Problem 159 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following substances has the highest proton affinity? [2003] (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{PH}_{3}\) (d) \(\mathrm{H}_{2} \mathrm{~S}\)

Step-by-Step Solution

Answer

Ammonia (n\mathrm{NH}_{3}n) has the highest proton affinity.

Step 1: Understanding Proton Affinity

Proton affinity measures how strongly a molecule attracts a proton. The higher the proton affinity, the stronger the attraction. Typically, proton affinity is related to the ability of the lone pair on the molecule to stabilize the added proton.

Step 2: Analyzing Nitrogen, Oxygen, Phosphorus, and Sulfur Compounds

The substances listed are \(\mathrm{NH}_{3}\), \(\mathrm{H}_{2} \mathrm{O}\), \(\mathrm{PH}_{3}\), and \(\mathrm{H}_{2} \mathrm{~S}\). Nitrogen and oxygen are in the same period, while phosphorus and sulfur are in the period directly below them. Generally, as we move down the group in the periodic table, the proton affinity decreases due to the larger atomic size and weaker hold on electrons.

Step 3: Comparing Electron Density

Among the options, \(\mathrm{NH}_{3}\) has a nitrogen atom with a higher electron density and more localized lone pair compared to the other atoms. This makes it more effective at stabilizing the added proton, thus having the highest proton affinity.

Key Concepts

Lone Pair Stability

Lone pairs are unshared pairs of electrons that reside on an atom's outermost shell. Their ability to stabilize added protons is a key factor in determining proton affinity. When a lone pair stabilizes a proton effectively, the resulting molecule becomes more stable. To assess lone pair stability, consider:

• The size of the atom: Smaller atoms, like nitrogen, can hold their electrons more tightly.
• Electron localization: More localized (or concentrated) lone pairs, such as those on nitrogen, can better stabilize additional protons.

Nitrogen, found in ammonia (\(\mathrm{NH}_{3}\)), has a localized lone pair on a relatively small atom, which contributes to its high proton affinity.

Periodic Table Trends

Understanding periodic table trends can help predict proton affinities. Generally, as you move down a group in the periodic table, proton affinity tends to decrease. This is due to the increase in atomic size and the corresponding decrease in the effective nuclear charge felt by the outer electrons. In the case of our molecules:

• Nitrogen and oxygen reside in the second period, while phosphorus and sulfur are in the third period.
• Moving down from nitrogen to phosphorus, and from oxygen to sulfur, the atomic size increases, causing the hold on electrons to weaken.

This explains why ammonia (\(\mathrm{NH}_{3}\)), with its nitrogen atom, has a higher proton affinity compared to phosphine (\(\mathrm{PH}_{3}\)) and also why water (\(\mathrm{H}_{2} \mathrm{O}\)) tends to have a higher proton affinity compared to hydrogen sulfide (\(\mathrm{H}_{2} \mathrm{S}\)).

Electron Density Comparison

Electron density refers to the concentration of electrons around an atom or molecule. High electron density around a lone pair can enhance a molecule's ability to attract and stabilize an incoming proton, thus increasing its proton affinity. Let's compare:

• The nitrogen atom in ammonia (\(\mathrm{NH}_{3}\)) has a higher electron density due to its smaller size and greater ability to localize electrons.
• In contrast, the phosphorus atom in phosphine (\(\mathrm{PH}_{3}\)) holds its electrons less tightly, resulting in a lower proton affinity.
• Similarly, oxygen in water (\(\mathrm{H}_{2} \mathrm{O}\)) maintains more electron density compared to sulfur in hydrogen sulfide (\(\mathrm{H}_{2} \mathrm{S}\)), thereby having a slightly higher proton affinity.

In summary, ammonia (\(\mathrm{NH}_{3}\)) has the highest electron density among the given substances, making it most effective at stabilizing an added proton.