Answer:
In
1911, Rutherford performed the gold foil experiment. He bombarded a stream of
a-particles on a gold foil, a thin sheet which was 0.00006 cm thick in an
evacuated chamber. An a-particle is a positively charged helium ion (He +).
A simplified picture of this experiment is shown in the figure.
In this famous experiment, the following observations were
made.
(i) Most of the a-particles passed straight through the
foil without any deflection.
This concluded that most of the space inside of an atom is
empty.
(ii) A few 
particles were
deflected through small angle and few through larger angles. This happened due
to positive charge on particles and core (nucleus)
of the atom. The heavy positively charged 'core' was named as nucleus.
(ii) The number of particles
which bounced back was very small. This concluded that the volume of the
nucleus is very small in comparison to the total volume of the atom.
On the basis of gold foil experiment, Rutherford concluded
that an atom consists of nucleus which has positive charge and it is surrounded
with electrons which are moving around the nucleus. The number of electrons and
protons are equal and the entire mass of the atom is concentrated at its nucleus.
Drawbacks in the Rutherford's model
(i) According to classical electro-magnetic theory, a
moving charged particle, such as an electron under the influence of attractive
force loses energy continuously in the form of radiations. As a result of this,
electron should lose energy and therefore, should move in even smaller orbits
ultimately falling into the nucleus. But the collapse does not occur. There is
no explanation for this behaviour.
(ii) Rutherford did not specify the number of orbits and
the number of electrons in each orbit.
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