Question

Which of the following is correct about V group Hydrides (from ammonia to Bismuthine).
A) Their thermal stability gradually increases.
B) Their ease of preparation gradually increases.
C) The electron pair donating Nature gradually decreases.
D) The bond energies gradually increase.

Answer 1

Hint: Consider various factors that affect the bond energy, bond strength, thermal dissociation and basicity. Consider how these factors vary when you move from top to bottom in group V.

Complete answer:
The group V elements include nitrogen, phosphorus, arsenic, antimony and bismuth. The group V elements have 5 valence electrons.
The general formula of group V hydrides is \[{\text{:E}}{{\text{H}}_3}\] . Here \[{\text{E}}\] represents a group V element. It has a lone pair of electrons and three bond pairs of electrons. Three bond pairs of electrons are formed when the element \[{\text{E}}\] shares 3 electrons with 3 electrons of 3 hydrogen atoms. The lone pair of electrons can be donated to suitable lone pair acceptors (Lewis acid).
The thermal stability of hydrides of group V elements gradually decreases on moving from top to bottom. This is because the size of the central atom \[{\text{E}}\] increases and the strength of \[{\text{E}} - {\text{H}}\] bond decreases.
Their ease of preparation gradually decreases as their thermal stability decreases.
The bond energies of hydrides of group V elements gradually decrease on moving from top to bottom. This is because the size of the central atom \[{\text{E}}\] increases and the strength of \[{\text{E}} - {\text{H}}\] bond decreases.
The following is correct about V group Hydrides (from ammonia to Bismuthine)
The electron pair donating Nature gradually decreases. On moving down the group V, the atomic size of group V elements increases. Due to this, the lone pair of electrons occupies more space and its electron density decreases. In other words, the electron pair is dispersed over a large volume and cannot be easily donated.

Hence, the correct option is the option (C).

Note: We can also call group V hydrides as Pnictogen hydrides as group V elements are also called Pnictogen. In these hydrides, the bonds are covalent in nature. Their valence shell has three electrons which are available for bonding.