Question

How does the reducing character of hydrides of group $15$ elements vary in the periodic table?

Answer 1

Reducing character of an atom or element is defined as the ability of that atom or element to reduce other atoms or elements that can be either addition or loss of hydrogen and gain or loss of electrons. We will see each hydride and observe their reducing character and arrange them accordingly.

Complete answer:
We are talking about a group of $15$ element which are Nitrogen, Phosphorus, Arsenic, Antimony, Bismuth. We will observe their hydrides and decide the trend of reducing character among them.
The reducing character depends upon some factors like
Strength of $E - H$ bond where E can be any of the group $15$ elements, more the strength of $E - H$ bond less will be the reducing character as it will require more energy to break.
Bond dissociation enthalpy, more the bond dissociation enthalpy of $E - H$ bond lesser will be the reducing character.
Stability of the hydrides, higher the stability of hydride lesser will be the reducing character.
Let’s write the hydrides of these elements
$N{H_3},\;P{H_3},\;As{H_3},\;Sb{H_3},\;Bi{H_3}$
We know the electronegativity decreases down the group due to increase in size so the most electronegative element is nitrogen. It will make a stronger bond than any other group $15$ element so the $E - H$ bond strength decreases down the group due to increase in size. So, bond dissociation enthalpy of $E - H$ bond also decreases from top to bottom of the group. So overall stability decreases down the group. From the above result we have the order of reducing character
$Bi{H_3}\; > \;Sb{H_3}\; > \;As{H_3}\; > \;P{H_3}\; > \;N{H_3}\;$
Hydride of Bismuth shows most reducing character and hydride of Nitrogen shows least reducing character.

Note:
If we talk about the structure of hydrides of these elements, they are pyramidal and have a lone pair in one of the orbitals. The bond angle of these hydrides decreases as we move down the group due to bond pair-bond pair repulsion.