A negatively-charged sub-atomic particle. It is an indivisible, elementary particle, and is usually to be found orbiting the nucleus of an atom. Electrons in an atom (which exist in the same quantity as the number of protons in the nucleus of the particular atom, so that the overall electric charge is zero) are constrained to occupy certain discrete orbital positions or “shells” around the nucleus. Interactions between the electrons of different atoms play an essential role in chemical bonding and phenomena such as electricity, magnetism and thermal conductivity. The discovery of electrons is credited to the British physicist J. J. Thomson in 1897.