Chemistry · Ch 10 — Chemical Bonding
Valence Bond Theory
Valence Bond Theory
Heitler and London gave the first theoretical, wave-mechanical treatment of covalent bond formation, working out the case of the hydrogen molecule; Pauling and Slater later developed this treatment further into full Valence Bond (VB) theory. (The full wave-mechanical mathematics is beyond this textbook's scope; only the simple, qualitative picture is given here.)
The qualitative picture for H₂. Imagine two hydrogen atoms, Hₐ and H_b, initially separated by an infinite distance. At this separation there is no interaction between them, so the potential energy of the system is (arbitrarily) taken as zero. As the two atoms are brought closer, in addition to each electron's own attraction to its own nucleus, NEW forces begin to operate (Fig 10.17(a)):
- New attractive forces arise between (i) the nucleus of Hₐ and the valence electron of H_b, and (ii) the nucleus of H_b and the valence electron of Hₐ.
- New repulsive forces arise between (i) the two nuclei (Hₐ and H_b), and (ii) the two valence electrons. …
What this figure shows. Two hydrogen atoms Ha and Hb drawn with their nuclei (+) and electrons (−); purple arrows mark each electron's attraction to its own nucleus, green arrows mark the new attractive forces (each nucleus to the other atom's electron), and red arrows mark the new repulsive forces (nucleus-nucleus and electron-electron) that appear as the atoms approach each other. …
What this figure shows. A potential-energy-versus-internuclear-distance curve: energy starts near zero at large separation, drops to a minimum of −432 kJ/mol at an internuclear distance of 74 pm (marked as the H₂ bond length), then rises sharply as the distance decreases further and repulsion dominates. …