An electron has the mass equal to 1/2000 of the mass of a common hydrogen atom. Electron was discovered in 1897 by Joseph Thomson. He performed a cathode rays experiment in which the rays with negative charge were identified and he explained that the charge on the rays was because of the negative particles called electron. Thomson gave plum-pudding model of electron and he identified four main characteristics of electrons. The very first characteristic was that mass of electron was equal to 1/2000 of the mass of a common hydrogen. His discovery is really appreciable because with the invention of the latest equipment it has found that mass of electron is nearly equal to 1/1836 mass of proton.