Orbitals and electron pairing in valence-bond theory

4 Orbitals and electron pairing in valence-bond theory 

The first electronic function to be defined in direct association with a formal chemical bond was, as expected, for the H2 molecule. It marked the beginning of the so called valence-bond theory, in 1927, and was suggested by the Austrian physicist Walter H. Heitler (1904—1981) and by the German physicist Fritz W. London (1900-1964). 

Firstly, it is recognized that, for two separate H atoms, A and B, with electrons 1 and 2, respectively, the electronic (wave) function can be expressed as the product of the respective Is orbital functions 

it is assumed that, when the atoms are associated to form H2, this fvmction would not need appreciable modification except for the possibility of assigning electron 1 to orbital ls(B) and electron 2 to orbital ls(A)  

This possibility arises because the electrons in the molecule are indistinguishable. 

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