Atomic Models

Ancient Greeks believed that atoms were like solid balls

'Raisin Pudding' Model - Thomson's model of atom

• J. J Thomson the discoverer of the electron believed that the atom is a uniform sphere of positive charge with electrons (raisins) embedded in it.

This model explained the electrical neutrality of atom, but it failed to explain how these fundamental particles were arranged in the atom.

Thomson's 'raisin pudding' model of the atom

Rutherford's model - discovery of nucleus

• Ernest Rutherford, an energetic Scientist From New Zealand working in England, was studying the properties of particles spontaneously emitted by radioactive substances. Of particular interest was a positively charged, relatively heavy emission named alpha particle. Quantities of such particles are emitted by certain radioactive elements and Rutherford's group had learnt how to control and detect them.

Alpha particles (a) are positively charged particles having 2 units of positive charge and 4 units of mass, i.e., particles are doubly charged helium atoms (He²⁺). They are emitted by radioactive elements like radium and polonium.

Rutherford's experiment

A stream of alpha particles (much like a stream of tiny bullets) was directed at a thin foil of gold atoms and a detector arranged to surround the sample completely except for a small hole for entry of the particles. The foil was several thousands of atoms thick.

Rutherford Scattering apparatus

What was expected?

Keeping the prevailing model in mind (Thomson's 'raisin pudding') the cloud of positive electricity should offer little resistance to the passage of an alpha particle even though the latter is charged. The reasoning is that alpha particles are moving at high speed and have a great amount of forward momentum (mass x velocity = momentum). Even though there would be some repulsion due to like charges, the momentum would overcome this and the particles ought to be deflected slightly or go straight through.

What was observed?

• Most of the alpha particles pass straight through the gold foil without any deflection from their original path.

• A few alpha particles are deflected through small angles and few are deflected through large angles.

• A very few alpha particles completely rebound on hitting the gold foil and turn back in their path (just as a ball rebounds on hitting a hard wall).

Explanation

• Since most of the alpha particles pass straight through the gold foil without any deflection it shows there is a lot of empty space in an atom.

• Some of the alpha particles are deflected through small and large angles, which shows that there is a 'centre of positive charge' in an atom, which repels the positively charged alpha particles and deflects them from the original path.

• Very few alpha particles rebound on hitting the gold foil, which shows the nucleus is very dense and hard which does not allow alpha particles to pass through it. The whole mass of the atom is centered at its nucleus.

Conclusion

• Nucleus of an atom is positively charged

• Nucleus is very dense and hard

• Nucleus is very small compared to the size of the atom.

Scattering of alpha particles by the atoms of a gold foil

A picture of the model as conceived by Rutherford

• The atom of an element consists of a small, positively charged nucleus in the centre, which carries almost the entire mass of the atom.

• The electrons are revolving around the nucleus at high speed.

• The number of electrons in an atom is equal to the number of protons. Hence it is electrically neutral.

• The volume of the nucleus is negligibly small compared to the volume of the atom.

• Most of the space in the atom is empty.

Rutherford's nuclear model of the atom

Rutherford compared the structure of an atom to the solar system i.e., just as in the solar system, the Sun is having the maximum mass and planets revolve around it, similarly in an atom, the nucleus forms the main mass of atom and electrons revolve around it.

Drawback of Rutherford's Atomic Model

• Rutherford proposed that electrons revolve at high speed in circular orbits around the positively charged nucleus. But according to the electromagnetic theory, if a charged particle were accelerated around another charged particle then there would be a continuous radiation of energy. The loss of energy would slow down the speed of the electron and eventually the electron would fall into the nucleus. But such a collapse does not occur. Rutherford's model was unable to explain it.

Diagram to show how an energy losing electron could fall into the nucleus.