Page 75 - ICSE Chemistry 8

• Some par cles were defl ected back, only one out of every 12,000 -par cles appeared to
rebounce. Very small frac on of -par cles were defl ected by large angles indica ng that all the
posi ve charge and mass of the gold atom were concentrated in a very small volume within the
atom.

Rutherford’s Conclusions

On the basis of these observa on, Rutherford made the following conclusions.

• A lot of space was empty in the atom as the alpha par cles passed straight without any defl ec on.

• Since the mass of alpha par cles is about 8000 mes that of an electron, it was evident that the
force which causes such large defl ec ons was also strong.
• Small defl ec ons of the alpha par cles could only be caused by a centre of concentrated posi ve
charge that accounts for most of the atom’s mass. He called this central body of concentrated
posi ve charge nucleus.

• Large defl ec ons of the alpha par cles meant that the nucleus is centrally located.

• The par cles that directly hit the nucleus bounced back.

As a result of this experiment, the plum pudding model was replaced by the Rutherford's model.

Rutherford’s Model of an Atom

Based on his experimental fi ndings, Rutherford in 1911 published his views on atomic theory. He suggested
the following model of the atom.
• Most of the mass of an atom was concentrated in the
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centre of the atom called the nucleus.

• The nucleus is the densest part of an atom and contains A large number of posi vely charged
protons confi ned in a small nucleus
posi vely charged par cles, the protons.
should lead to repulsion, thus, making
• The size of the nucleus is extremely small as compared the nucleus unstable. But a nucleus
to the size of atom as a whole, as there is a lot of empty is stable. Rutherford explained that
space around the nucleus. protons in the nucleus are acted
upon by an a rac ve nuclear force
• An atom is electrically neutral, i.e., the number of
that balances the repulsion in
electrons is equal to the number of protons.
protons. Thus, a nucleus is stable.
• Electrons revolve in circular orbits (shells) available
around the nucleus.

Rutherford model of an atom did not explain the stability of an atom. Posi vely charged nucleus and
nega vely charged electrons would a ract each other. An electrically charged par cle in mo on releases
energy and therefore, the electrons should lose energy and get a racted towards the nucleus, leading to
the total collapse of the atom. But this does not happen. An explana on to this problem was provided by
Niels Bohr, a Danish physicist. He suggested that electrons possess a specifi c amount of energy and are
arranged in a succession of energy levels or orbits that allow them to revolve around the nucleus.

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