Characteristic Feature of Covalent Compounds:
Nature of the Bond:
The bonds are obtained by mutual sharing of electron and may be intramolecular or intermolecular. The intermolecular forces in solid compounds having covalent bonds are weak (Van derwaals’forces) Therefore, the solid compounds are soft and can be easily broken.
Melting and Boiling Points:
The melting and boiling points are relatively low due to weak intermolecular forces. If intermolecular forces are larger, the crystals will be somewhat harder and have higher melting points e.g., Diamond, SiC etc.
Since covalent compounds do not contain ions they will not be able to conduct heat and electricity i.e., they are non-electrolytes.
They are soluble in non-polar solvents such as benzene, ether etc, but insoluble in polar solvents such as water unless some interaction occur with the solvent molecules. For example, the solubility of sugar (a covalently bonded organic compound) in water (a polar solvent) is attribute to the hydrogen bonding between the –OH groups in this organic molecule and water.
Reactions between covalent compounds are generally much slower than those of ionic compounds because they involve the breaking and reforming of bonds.
Conditions of Formation:
The covalent molecules are obtained from atoms of the elements having very similar ionization potential and electron affinity values. Under such conditions the possibility for the formation of ionic bond is ruled out.
Directional Character of the bond:
Whereas the ionic bonds are non-directional, the covalent bonds are directional in character. Hence individual covalent compounds possess definite shapes of their molecules. This factor may also be responsible for slow rate of reaction and may even determine the nature of the reaction in some cases.
The dielectric constant (∈, the ratio of the coulombs for a substance compared to that of a vacuum) values of covalent molecules are low (benzene = 2.3) The molecules of compounds having dielectric constant of unity must have a dipole moment zero.
More than one conceivable structure may be achieved while considering the possible distribution of electron between atoms within a molecules. Out of all the possible electronic structures obtained by rapid interchange (resonance), the one which actually exists (resonance hybrid) will possess the minimum energy of the system. The minimum energy is the criteria of maximum stability. Resonance is very common phenomenon occurring in organic compounds.
Sidgwick pointed out in 1933 that the maximum number of covalent bonds formed by an element would be related to its position in the Periodic Table. Thus carbon, a member of the IVth group will be able to form four covalent bonds. Similarly, phosphorus (a member of the Vth group) would be able to form at the most 5 covalent bonds and sulphur (a member of the VIth group) would from 6 covalent bonds. The maximum number of covalent bonds formed by an atom will depend upon the number of paired an unpaired electrons present as well as upon the number of vacant orbitals available in the atom of that element.