General Properties of Covalent Compounds

The main characteristic properties of covalent compounds are:

State of existence

The covalent compounds do not exist as ions but exist as molecules. There are weak intermolecular forces between the molecules and hence they exist as liquids or gases at room temperature. However, a few compounds also exist in the solid state e.g. urea, sugar etc.

Low melting and boiling points

The melting and boiling points of covalent compounds are generally low. This is because of the fact that the forces between the molecules are weak and thus are easily overcome at low temperatures.

Solubility

Covalent compounds are generally insoluble or less soluble in water and in other polar solvents. They are however, soluble in non-polar solvents such as benzene, carbon tetrachloride etc.

Non-conductors

Since covalent compounds do not give ions in solution, these are poor

conductors of electricity in the fused or dissolved state.

Molecular reactions

The reactions between covalent compounds occur between their molecules. These involve the breaking of covalent bonds in reacting molecules and forming new covalent bonds to give molecules of the products. These reactions are quite slow because energy is required to break covalent bonds.

Directional character of bond

Since the covalent bond is localised in between nuclei of atoms, it is directional in nature.

Single covalent bond

A covalent bond formed by mutual sharing of one pair of electrons is called a 'single covalent bond', or simply a single bond. A small line () between the two atoms is represents a single covalent bond.

Formation of single covalent bond is illustrated through the following examples.

Formation of hydrogen molecule

Hydrogen atom has only one electron in its outermost shell, and requires one more to acquire the nearest noble gas configuration of helium (He: 1s²).

To do so, two hydrogen atoms contribute one electron each to share

one pair of electrons between them. This leads to the formation of a single covalent bond between the two hydrogen atoms.

Formation of chlorine molecule

Chlorine atom has seven valence electrons. Thus, each Cl atom requires one more electron to acquire the nearest noble gas configuration (Ar:2, 8, 8). They do this by mutual sharing of one pair of electrons as shown below.

Formation of methane (CH₄)

 

Carbon atom has four electrons in its outermost shell. Thus, it requires four more electrons to acquire a stable noble gas configuration. Each hydrogen atom has only one electron in its outermost shell and requires one more electron to complete its outermost shell (to acquire He configuration). This is done as follows:

Problems

4. Represent the formation of phosphorus trichloride in terms of Lewis structure.

Solution

Phosphorus atom has five electrons in its outermost shell. It needs three more electrons to complete its shell. One electron is required to complete a chlorine atom, as its outer shell has seven electrons. Thus, one phosphorus atom can share one electron pair with each of the three chlorine atoms. This gives the following Lewis structure for PCl₃

5. Show how nitrogen forms covalent bonds with three hydrogen atoms in the formation of ammonia (NH₃). Are all electrons involved in the bonding? Give the Lewis structure of the covalent bond also.

Solution

The electronic configurations of nitrogen and hydrogen are

Thus, each nitrogen atom requires three more electrons to acquire a stable noble gas configuration. On the other hand, each H-atom requires only one electron to achieve the stable helium configuration. This is done by mutually sharing of three pairs of electrons between one nitrogen and three hydrogen atoms, as shown below:

The unshared pair of electrons on the nitrogen atom (in ammonia molecule) is not involved in bond formation and is called a lone pair of electrons.