Can we imagine a world without oxygen, nitrogen, water or carbon dioxide? Without any of these there would be no life at all. All the above mentioned compounds are covalent compounds formed by sharing of electrons. Non-metal atoms share the valence electrons and form molecules. These shared pairs of electrons contributed by each of the atoms involved in bonding are located between the nuclei of atoms.
Covalent bonds arise from the sharing of electrons between two atoms. Simple examples are found in water and carbon dioxide. Each single covalent bond by definition consists of two shared electrons. Double covalent bonds possess four shared electrons and triple covalent bonds, six shared electrons.
The greater the multiplicity of the covalent bond, the greater the energy of the bond and the closer together of the two bonded atomic nuclei.
Covalent bonding may take place between atoms of the same element as in a hydrogen molecule or a chlorine molecule.
It may also take place between atoms of different elements such as:
(i) Hydrogen and chlorine combine to form hydrogen chloride (HCl).
(ii) Carbon and oxygen combine to form carbon dioxide (CO2).
Covalent bonds are formed between non-metal atoms. Each of the atoms involved in bonding contribute one, two, three or more electrons to form the shared pair.
- electron present in K shell of the hydrogen atom
Here the shared pair consists of two electrons, one electron being contributed by each of the hydrogen atoms. This is called a shared pair.
Depending on the number of electron pairs shared between atoms which participate in bonding, covalent bonds are classified as follows:
There are many examples of compounds having covalent bonds, including the gases in our atmosphere, common fuels and most of the compounds in our body. The molecules and ions just mentioned are composed entirely of nonmetals atoms. A point that needs special emphasis is that in molecules or ions made up only of nonmetals atoms, the also are attached by covalent bonds.
Methane molecule (CH4)
The electronic configuration of carbon is 2,4. It needs 4 more electrons in its outer shell to be like the noble gas neon. To do this one carbon atom shares four electrons with the single electrons from four hydrogen atoms. The methane molecule has four C-H single bonds.
Water molecule (H2O)
One oxygen atom joins with two hydrogen atoms. The water molecule has two O-H single bonds.
Carbon dioxide (CO2)
One carbon atom joins with two oxygen atoms. The carbon dioxide molecule has two C=O bonds.
The properties of covalent compounds are listed below.
- Covalent compounds are mostly gases and liquids.
- Covalent crystals 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. This is 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. Which are quite weaker.
- 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, carbon in the diamond structure is 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.