Valence Spanning Electron Pair

Some comments are now appropriate. Firstly, the ionization potential of a doubly occupied AO is set equal to the ionization potential of the lone pair of an appropriate model system. Secondly, the ionization potential of a singly occupied AO is set equal to the valence state ionization potential of the atom in the appropriate electronic configuration. Thirdly, due to the small splitting of the symmetric and antisymmetric MO s spanning the ligands, e,g. 0g and 0a > the corresponding electron affinities or ionization potentials are set equal to the appropriate valence state electron affinities or ionization potentials. In the cases at hand, the following data have to be used. 

According to the VSEPR model the five electron pairs in CLF3 or BrF3 should arrange themselves in a trigonal bipyramidal manner with the two lone pairs in equatorial positions. Since the electron lone pairs require more space around the central atom they are expected to deform the molecule is such a way that the valence angles spanned by one axial and one equatorial F atom are reduced to values below 90" . This is indeed what is observed. 

First, we need to see why two electrons constitute a bond, and not one electron, or three electrons, or some other number. The reason can be traced back to the Pauli exclusion principle, which limits the occupation of atomic orbitals to no more than two electrons. When two atoms come together, the distribution of the electrons of their valence shells is no longer confined to each atom alone but now spreads over both of them like a net. Analogous to the distributions in atoms—the atomic orbitals— the distributions in molecules, which spread over the constituent atoms, are called molecular orbitals. However, even though they have a wider span and a more complicated form, these orbitals are still orbitals, and the exclusion principle still applies only two electrons can be accommodated in one of these orbitals, and this is why a covalent bond consists of a pair of electrons. 

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