Solid materials are made up of large collection of atoms where the individual atoms are bonded together. The type of bonding which exists between atoms is different for all material.
An atom with a completely filled outer electron shell has a high degree of stability. The atoms of other elements with incomplete outer shells attempts, by combination with other atoms, to satisfy this condition for stability. The principal types of inter atomic bonds which may be formed are the ionic, covalent, metallic, Van Der Wall’s or molecular bonds.
The solids are also classified as per the bond structure found in them. They are Ionic, Metallic, Covalent and Molecular.
Covalent Bond. A covalent bond is formed when the electron or electrons of an atom in its outermost orbit are shared by the electrons of another atom in its outermost orbit in order to fulfil the octet configuration (8 electrons)
Where a compound only contains non-metal atoms, a covalent bond is formed by atoms sharing two or more electrons. Non-metals have 4 or more electrons in their outer shells (except boron). With this many electrons in the outer shell, it would require more energy to remove the electrons than would be gained by making new bonds. Therefore, both the atoms involved share a pair of electrons. Each atom gives one of its outer electrons to the electron pair. Consequently, both atoms are held near each other since both atoms have a share in the electrons.
More than one electron pair can be formed with half of the electrons coming from one atom and the rest from the other atom. An important feature of this bond is that the electrons are tightly held and equally shared by the participating atoms. The atoms can be of the same element or different elements. In each molecule, the bonds between the atoms are strong but the bonds between molecules are usually weak. This makes many solid materials with covalent bonds brittle. Many ceramic materials have covalent bonds.
Covalent Solids. It consists of atoms that share electrons with neighbouring atoms so that atoms are linked with one another by a continuous system of covalent bond. Diamonds, SiC, SiO2 are some example of covalent solids.
Properties of Covalent Solid. The covalent solids are electrically neutral so they are bad conductors of electricity (except graphite). Because of the strongly bonded rigid structures, most covalent solids are very hard and melt at high temperature.