We begin with H2, the simplest molecule of all, and start by thinking about the two
hydrogen atoms from which it is formed.
Each hydrogen atom in its ground state has one electron in a 1s-orbital. In valencebond
theory, we suppose that, as two H atoms come together, their 1s-electrons pair
(denoted cT, as in the discussion of atomic structure in Section 2.3), and the atomic
orbitals merge together (FIG. 4.8). The resulting sausage-shaped distribution of
electrons, with an accumulation of electron density between the nuclei, is called a
“-bond” (a sigma bond). More formally, a -bond is cylindrically symmetrical
(the same in all directions around the long axis of the bond), with no nodal planes
containing the internuclear axis. A hydrogen molecule is held together by a -bond.
The merging of the two atomic orbitals is called the overlap of orbitals. A general
point to keep in mind throughout this section is that, the greater the extent of
orbital overlap, the stronger is the bond.
Much the same kind of -bond formation (“-bonding”) occurs in the hydrogen
halides. For example, before an H and F atom combine to form hydrogen fluoride,
the unpaired electron on the fluorine atom occupies a 2pz-orbital, and the
unpaired electron on the hydrogen atom occupies a 1s-orbital. These two electrons
are the ones that pair to form a bond (32). They pair as the orbitals that they
occupy overlap and merge into a cloud that spreads over both atoms (FIG. 4.9).
When viewed from the side, the resulting bond has a more complicated shape than
that of the -bond in H2; however, the bond looks much the same—it has cylindrical
symmetry and no nodal planes containing the internuclear axis—when viewed
along the internuclear (z) axis; hence it too is a -bond. All single covalent bonds
are -bonds