Valency is governed by the number of electrons in the outermost electronic shell of the atoms of that element (i.e. the valency electrons). These valency electrons are given up to other atoms or are received from other atoms to make Ionic Bonds, or the valency electrons are shared with other atoms to make Covalent Bonds. The process of transferring electrons between atoms or of sharing electrons between atoms results in the formation of compounds, where the atoms in the compound achieve the stable electronic configuration of the Nobel Gas elements.
In order to achieve the electronic configuration of the nearest inert gas, which is a stable electronic configuration, atoms may lose, gain or share electrons. When these valency electrons are given up by an atom, or are received by an atom, to make ionic bonds are formed. When these valency electrons are shared with other atoms, covalent bonds are formed. Thus, the electron transfers result in the formation of bonds between atoms, and in the formation of compounds.
In general, the normal valency of an atom is thus equal the smaller of the following two numbers
In complex covalent compounds, an attraction occurs between the protons of one molecule and the electrons of another. As two molecules of the compound approach one another, the electrons in the outer shells of both molecules which have a negative charge begin to repel each other. When the molecules are a certain distance apart, the forces of attraction and repulsion are equal and opposite (i.e. the forces are balanced at that distance).
Graphite, which is a crystalline form of carbon, consists of flat planes of carbon atoms. The carbon atoms in each plane are held together by covalent bonds. However, there are no covalent bonds between the planes, and the only forces between the planes are the weak intermolecular interactions (i.e. the Van der Waal's Force). The planes can slip and slide over each other with ease, a fact which helps to explain why graphite is soft and a good lubricant.
The Van der Waal's Force give rise to intermolecular attractive forces between the molecules of gases, and are responsible for the deviation of the behaviour of real gases from the ideal gas laws.
The shapes of simple covalent molecules is determined primarily by the geometry of the bonds formed between the atoms in the molecule. In any molecule, the electrons occur in pairs in the orbitals of the atoms that make up the molecule. The pairs of electron which are in each bond are called a bonding pair. There are interactions between these electron pairs, so that an electron pair in one bond repels those of another bond. This interaction is called
bonding pair : bonding pair repulsion.
Some molecules have orbitals which are not involved in bonding and which contain an electron pair. This pair of electrons is called a lone pair. There are interactions between these lone pairs and other electron pairs, so that an lone pair repels the electron pairs in bonds. This is called
lone pair : bonding pair repulsion.
Similarly, a lone pair repels other lone pairs, which is called
lone pair : lone pair repulsion.
The strength of the repulsion between electron pairs depends on the proximity of each pair to the central atom. Lone pairs lie closer to the central atom than bond pairs, since the lone pairs have no other nearby positive nucleus to which they are attached to attract them away from the central atom. Bond pairs are attracted by a second nucleus and so they are drawn further away from the central atom. The repulsion between electron pairs is inversely proportional to the distance between them. Thus,