Question

Which element from following combines with hydrogen to form compound having lowest acidic strength?

• A. $\mathrm{Cl}$
• B. $\mathrm{Br}$
• C. $\mathrm{F}$
• D. $\mathrm{I}$

Hint:

Down a group, bond strength dominates over electronegativity when determining acidity. Because $\mathrm{F}$ is the smallest halogen, the $\mathrm{H-F}$ bond is the strongest and hardest to break, making $\mathrm{HF}$ the weakest acid in the series!

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The problem requires us to determine which of the given halogen elements ($\mathrm{Cl}$, $\mathrm{Br}$, $\mathrm{F}$, $\mathrm{I}$) reacts with hydrogen to form a binary hydride ($\mathrm{HX}$) with the weakest or lowest acidic strength.

Step 2: Key Formula or Approach:
For binary hydrides of the same group (Group 17), the acidic strength depends primarily on the hydrogen-halogen bond dissociation enthalpy ($\Delta_{\mathrm{diss}}H$) rather than electronegativity. As the size of the halogen atom increases down the group, the orbital overlap between hydrogen and the halogen weakens, making the $\mathrm{H-X}$ bond easier to break and thus increasing its acidity: $$\text{Acidic Strength: } \mathrm{HF < HCl < HBr < HI}$$

Step 3: Detailed Explanation:
Fluorine ($\mathrm{F}$) is the smallest element in Group 17 and possesses a very high electron density. The bond formed between the small $1s$ orbital of hydrogen and the small $2p$ orbital of fluorine is exceptionally strong because of excellent orbital overlap.
Because the $\mathrm{H-F}$ bond dissociation energy is remarkably high, hydrofluoric acid ($\mathrm{HF}$) dissociates to a much lower extent in aqueous solution compared to $\mathrm{HCl}$, $\mathrm{HBr}$, or $\mathrm{HI}$.
Consequently, $\mathrm{HF}$ behaves as a weak acid in water, whereas $\mathrm{HCl}$, $\mathrm{HBr}$, and $\mathrm{HI}$ completely dissociate as strong acids. Therefore, fluorine forms the hydride with the lowest acidic strength.

Step 4: Final Answer:
The element that forms the compound with the lowest acidic strength is fluorine, which corresponds to option (C).