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electrons have to be neutralized by positive charge inside an atom.The presence of this positive

charge had been suggested by Eugene Goldstein in 1886 . In general , Thomson‟s model can be

stated as follows.

Thomson’s atomic model

Atom is a sphere in which the positive
charges spread out evenly in the atom are
neutralized by negatively charged electron
positioned among positive charges.
Electrons in an atom are like raisins in plum
pudding.
Figure 6. Thomson’s Atomic Theory

The Thomson‟s model lasted for more than a decade before being replaced by Rutherford’s

atomic model after the discovery of another subatomic particle alled proton.

Rutherford’s Atomic Theory

In 1991, Ernest Rutherford resumed Lenard‟s experiment

using α particle instead of electron. The experiment was

conduscted by two of his assistants, Hans Geiger and Ernest

Marsden.

From the experiment, Rutherford concluded that :

a. A large fraction of the room in an atom is empty. This is

indicated by a large number of α particles pass straight

through, not deflected.
Figure 7. Rutherford
b. Atom has a tiny yet very dense core called the nucleus.

This is indicated by a small fraction of α particles that are bounced back by the nucleus.

c. The charge of the nucleus is the same as that of the particle, which is positive. This is

because a small number of the α particles are deflected and that deflection occurs due to

repulsion forces between similar charges.

The results of the experiment brought down the Thompson‟s atomic model and replaced it

with the Rutherford‟s atomic model.

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