atom is made of a central nucleus containing protons (positively-charged) and neutrons (with no charge). The electrons (negatively-charged) revolve around the nucleus in different imaginary paths called orbits or shells.
Bohr’s Model of the Atom