Atomic Orbital Approach To Bonding
This second approach is also called the Atomic Orbital Approach to Bonding. The basic premise of this theory is that bonds are formed when atoms get close enough so that atomic orbitals on the individual atoms will be able to overlap so that the three dimensional probability regions share a common volume. This effectively increases the probability of finding bonding electrons between the two atoms. It also effectively results in the lowering of the energy state of the molecular system making the molecule more stable as a result of the overlap. The greater the overlap, the greater is the strength of the bond.
Pure Atomic Orbital Overlap
The simplest of these bonds involve the overlap of two "s" orbitals as in the example when two Hydrogen atoms get close enough to bond. The "s" orbitals overlap to form a "sigma" bond between the two "s" orbitals.
A second type of overlap is between two "p" orbitals to form a sigma bond between two "p" orbitals. An example is the p-p overlap between two Chlorine atoms.
A third type of sigma overlap is the overlap between an "s" orbital and a "p" orbital such as when a Hydrogen atom's "s" orbital overlaps with a "p" orbital of another atom like a Chlorine atom.