The Shared-Electron Chemical Bond.

With the development of the quantum mechanics it has become evident that the factors mainly responsible for chemical valence are the Pauli exclusion principle and the Heisenberg-Dirac resonance phenomenon. It has been shown that in the case of two hydrogen atoms in the normal state brought near each other the eigenfunction which is symmetric in the positional coordinates of the two electrons corresponds to a potential which causes the two atoms to combine to form a molecule. This potential is due mainly to a resonance effect which may be interpreted as involving an interchange in position of the two electrons forming the bond, so that each electron is partially associated with one nucleus and partially with the other. The so-calculated heat of dissociation, moment of inertia, and oscillational frequency of the hydrogen molecule are in approximate agreement with experiment. London has recently suggested that the interchange energy of two electrons, one belonging to each of two atoms, is the energy of the non-polar bond in general. He has shown that an antisymmetric (and hence allowed) eigenfunction symmetric in the coordinates of two electrons can occur only if originally the spin of each electron were not paired with that of another electron in the same atom. The number of electrons with such unpaired spins in an atom is, in the case of Russell-Saunders coupling, equal to 2s, where s is the resultant spin quantum number, and is closely connected with the multiplicity, 2s + 1, of the spectral term. This is also the number of electrons capable of forming non-polar bonds. The spins of the two electrons forming the bond become paired, so that usually these electrons cannot be effective in forming further bonds.