Electron - a very low-mass subatomic particle with a negative charge. A fluffy cloud of electrons makes up the outer part of atoms and molecules. Atoms can share electrons, and this binds the atoms into molecules. The unique way that atoms of each element share electrons defines their chemistry. Electrons can be knocked loose from atoms by ultraviolet light, by impact from other electrons moving at high speed, or by a kind of group sharing of electrons by the atoms in some solids (especially metals). Freely moving electrons can be driven into motion by electric fields, forming an electrical current. In the presence of a magnetic field, free electrons and ions move in circles about the field lines so that they tend to move along with the magnetic field.