A subatomic particle with negative elementary electric charge. An electron has a mass of 9 × 10⁻³¹ kg, which is 1/1836 the mass of a proton. Electrons form the shells of atoms, but they can also be found loose. In the electron shells of atoms, they occupy so-called atomic orbitals, with the electrons in the highest orbitals, the valence orbitals, taking part in chemical reactions (e.g. the bonding of oxygen with carbon during combustion). A neutral atom has the same number of electrons as it has protons in the nucleus, and so its total charge is neutral. If the atom loses some electrons, it becomes a positive ion, and its charge will be positive. There are also negative ions, where the atom has more electrons than protons, and its total charge is negative.

Because it is a charged particle, the path of the electron can be affected by a magnetic field. The movement of free electrons is also referred to as an electric current. Electrons exhibit corpuscular-wave dualism, i.e. they can behave as both a wave and a particle. High-energy electrons released from the nucleus of an atom are referred to as beta radiation.