A fundamental particle with negative electric charge of 1.602*10-19 coulombs and mass 9.109*10-31 kg. Electrons are a basic constituent of the atom; they are distributed around the nucleus in shells and the electronic structure is responsible for the chemical properties of the atom. Electrons also exist independently and are responsible for many electric effects in materials. Due to their small mass, the wave properties and relativistic effects of electrons are marked. The positron, the antiparticle of the electron, is an equivalent particle but with positive charge. Either electrons or positrons may be emitted in ß-decay. Electrons belong to a group of fundamental particles called leptons.