Electrons are one of the three sub-atomic particles of an atom. They orbit the nucleus which is made of protons and neutrons. They have a negative charge, and have a mass of very very little - about 1/1837 of a proton. The symbol for an electron is e-. As atoms have equal numbers of electrons and protons, their overall charge is always neutral. However, when atoms form ions, their charge becomes either positive or negative (e.g. Hydrogen ions are positvely charged.)