Page 104 - Chemistry ICSE Class IX

92 ICSE Chemistry – 9

What is the J. J. Thomson Model of an Atom
In 1898, Sir J. J. Thomson proposed a model of the atom. This model is
known as J. J. Thomson’s Plum-Pudding model of the atom (Fig. 4.7).
According to this model,
ƒ An atom is considered to be a sphere of uniform positive charge and
electrons are embedded into it like raisins in a plum pudding.
ƒ In an atom, total positive charge is equal to the total negative charge
due to the electrons.
ƒ Mass of the atom is considered to be uniformly distributed.

What is Rutherford’s scattering experiment

Fig. 4.7 J.J. Thomson¶s plum-pudding In 1911, Rutherford performed his famous Scattering Experiment.
model of the atom In his experiment, Rutherford bombarded a thin sheet (0.00006 cm
thick) of gold foil with alpha (D) particles in an evacuated chamber. A
ůƉŚĂ ;DͿ ƉĂƌƟĐůĞƐ ĂƌĞ ĚŽƵďůLJ ŝŽŶŝƐĞĚ simpli ed picture of an D-particle scattering by a thin gold foil is shown
Ϯн
ŚĞůŝƵŵ ĂƚŽŵƐ ;,Ğ Ϳ͘ in Fig. 4.8.
The following observations were made on the results obtained:
ƒ Most of the alpha (D) particles passed through
the foil straight without suffering any change
in their direction.
ƒ A small fraction of alpha-particles were
de ected through small angles and a few
through large angles.
ƒ One particle in every 10,000 returned back.
This observation surprised everyone in the
laboratory.
This discovery was so fascinating that
Rutherford wrote the following comments
regarding the scattering experiment:
I remember Geiger (Rutherford’s student
Fig. 4.8 The scattering of D-particles by a thin gold foil. 0ost D-particles
pass straight through the foil (A , A , A ), but some suffer deÀections from working on the experiment) coming to me in great
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their normal path (B , B , B ), and only a few are repelled back (C , C ). An excitement and saying, “We have been able to get
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D-particle is about 500 times heavier than an electron. some of the alpha particles coming back ...”.
It was the most incredible event that ever happened to me in my life.
It was almost as incredible as if you red a fteen-inch shell at a piece of
tissue paper and it came back to hit you.
What conclusions were derived from the scattering
experiment
The following conclusions were drawn from the Rutherford’s scattering
experiment:
1. Most of the D-particles passed straight through the foil without
suffering any de ection. This showed that most of the space inside
the atom is empty or hollow.
2. Some of the D-particles suffered de ection by 90° or even larger angles.
For this to happen, D-particles (positively charged) must approach a
heavy positively charged core inside the atom (like charges repel each
other). This heavy positively charged ‘core’ inside the atom was
named nucleus.
3. Since the number of D-particles which bounced back was very small,
the volume occupied by the nucleus is very small as compared to the
Fig. 4.9 Rutherford¶s model of the atom total volume of the atom.