Electron pair
Two electrons grouped together as a pair. When two atoms share an electron pair, a chemical bond forms.
In 1916 Gilbert Lewis drew an atom's outer electrons as dots and proposed that a bond is two atoms each contributing one electron to a pair that they share[1]. The picture proved enormously useful to chemists and is still a basic tool for thinking about the shapes of molecules.
Why electrons should go in twos, though, remained a mystery. In 1927 Walter Heitler and Fritz London showed with quantum mechanics that two atoms attract only when their two electrons have opposite spins, giving the electron pair a physical reason[2]. Because spin has only two values, at most two electrons, with opposite spins, can occupy the same state.
Some electrons stay paired on a single atom without taking part in a bond (lone pairs). The bent shape of the water molecule comes from the room these pairs take up.
Related terms
- Chemical bondThe link that holds atoms together in molecules and crystals, arising from electrons being shared or handed over.
- SpinA property that electrons and other particles are born with, making them behave like tiny magnets. For an electron it takes only two values: "up" or "down".
- Pauli exclusion principleThe rule that no two electrons in the same atom can be in exactly the same state. Set out by Wolfgang Pauli in 1925.