Formation of Substances

Covalent Bond: They are formed by sharing of electrons and have low melting point and boiling points. These are soluble in organic solvents and are non-conductor of electricity. However graphite and diamond are covalent compounds but they have very high melting point because of their giant structure. The properties of covalent bonds are given below:

• They are mostly gases and liquids.
• They tend to be hard and brittle, and incapable of appreciable bending. These facts are understandable in terms of the underlying atomic forces. Since the bonds have well defined directions in space attempts to alter them are strongly restricted by the crystals.
• The melting and boiling points are usually low as compared to those of ionic crystals because the covalent bond is not so strong as ionic bond and also because the atoms are less powerfully attracted towards each other, the force that attracts them towards each other is called van der waals forces, dipoles etc.
• Most of the covalent substances do not conduct electricity because of the non-availability of free electrons or charged ions to carry the current. However certain substances like HCl which exhibit polarity in aqueous solutions behave like ionic substances and allow the passage of electricity through them accompanied by their own decomposition thereby acting as electrolytes.
• Covalent substances are insoluble in polar solvents like water. However they are soluble in non-polar solvents like benzene, carbon disulphide etc. This is because of the covalent nature of the solvent. However the giant molecules are not soluble in any solvent because of the large size of the molecules.
• A very interesting property of covalent crystals is the apparent lack of sensitivity of their physical properties of their bonding type. For example, carbons in the diamond structure are the hardest substance and have a very high melting point 3280K. The hardness and melting point decreases as we proceed to other elements. For example, Tin is the very soft element and has very low melting point. Depending on the number of electrons shared, the bond length and bond energy vary. When the number of electrons shared is more, the bond length between the atoms is decreased and bond energy is increased. Diamond, silicon, germanium, silicon carbide, tin and rutile are some examples of covalent crystals.