A group 15 element, which is a metal and forms a hydride with strongest reducing power among group 1

Q: Why does metallic character increase down the group?

As atomic size increases down the group, the outermost electrons are further from the nucleus and experience less effective nuclear charge. This makes it easier for the atom to lose electrons, which is the defining trait of a metal.

Q: Why is ammonia ($NH_3$) a poor reducing agent compared to $BiH_3$?

Nitrogen is a very small atom, so the N-H bond is short and strong. It requires a lot of energy to break this bond and release hydrogen. Bismuth is large, making the Bi-H bond long and weak, so it releases hydrogen easily.

Q: Does thermal stability follow the same trend as reducing power?

No, they are inversely related! Because the Bi-H bond is weak, $BiH_3$ is the least thermally stable hydride in the group. However, this exact weakness is what makes it the strongest reducing agent.

Q: Are Arsenic (As) and Antimony (Sb) considered metals?

No, Arsenic and Antimony are classified as metalloids because they exhibit properties intermediate between metals and non-metals. Bismuth (Bi) is the only true metal in Group 15.

Q: What is the oxidation state of the Group 15 element in these hydrides?

In hydrides like $NH_3$, $PH_3$, etc., the Group 15 element is formally considered to be in a -3 oxidation state, as hydrogen is generally less electronegative (though the electronegativity difference becomes very small down the group).

Question

A group 15 element, which is a metal and forms a hydride with strongest reducing power among group 15 hydrides. The element is

Options:

(A) Sb

(B) P

(C) As

(D) Bi

Sigma Percentile · Accepted Answer

### Unlocking the Secrets of Group 15 Hydrides

Imagine you are a detective trying to identify a mystery element. The question gives you two massive clues hidden in plain sight: the element must belong to Group 15 and be a **metal**, and its hydride must have the **strongest reducing power**. Let's break down these clues one by one to unmask our culprit.

### The Metallic Ladder of Group 15

First, let's recall the layout of Group 15 in the p-block of the periodic table. As we travel down the group from Nitrogen to Bismuth, the atomic size increases. Because the outermost electrons get further away from the pull of the nucleus, it becomes easier for the atoms to lose electrons. This ease of losing electrons is the very definition of metallic character.

At the top, Nitrogen (N) and Phosphorus (P) hold onto their electrons tightly; they are strictly **non-metals**. Moving down, Arsenic (As) and Antimony (Sb) sit on the fence as **metalloids**. Finally, at the very bottom, we find Bismuth (Bi), which is a true **metal**.

Right away, just by knowing the metallic trend, Bismuth stands out as the only candidate that fits the first half of our criteria!

### The Anatomy of a Reducing Agent

Now, let's verify the second clue: reducing power. What exactly makes a hydride a good reducing agent? A reducing agent's job is to reduce another substance, which often means donating hydrogen atoms to it. Therefore, the easier a hydride can give up its hydrogen, the stronger its reducing power.

This ability to donate hydrogen boils down to one critical factor: the strength of the metal-hydrogen ($M-H$) bond. If the bond is weak, hydrogen is released easily. If the bond is strong, the hydride holds onto its hydrogen stubbornly.

### The Size Factor and Bond Energy

How does the $M-H$ bond strength change as we go down Group 15? It's all about atomic size. Nitrogen is a tiny atom, so the $N-H$ bond in ammonia ($NH_3$) is short and incredibly strong. It requires a massive amount of energy (bond dissociation energy) to break it.

However, as we move down the group to Bismuth, the central atom becomes huge. The bond between the giant Bismuth atom and the tiny Hydrogen atom becomes very long. In chemistry, a longer bond is almost always a weaker bond. Consequently, the $Bi-H$ bond dissociation energy is the lowest in the entire group.

Because the $Bi-H$ bond is so weak, $BiH_3$ readily snaps and releases hydrogen gas, making it the **strongest reducing agent** among all Group 15 hydrides.

### The Final Verdict

Bismuth perfectly satisfies both conditions: it is the only metal in Group 15, and its hydride, $BiH_3$, has the weakest bond, granting it the strongest reducing power.

**Bonus Insight:** This exact same logic applies to thermal stability. Because $BiH_3$ has the weakest bonds, it is also the least thermally stable hydride in the group. Always remember that reducing power and thermal stability are inversely related in these hydride groups!

Animated Solution for Chemistry - s and p-Block Elements: A group 15 element, which is a metal and forms a hydride with strongest reducing power among group 15 hydrides. The element is

Select Answer:

Visualized Solution

\text{Analyzing the Given Conditions}

\text{Metallic Character Trend}

\text{Concept of Reducing Power}

\text{Bond Energy Trend}

\text{Final Conclusion}

\text{Bonus: Thermal Stability}

The Sigma Insight: Group 15 Elements

Unlocking the Secrets of Group 15 Hydrides

The Metallic Ladder of Group 15

The Anatomy of a Reducing Agent

The Size Factor and Bond Energy

The Final Verdict

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