Abstract
In 1811, A. Avogadro explained that elementary hydrogen exists as H2 molecules. Avogadro's findings were almost completely ignored until S. Canizarro presented them at the Karlsruhe Conference in 1860. In 1916, G. N. Lewis described the sharing of electron pairs between atoms. This idea provides a picture of covalent bonding, however, quantum mechanics is needed to understand the nature of the chemical bond. In 1927, W. Heitler and F. London were succeeded in the first explanation of the chemical bond of molecular hydrogen based on the valence bond method. Molecular orbital (MO) method was also proposed to explain theoretically the nature of the chemical bond. Similar to photons, electrons show properties of particles and waves. A. Tonomura revealed that the wave character of electron is displayed when a beam of electron is passed through parallel slits thereby creating interference patterns [5, 7, 8]. Formation of two molecular orbitals φ1 (bonding MO) and φ2 (antibonding MO) from the interference of two hydrogen atomic orbitals χ1 and χ2 is shown in Figure 4. Electron density distribution in the 3-dimensional representation of the squares of hydrogen MOs are given in Figure 5. Difference electron density between |φ1|2 and (|χ1|2 + |χ2|2) in Figure 6 (c) and 7 (b) shows the internuclear region having high probability densities of finding an electron, to explain electron densities (-δe) in this region could attract the hydrogen nuclei having plus electric charges.
