Atomic Models

Thomson’s Model of Atom(1904)

1. Thomson was the first to proposed a detailed model of an atom.

2. This model can be visualized as a watermelon in which the black seeds act as electron & large reddish part represents proton.

Drawbacks:

1. It could not explain how the electrons an proton interact with each other to stabilize the atom.

2. This model was not consistent with the results of later experiments conducted by other scientists.

3. It is a static model. It does not reflect the movement of electrons.

Rutherford’s Nuclear Model of Atom

In this experiment Rutherford allowed a narrow beam of ∝-particles to fall on a very thin gold foil. This gold foil had circular fluorescent zinc sulphide screen around it. The ∝ -particles emitted by radioactive substances are dipositive helium ions (He++) having a mass of 4 units and 2 units of positive charge.

A tiny flash of light was produced at the point where ∝– particles struck.

The observations and conclusions made from the experiment by Rutherford are as following:

	Observation	Conclusion
1	Most of the – particles passed through the foil without any deflection.	Presence of large empty space in the atom.
2	Few – particles were deflected by small angles.	Positive charge is concentrated at a very small region and not uniformly distributed in whole atom. (If not then large number of – particles would have been deflected b experiencing the enormous repulsive force from positive charge of the atom).
3	Very few – particles (1 out of 20,000 particles) completely rebound i.e. deflected at 1800.	Positively charged core is known as nucleus.

Based on the a-Scattering Experiment Rutherford gave Nuclear Model of Atom

The main features of this model are:

1. Nucleus: In an atom, the mass and positive charge is centrally located in extremely small region called nucleus.

2. The volume of nucleus is negligible as compared to the total volume of the atom. As the radius of atom is about 10^-10 m and the radius of nucleus is 10^-15 m.

3. Both protons and neutrons present in the nucleus are collectively called nucleons.

4. Extra-nuclear part: The nucleus is surrounded by revolving electrons. Electron revolve with very high speed in circular paths called orbits, so as to counterbalance the electrostatic force of attraction between protons and electrons.

5. Rutherford’s model of atom resembles the solar system in which the nucleus plays the role of sun and the electrons of revolving planets.

6. The centrifugal force acting outwards balances the inward coulombic attraction by the nucleus. This stabilises the electrons in their orbits.