The understanding of the nature of the chemical bond is a relatively recent development that dates back only to several decades. Before the understanding of the chemical bond could be achieved there was a need for the understanding of the nature of atomic structure, which was not completely understood at that time. By the development of quantum mechanics in general, and the Schroedinger equation in particular, the understanding of the atomic nature began to be more understood.
The complete solution of the Schroedinger equation of the hydrogen atom gave a complete description of its electronic energies and also gave a description of the radial and angular wave functions of this atom. This achievement paved the road to the understanding of the structure of more complicated atoms with more than one electron in its structure.
The solution of the Dirac equation for the hydrogen atom uncovered the existence of the electronic spin which is a very fundamental property to the description of the chemical bond. Also the principle of exclusion by Pauli led to the understanding that no more than two electrons with opposite spin could occupy the same orbital at the same time.
The functions of the hydrogen atom, as obtained from the solution of the Schroedinger equation for this atom, were used for a long period of time to describe the orbitals of many electrons atoms by giving the many electrons atom orbital functions that are hydrogenlike wave functions. The solution of the Schroedinger equation for the hydrogen atom gave, as one of its many results, quantum numbers that designate the energy levels of the electronic levels in the atom. This is in addition to describing the various orbitals shape, energy and symmetry by other quantum numbers that are also used for other atoms. These quantum numbers also gave the amount of degeneracy of the orbital levels in the hydrogen atom. For example p orbital in the hydrogen atom is triply degenerate while d orbitals are five degenerate orbitals.
All these results contributed to the attempts to understand how chemical bonds occur based on the quantum mechanical data that were obtained for the hydrogen atom. An orbital is a wave function that is obtained by the solution of the radial part of the Schroedinger equation of the hydrogen atom. This orbital describes the movement of the electrons that reside in it. This orbital can accomodate only two electrons with opposite spins of 1/2 and -1/2.
The attempts to explain the bonding nature in the hydrogen molecule were achieved relatively recently at the early years of the 19th century. The driving force for the formation of the hydrogen molecule and any chemical bond in general can be obtained using the theory of molecular orbitals, which states that chemical bonds between atoms are driven by the favorable formation of a molecular orbital that is lower in energy than both atomic orbitals of the two separate atoms.
Every chemical bond has two orbitals. One of them is bonding with positive overlap between the two overlapping orbitals. The other molecular orbital is anti-bonding with a nodal plane between the overlapping orbitals. The two electrons of the chemical bond occupy the bonding orbital which can explain the extra-stability of the formed molecule.
Today it is known that only certain number of electrons in an atom can participate in the bonding process of making chemical bonds. These electrons are located in the outer electronic shell of the atom. The inner electrons do not participate in the bonding process. These electrons that are located in the outer electronic shell of the atom are called the valence electrons and confer the reactivity characteristics of the atom. The octet rule of a closed valence shell was used qualitatively to rationalize why bonds form.
There are several types of chemical bonds. These are: the covalent bond and the ionic bond in addition to the dative bond and hydrogen bonding. The covalent bond can be fairly explained with the help of the molecular orbitals theory which states that the driving force for the formation of a chemical bond is the formation of a more stable molecule that has molecular orbital that is energetically lower than the atomic orbitals.
The ionic bond can be easily understood based on the octet rule in which the two atoms are happy with their closed shell electronic configuration. Ionic bonds are typical for atoms with completely different electronegativities. This is in contrast to covalent bonds in which the participating atoms in the bond have similar electronegativities. Ionic bonds are an example of an electrostatic interaction between a positive and a negative charge. An example of an ionic bond occurs in the molecule of calcium phosphide. An example of a covalent bond occurs in the hydrogen molecule.
Dative bonds are the third type of chemical bonds and are typical of transition metals complexes which occur between a transition metal and ligands that can be neutral with electron lone pair or can bear a negative charge. In this type of bonding the d-orbitals of the metal are involved with sigma donor ligands.
The last type of chemical bonds that is discussed here is hydrogen bonding. It occurs mainly between molecules that bear a hydrogen atom that is bonded to an electronegative atom and an electron donor atom such as oxygen. An example of hydrogen bonding occurs in a solution of pure water. Hydrogen bonding makes the liquid that is involved have a high boiling point. Liquids that do not have hydrogen bonding between their molecules are low boiling liquids such as ethers.
