Hydrides

The elements of groups 15 form trihydrides of the general formula MH₃ such as

These hydrides can be obtained by different chemical reactions:

Structure

All these hydrides are covalent in nature and have pyramidal structure. These involve sp³ hybridization of the central atom and one of the tetrahedral position is occupied by a lone pair.

Shape of NH₃ molecule

Due to the presence of lone pair, the bond angle in NH₃ is less than the normal tetrahedral angle. It has been found to be 107^o. Down the group the bond angle decreases as:

Explanation:

In all these hydrides, four electron pairs, three bond pairs and one lone pair surround the central atom. Now, as we move down the group from N to Bi, the size of the atom goes on increasing and its electronegativity decreases. Consequently, the position of bond pair shifts more and more away from the central atom in moving from NH₃ to BiH₃. For e.g., the bond pair in NH₃ is close to N in N-H bond than the bond pair in P-H bond in PH_3. As a result, the force of repulsion between the bonded pair of electrons in NH₃ is more than in PH_3. In general, the force of repulsion between bonded pairs of electrons decreases as we move from NH₃ to BiH₃ and therefore, the bond angle also decreases in the same order.

Basic strength

All these hydrides have one lone pair of electrons on their central atom. Therefore, they act as Lewis bases. They can donate an electron pair to electron deficient species (Lewis acids). Down the group, the basic character of the hydrides decreases. For e.g., NH₃ is distinctly basic; PH₃ is weakly basic; AsH₃, SbH₃ and BiH₃ are very weakly basic.

Explanation:

Nitrogen atom has the smallest size among the hydrides. Therefore, the lone pair is concentrated on a small region and electron density on it is the maximum. Consequently, its electron releasing tendency is maximum. As the size of the central atom increases down the family, the electron density also decreases. As a result, the electron donor capacity or the basic strength decreases down the group.

Thermal stability

Thermal stability of the hydrides of group 15 elements decreases as we go down the group. Therefore, NH₃ is most stable and BiH₃ is least stable. The stability of the hydrides of group 15 elements decreases in the order:
NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃

Explanation:

This is due to the fact that on going down the group, the size of the central atom increases and therefore, its tendency to form stable covalent bond with small hydrogen atom decreases. As a result the M~H bond strength decreases and therefore thermal stability decreases.

Reducing character

The reducing character of the hydrides of group 15 elements increases from NH₃ to BiH_3. Thus, increasing order of reducing character is as follows: NH₃< PH₃< AsH₃ < SbH₃ < BiH₃

Explanation:

The reducing character depends upon the stability of the hydride. The greater the instability of an hydride, the greater is its reducing character. Since the stability of group 15 hydrides decreases from NH₃ to BiH₃, hence the reducing character increases.

Boiling points

Ammonia (240 K) has a higher boiling point than phosphine (190 K) and then the boiling point increases down the group because of increase in size.

Explanation:

The abnormally high boiling point of ammonia is due to its tendency to form hydrogen bonds.

In PH₃ and other hydrides, the intermolecular forces are Van der Waals' forces. These van der Waals' forces increase with increase in molecular size and therefore, boiling points increase on moving from PH₃ to BiH₃. The main trends are summarized below.