Molecules result by the union of atoms. Before a comprehensive study of the structure of organic molecules could be made, it is necessary to know what an atom is; why and how combines with atoms to form molecules.
Early Concept
According to Bohr's atomic model (1913) an atom consists of a central positively charged nucleus containing positively charged particles called proteins and neutral particles called electrons which carry unit mass. The nucleus is surrounded by negatively charged particles called electrons which carry unit negative charge and negligible weigh.
Since an atom is electrically neutral, the number of protons in the nucleus equals the number of electrons around the nucleus.
The surrounding electrons often called the extra nuclear electrons, are said to be moving about the nucleus in closed concentric orbits or energy levels. While an electron moves in such a level it possess a definite amount of energy and it neither emits nor absorbs energy as long as it remains in that level or orbits.
The electrons are arranged in these orbits so that the maximum number of electrons in the various orbits starting from the first one is 2,8,18,32,18,8.
The outermost orbit cannot have more than eight electrons and the penultimate orbit cannot have more than eighteen elections.
The outermost orbit of electrons in different atoms (except those of inert gases) is incomplete and the electrons in it are known as valency electrons.
Electronic Theory of Bonding. Kossel and Lewis (1916) gave their Electronic theory of binding to explain the formation of bonds or unions between atoms. They visualised that the atoms of inert gases have either two (helium) or eight electrons in the outermost orbit. These gases do not enter into chemical combination and, therefore, they are assumed to have complete them by chemical combination with other atoms. This inert gas electronic grouping of two or eight elections can be achieved by the combining atoms by losin, gaining or sharing electrons among themselves.
Hence it must be emphasized here that it this tendency of atoms to acquire inert gas electronic configuration which causes the atoms to unite to form the chemical bonds.
The different modes of bond formation arise as a result of different manners in which the electrons distribution occurs between the combination atoms to attain stable inert gas electronic configuration.
