Electron is a negatively charged particle with an absolute mass of -1.6×10-19 coulombs. This is taken as the unit of negative charge as this is the smallest negative charge carried by a particle. The charge of the electron is equal in magnitude but opposite in sign to the charge of a proton. An atom has equal number of protons and electrons, so an atom, on the whole, is electrically neutral. Electron is the lightest os three atomic particles. The mass of an electron is equivalent to 1/1836 of the mass of a hydrogen atom. The electron in an atom possesses a certain amount of energy to maintain its distance from the oppositely charged nucleus. Without this energy, the electron would collapse into the nucleus.