Page 71 - ICSE Chemistry 8

SUBATOMIC PARTICLES AND THEIR DISCOVERY

John Dalton proposed that an atom is a solid, indestruc ble and indivisible sphere, but his theory le
many ques ons unanswered such as ‘what is an atom made up of?’. These unanswered ques ons led
to studies and discoveries in the end of the 19th century. These studies changed the en re data about
atoms.

It was proved that atoms can be divided into electrons, protons, and neutrons. These are known as
subatomic par cles. In 1897, the English scien st Sir J.J. Thomson proved that an atom can be split into
even smaller parts. His discovery of the electron was the fi rst step towards the detailed model of an atom.

Know Your Scientist

Joseph John Thomson was born on December 18, 1856 near Manchester, England.
He won the scholarship at Trinity College, one of the most pres gious colleges at
Cambridge University. In 1906, he was awarded the Nobel Prize in Physics for his
researches on the discharge of electricity in gases.

Discovery of Electrons

An English chemist, Sir William Crookes, in 1879 conducted an high voltage generator
– +
experiment to inves gate electric discharge through gases.
air at very
We know that under ordinary condi ons, gases are poor discharge low pressure
tube green
conductors of electricity. But gases become good conductors
– + glow
of electricity when cathode rays anode
• the gas is stored at very low pressure (0.01–0.001 mm cathode
of mercury) and to vacuum pump
• a very high voltage is applied through the gas (more Fig. 4.1: Cathode rays in discharge tube
than 10,000 volts).

The experiment was conducted in a discharge tube which he had designed. The discharge tube was a
– +
cylindrical glass tube with sealed electrodes at each end.
Electrodes were the metal plates connected to a high voltage cathode electric fi eld anode
source. A small tube at the side of the cylindrical glass tube was
_ _
connected to a vacuum pump to reduce the pressure of the gas,
as desired. The electrode connected to the nega ve terminal + +
of the voltage source served as the cathode and the electrode
connected to the posi ve terminal served as the anode. Fig. 4.2: Emission of cathode rays

When high voltage electric current was passed through the discharge tube containing a gas or air at
very low pressure, Crookes found that the opposite end of the cathode in the discharge tube began to
show a green fl uorescence. He observed that on passing the electric current, rays were emi ed from
the cathode. This fl uorescence was observed due to a stream of rays that originated from the cathode,
hence called the cathode rays. He showed that the emi ed rays were nega vely charged by studying the
direc on in which these rays were defl ected by a magnet.

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