Electron pair 电子对
In chemistry, an electron pair or a Lewis pair consists of two electrons that occupy the same orbital but have opposite spins. The electron pair concept was introduced in a 1916 paper of Gilbert N. Lewis.
Because electrons are fermions, the Pauli exclusion principle forbids these particles from having exactly the same quantum numbers. Therefore the only way to occupy the same orbital, i.e. have the same orbital quantum numbers, is to differ in the spin quantum number. This limits the number of electrons in the same orbital to exactly two.