Formation of Hydrides:
The elements of group – 15 form volatile hydrides having general formula MH3 e.g. NH3, PH3 AsH3 SbH3, BiH3 or ammonia, phosphine, arsine, stibine and bismuthine respectivily.
(A) Thermal stability:
On moving down the group, the thermal stability of hydrides change in the following order.
| Hydrides |
NH3 |
PH3 |
AsH3 |
SbH3 |
BiH3 |
| Temparature (k) |
1573 K |
673 K |
733 K |
423 K |
Very unstable |
In Contrast, P and H both have an electronegativity of 2.1. Therefore, P – H bond is not polar and hence PH3 does not exhibit H – bonding.
(B) Bond angles:
The hydrides of these elements are pyramidal in shape with a lone pair of electrons in one of the orbital. On moving down the group the bond angles gradually decreases due to decrease in bond pair – bond pair repulsion.
| Hydrides |
Bond angle |
| NH3 |
107° |
| PH3 |
93.5° |
| AsH3 |
91.8° |
| SbH3 |
91.3° |
| BiH3 |
90° |
(C) Reducing Character:
Due to decrease in thermal stability, the reducing character of these hydrides gradually increases in the order.
NH3 < PH3 < AsH3 < BiH3.
