Electron
A negatively charged fundamental particle that occupies orbitals around the nucleus and governs chemical bonding.
Formula / equation
m_e = 9.109 × 10⁻³¹ kg
Unit
u
In depth
With a mass only 1/1836 that of a proton, the electron contributes almost nothing to atomic mass yet determines essentially all chemical behaviour. It is a lepton with spin ½, so it obeys the Pauli exclusion principle. Its wave nature, confirmed by diffraction, is what forces the orbital description of atoms.
Examples in the real world
The single electron of hydrogen; the sea of delocalised electrons that makes copper conduct electricity.