DISCOVERY OF ELECTRON...
In the 18th century many scientists carried out studies about atom but in 1897 JJ THOMSON finally discovered the particle namely electron present in the atom successfully. The experiment of Thomson is described below:
Thomson passed electricity at high voltage through a gas at very low pressure taken in a discharge tube. and he saw that streams of minute particles were given out by cathode(negative electrode). These stream of particles are called cathode rays (because they came out of cathode). the mas and charge of cathode ray particles does not depend on the gas taken in the discharge tube. Cathode rays consists of small, negatively charged particles called electrons. Since all gases form cathode rays, it was concluded that all atoms contain negatively charged particles namely electrons.
Thomson explained the formation of cathode rays as follows. The gas taken in discharge tube consists of atoms, and all the atoms consists of electrons. When high electrical voltage is passed through the gas taken in the tube the high electrical energy pushes out some of the electrons present in the atoms of the gas. These fast moving electrons form cathode rays . The formation of cathode rays shows that one of the subatomic particle present in all atoms is the negatively charged "electron".
DISCOVERY OF PROTON...
After the the discovery of electron scientists tried to carry out more information about atom and one scientist namely E.Goldstein was successful in discovering the positively charged particle present in atom known as proton. He described his experiment as follows:
When passed electricity at high voltage through a gas at very low pressure taken in a discharge tube , streams of heavy particles were given out by the anode(positive electrode). These streams of particles are called anode rays. Anode rays consists of positively charged particles. The mass and charge of the anode rays depends on the nature f gas taken in the discharge tube.
Goldstein explained the formation of protons as follows. Hydrogen gas consists of hydrogen atoms. When high electricity is applied to hydrogen gas the electrical energy removes the electrons from the atom after the removal of electrons from the atom a positively charged particle called proton is formed the fast moving protons form anode rays. It should be remembered that hydrogen is lightest of all the atoms, so it forms lightest charged particles called protons.
DISCOVERY OF NEUTRON...
After
the discovery of proton and electron , it was noticed that their are different variations
in the mass of atom as we know that the the mass of electron is so small that it
can be ignored easily so the mass of carbon atom should be 6u (u=atomic mass unit)
i.e is the mass of six protons present in it but this however proved wrong because
the actual atomic mass of carbon atom is 12 units. Then how do we explain this extra mass of six units ?
This
problem was solved by the discovery of another subatomic particle by Chadwick in
1932. this subatomic particle came to be know as neutron. The neutron is a neutral
particle found in the nucleus of the atom. Atoms of all the elements contain neutrons
except the ordinary hydrogen atom which does
not contain neutron at all. Thus the subatomic particle not present in the hydrogen
atom is neutron. A hydrogen atom only contains one proton and one electron. A neutron
is represented by the symbol n. this sub atomic particle has no charge on it. It is electrically neutral.
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