Atoms have three types of sub atomic particles. They are electrons, protons and neutrons.
Electrons are negatively charged (e-), protons are positively charged (p+) and neutrons have no charge (n).
The mass of an electron is 1/2000 the mass of a hydrogen atom. The mass of a proton is equal to the mass of a hydrogen atom and is taken as 1 unit. The mass of a neutron is equal to the mass of a hydrogen atom and is and is taken as 1 unit.
b) Discovery of sub atomic particles :-
In 1900, J.J.Thomson discovered the presence of the negatively charged particles called electrons in the atom.
In 1886, E.Goldstein discovered new radiations in gas discharge and called them canal rays. These rays were positively charged. This later led to the discovery of the positively charged particles called protons in the atom.
In 1932 Chadwick discovered the presence of particles having no charge in the atom called neutrons.
a) Thomson’s model of an atom :-
According to Thomson an atom is similar to a Christmas pudding. The pudding had positive charge and the electrons having negative charge were like plums on the pudding.
He proposed that :-
i) An atom consists of a positively charged sphere and the electrons are embedded in it.
ii) The negative and positive charges are equal in magnitude So the atom as a whole is electrically neutral.
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