Chemical bonding : The present matter of earth contains atoms bound in groups of two or more called molecules. Their binding capacity is called chemical reactivity; It depends upon the number of electrons in their outermost shell. When the number of shells  is more than one maximum seven the first shell nearest to the nucleus can have maximum two electrons the second one can have maximum 8,third one as many as 18 fourth 32,fifth 32,sixth 18and seventh only two. Whichever it may be the outermost shell can never have more than 8electrons.Thus the presence of 8 electrons in the outermost shell represents the stability of an atom. In chemical bonding, the reactive atoms tend to gain stability either by covalent bonds electrons of their outermost shells, or by transferring ionic or electrovalent bonds electrons from one to the other. For instance, a molecule of water is formed by covalent bonding of two hydrogen atoms with an atom of oxygen atom has two electron shells. That of hydrogen atom has a single electron instead of the maximum two it can have. Similarly, the outermost shell of oxygen has only 6 electrons instead of 8 it can have. Obviously, an oxygen atom binds with two hydrogen atoms to complete the electron the electron number of8 in its outermost shell, while each hydrogen atom completes the number of two electrons in its shell, all by mutual sharing.

In another example, an atom of sodium combines with an atom of chlorine to form common salt (NaCI) sodium atom contains 11 electrons in 3 shells (2,8,1,) Chlorine atom contains 17 electrons in 3 shells(2,8,7,)  To gain stability, chlorine atom takes up the lone electron of the outermost shell of sodium atom, while the latter easily loses this shell. Thus due to change in their electrons both atoms become electrically charged ions. Losing of an electrons by an atomic such electrovalent bonding is called its oxidation, and gaining of an electron by an atom is called its reduction.