Fundamental Particles of Atom

In 1897 J.J. Thomson discovered electron as a constituent of atom. He determined that an electron had a negative charge and had very little mall as compared to that of the atom. Science an atom was found to be electrically neutral it was inferred that some source of positive charge must be present in the atom. This soon led to the experimental discovery of the proton, which is a positively charged subatomic particle. Proton was found approximately 1840 times heavier than an electron. Further' experiments revealed that the atomic masses were more than that expected from the presence of just protons and electrons in the atom. For example, the mall of helium atom was expected to be double that of hydrogen atom but was actually found to be almost four times the mass of hydrogen atom. this suggested the presence of neutral particles with mass comparable to that of protons in the atom. Sir James Chadwicd discovered this neutral particle and called it neutron subsequently in 1932. Thus we may conclude that atoms are not indivisible but are made up of three fundamental particles they are given belo.
1. Electron
3. Neutron

                                                                                                                                                               Fundamental particles of atom and their characteristics                         

Particle    symbol           mass/kg                     actual charge/c            relative charge

Electron    e           9.109389×10-31          -1.602177×10-19                                                  -1 Proton       p          1.672623×10-27           -1.602177×10-19                                               +1 Neutron    n           1.674928×10-27                      0                                                   0

                                                                
                                                                      sccience atoms are made up of still smaller particles, they must have an internal structure.in the next section we shall take up some of the earlier ideas about internal strecture of atom