ABSTRACT
The concept of valence and bonding are central to chemistry and helps to understand molecular structure and reactivity in a systematic way. The principles that govern the formation of molecules from atoms and intermolecular interactions are of paramount interest and have attracted much attention. Initially it was believed that certain types of chemical species were joined together by means of chemical affinity to form a chemical bond. A clear concept of a chemical bond emerged only after Lewis introduced the concept of electron pair bond in his landmark paper in 1916 [1] and his ideas continue to dominate a chemist’s perception toward chemical bonding and molecular structure. The theory of chemical bonding has received considerable attention ever since and several theories have been put forward to understand the nature of atoms and how atoms come together to form molecules, followed by the Lewis theory of paired electron bond [2-9]. Linus Pauling, in his highly influential book The Nature of the Chemical Bond, has orchestrated rules for the shared electronbond on the basis of electron paired bond on which the valence bond (VB) theory was built [2]. This chapter aims to introduce chemical bonding at the molecular and supramolecular levels starting from the Lewis concept. Early ideas are presented as a historical note and the development of chemical bonding models after the advent of quantum mechanics are described in a logical way. The importance and computation
of electron correlation are discussed and then various kinds of noncovalent interactions are outlined quoting a few standard examples. This will form a basis on how the early concepts, which are mostly wave function based, describe bonding and reactivity in greater detail. This would naturally be a prelude to understand reactivity from density functional theory (DFT) point of view.
