Elements of group- 15, 16, and 17 form compounds with hydrogen with general formula \(MH_3\), \(H_2M\), and \(HX\) (called hydrogen halides). The stability of these hydrides, along with \(M-H\) bond strength and basic nature decreases down the group. Boiling point of these hydrides mainly depends on the strength of their intermolecular forces.

Which of the following is the strongest reducing agent?

\(BiH_3\)

The correct answer is option 4. \(BiH_3\).

The reducing ability of a substance is related to its tendency to donate electrons. Substances that readily donate electrons are strong reducing agents. In this context, let's compare \(NH_3\), \(PH_3\), \(AsH_3\), and \(BiH_3\) as reducing agents.

1. \(NH_3\) (Ammonia):

Ammonia is a weak reducing agent. While it can donate electrons, its reducing ability is limited compared to the other hydrides in the list.

2. \(PH_3\) (Phosphine):

Phosphine is a stronger reducing agent than \(NH_3\) due to the larger size of phosphorus compared to nitrogen. Larger atoms generally have more loosely held electrons, making them easier to donate.

3. \(AsH_3\) (Arsine):

Arsine is a stronger reducing agent than both \(NH_3\) and \(PH_3\). As we move down Group 15 of the periodic table, the size of the atoms increases, leading to easier electron donation.

4. \(BiH_3\) (Bismuthine):

Bismuthine (\(BiH_3\)) is expected to be the strongest reducing agent among the options. Bismuth is a heavy element with a larger atomic size, and its hydride is likely to exhibit strong reducing properties. The large size of bismuth makes it easier for the central atom to release electrons.

In summary, the trend in reducing ability follows the increase in size down Group 15, with \(BiH_3\) being the strongest reducing agent among the given options. The larger atomic size facilitates easier electron donation, making \(BiH_3\) more effective as a reducing agent.