Page 51 - Understanding NCERT Science 09
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He will hear a sound when each stone strikes
the wall. If he repeats this ten times, he will
hear the sound ten times. But if a blind-folded
child were to throw stones at a barbed-wire
fence, most of the stones would not hit the
fencing and no sound would be heard. This is
because there are lots of gaps in the fence
which allow the stone to pass through them.
Following a similar reasoning, Rutherford
concluded from the α-particle scattering
experiment that—
(i) Most of the space inside the atom is
empty because most of the α-particles
passed through the gold foil without
getting deflected.
(ii) Very few particles were deflected from
their path, indicating that the positive
charge of the atom occupies very little
Fig. 4.2: Scattering of α-particles by a gold foil space.
(iii) A very small fraction of α-particles
But, the α-particle scattering experiment were deflected by 180 ,indicating that
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gave totally unexpected results (Fig. 4.2). The all the positive charge and mass of the
following observations were made: gold atom were concentrated in a very
(i) Most of the fast moving α-particles small volume within the atom.
passed straight through the gold foil. From the data he also calculated that the
(ii) Some of the α-particles were deflected radius of the nucleus is about 10 times less
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by the foil by small angles. than the radius of the atom.
(iii) Surprisingly one out of every 12000 On the basis of his experiment,
particles appeared to rebound. Rutherford put forward the nuclear model of
In the words of Rutherford, “This result an atom, which had the following features:
was almost as incredible as if you fire a (i) There is a positively charged centre in
15-inch shell at a piece of tissue paper and it an atom called the nucleus. Nearly all
comes back and hits you”. the mass of an atom resides in the
nucleus.
E. Rutherford (1871–1937) (ii) The electrons revolve around the
was born at Spring Grove nucleus in circular paths.
(iii) The size of the nucleus is very small
on 30 August 1871. He was
known as the ‘Father’ of as compared to the size of the atom.
nuclear physics. He is Drawbacks of Rutherford’s model of
famous for his work on the atom
radioactivity and the
discovery of the nucleus of an atom with The revolution of the electron in a circular orbit
the gold foil experiment. He got the Nobel is not expected to be stable. Any particle in a
circular orbit would undergo acceleration.
prize in chemistry in 1908.
During acceleration, charged particles would
radiate energy. Thus, the revolving electron
Let us think of an activity in an open field would lose energy and finally fall into the
to understand the implications of this nucleus. If this were so, the atom should be
experiment. Let a child stand in front of a highly unstable and hence matter would not
wall with his eyes closed. Let him throw exist in the form that we know. We know that
stones at the wall from a distance. atoms are quite stable.
40 SCIENCE
Rationalised 2023-24
