Animated Solution for Chemistry - Ionic Equilibrium: Which one of the following substances has the highest proton affinity ?

When we talk about proton affinity, we are essentially discussing a molecule's "love" for a proton ($H^{+}$). In chemical terms, it is the tendency of a molecule to accept a proton by donating its lone pair of electrons. This concept is deeply intertwined with basicity—the more readily a molecule donates its lone pair, the stronger a base it is, and the higher its proton affinity.

In this problem, we are presented with four classic hydrides: water ($H_{2}O$), hydrogen sulfide ($H_{2}S$), ammonia ($N{H}_3$), and phosphine ($P{H}_3$). Our mission is to determine which of these is the most generous with its electrons.

The Size Factor

Let's first look at the central atoms of these molecules: Nitrogen ($N$), Oxygen ($O$), Phosphorus ($P$), and Sulfur ($S$).

Nitrogen and Oxygen reside in the second period of the periodic table, making them relatively small atoms. Phosphorus and Sulfur, however, are in the third period, meaning they are significantly larger.

Why does size matter? Imagine a lone pair of electrons as a cloud of negative charge. On a small atom like Nitrogen or Oxygen, this cloud is compact and dense—a highly concentrated target for a positively charged proton. But on larger atoms like Phosphorus and Sulfur, this electron cloud is spread out over a much larger volume. The electron density is diffuse. Because the charge is so spread out, it cannot effectively attract and bond with a tiny incoming proton.

Consequently, $P{H}_3$ and $H_{2}S$ are poor electron donors compared to their second-period counterparts. We can safely eliminate them from the race.

Electronegativity

Now, the battle for the highest proton affinity is between $N{H}_3$ and $H_{2}O$. Both central atoms are small, so their lone pairs are concentrated. How do we break the tie?

We must look at electronegativity. Oxygen is significantly more electronegative than Nitrogen. Electronegativity is essentially an atom's greed for electrons. Because Oxygen is highly electronegative, it holds onto its lone pairs with an iron grip. It is reluctant to share its precious electrons with an incoming proton.

Nitrogen, being less electronegative, is much more generous. It doesn't hold its lone pair as tightly as Oxygen does, making it far more willing to donate those electrons to form a bond with $H^{+}$, resulting in the ammonium ion ($N{H}_4^{+}$).

Because Nitrogen is small enough to have a concentrated electron density, yet less electronegative than Oxygen, it strikes the perfect balance for electron donation. Therefore, ammonia ($N{H}_3$) donates its lone pair most readily and boasts the highest proton affinity among the given choices.