Under an electron pair is understood to two electrons with opposite spin occupying the same atomic or molecular orbital.
Due to the Pauli principle, electrons can within an atom (more precisely, within an electronically closed system ) do not coincide in all quantum numbers. Per orbital, which is already defined principal quantum number, azimuthal quantum number and magnetic quantum number of the electrons contained, the only distinguishing feature of the spin quantum number is left, that can take only the values 1 / 2 and -1 / 2. Therefore, the maximum number of different electrons per orbital is limited to two.
The electrons are distributed according to specific rules on the available orbitals (see electron configuration or Hund's rules). Rule of thumb: a new, energetically higher lying orbital is only occupied when the energy difference is low; larger difference in the lower-energy orbitals are initially occupied by a second electron, the formation of electron pairs.
Play a special role pairs of electrons in the chemical bond: the nuclear binding is mediated by binding electron pairs that are between the two involved atoms and two are together. This type of binding is next to the Ionian and Metallic bonding is the most important.
In contrast, lone pairs are only one atom belonging to and be represented by IUPAC recommendation, to better differentiate by two points.
- Chemical Bonding