Inchemistry, anelectron pair orLewis pair consists of twoelectrons that occupy the samemolecular orbital but have oppositespins.Gilbert N. Lewis introduced the concepts of both the electron pair and thecovalent bond in a landmark paper he published in 1916.[1][2]
Because electrons arefermions, thePauli exclusion principle forbids these particles from having all the samequantum numbers. Therefore, for two electrons to occupy the same orbital, and thereby have the sameorbital quantum number, they must have differentspin quantum numbers. This also limits the number of electrons in the same orbital to two.
The pairing of spins is often energetically favorable, and electron pairs therefore play a large role inchemistry. They can form achemical bond between two atoms, or they can occur as alone pair ofvalence electrons. They also fill thecore levels of an atom.
Because the spins are paired, themagnetic moment of the electrons cancel one another, and the pair's contribution to magnetic properties is generallydiamagnetic.
Although a strong tendency to pair off electrons can be observed in chemistry, it is also possible for electrons to occur asunpaired electrons.
In the case ofmetallic bonding, the magnetic moments also compensate to a large extent, but the bonding is more communal, so that individual pairs of electrons cannot be distinguished and it is better to consider the electrons as a collective 'sea'.