PROTON:-

The discovery of proton brought the concept that atomic mass should be equal to the total mass of proton of the atoms since the mass of electron negligible i.e., mass of an electron is very small with respect to mass of a proton. However it has been found that in all atoms, expect hydrogen, the mass of an atom is more than total mass of protons present in the atom.

                      To solve this problem, Chadwick performed an experiment, in 1932, (who was awarded by Nobel Prize for this work) by bombarding alpha particles on thin sheet of beryllium element. He observed highly penetrating rays consisting of streams of neutral particles were found to have a mass of 1.675*10^-24gm which is nearly equal to the mass of hydrogen atom. These neutral particles were named as neutrons. The fundamental particle which carries no charge but has a mass nearly equal to that of proton is known as neutrons.

         

Neutron

The discovery of negatively charged particles in an atom suggests that there must be some positively particles in it since an atom is electrically natural. The existence of positively charged particles was concluded by E. Goldstein. He repeated the discharge tube experiment using a perforated cathode. When the high voltage was applied, new kind of rays were emerged from the holes in the cathode travelling towards anode opposite side i.e., these rays travelled on the opposite direction to the cathode rays. These rays were names as canal rays. When an external electric field was applied, they deflected towards the negatively charge electrode. Hence, they are also called anode rays or positive rays. Anode ray posses the flowing characters:

1.        The value of e/m for the particles is smaller than that for electrons.

2.       The e/m ratio depends upon the nature of the gas taken in the tube.

3.       The anode rays travel in straight lines and cast shadow of the object placed in their path.

Electron

J.J.Thomson discovered the electron in 1897 and won the Nobel Prize in 1906 for this work. His discovery was as a result of discharge tube experiment. The experiment was performed by passing high voltage (10000 Volts) electricity through gases at extremely low pressure .the gas was taken in a well designed glass tube using metal electrodes known as discharge tube. As the high voltage electricity was through the gases, they became conductor and electricity began to flow from the cathodes side in the form of rays. These rays were called cathode rays. J.J.Thomson studied the properties of the cathode rays which lead to the discovery of an electron. Cathode rays possess the following properties:

        1.    These rays ionize the gas through which they pass.

        2.    They have capacity to penetrate the thin sheet of metals.

        3.   They cause the materials like glass to glow upon striking on them.

                                    All the above observation led to the conclusion that cathode rays consists of the negatively charged particles having a certain mass. These                           particles were named electrons by J.J.Thomson and Stoney.

ATOMIC STRUCTURE:-

It deals with the important particles of atom and arrangement of these atoms in atomic structure. John Dalton in 1808, proposed that matter is made up of extremely small particles and invisible particles called atom (Greek atom means these atoms cannot be cut). John Dalton in 1808 put forward the first theory about the structure of matter which is known as Dalton’s atomic theory. It state that “all mater is composed of extremely small particles called atoms which are regarded to be invisible and spherical particles. “ This theory remained undisputed up to the 19^th century. But, the theory was discarded when the further researchers provide that atom consists of still smaller particles like electron, proton and neutrons.                         

                                   By many scientists like J.J.Thomson, Goldstein, Rutherford, Chadwick the research was done and find that the atom is not smallest particles and invisible particles but it has complex structure of its atom. Atom was made up of smallest particles like electron, proton and neutron and present about 35 sub-atomic particles.