Statement I : Acid strength increases in the order given as HF << HCl << HBr << HI.

Statement II : As the size of the elements F, Cl, Br, I increases down the group, the bond strength of HF, HCl, HBr and HI decreases and so the acid strength increases.

In the light of the above statements, choose the correct answer from the options given below.

**Understanding the trend in acid strength**

When we compare the hydrogen halides HF, HCl, HBr and HI, the experimentally observed order of acid strength in aqueous solution is:

In other words, hydroiodic acid (HI) is the strongest acid and hydrofluoric acid (HF) is the weakest.

**Why does the strength increase down the group?**

Acid strength depends mainly on how easily the H–X bond can break to release a proton (H⁺). Two key factors influence this:

1. **Bond dissociation energy (BDE)** – the energy required to break the H–X bond.
2. **Stability of the resulting anion (X⁻)** – larger anions can better delocalize the negative charge.

As we go from fluorine to iodine, the atomic radius of the halogen increases:

F < Cl < Br < I

Because of this increase in size:

• The overlap between the H 1s orbital and the halogen’s valence orbital becomes poorer, so the H–X bond becomes weaker (lower BDE).
• The larger halide ion can spread out its negative charge over a larger volume, stabilizing the anion.

Both effects make it easier for the bond to break and for the anion to remain stable, which enhances the acid’s ability to donate a proton.

**Statement I** says:

“Acid strength increases in the order given as HF << HCl << HBr << HI.”

This matches the observed trend, so Statement I is correct.

**Statement II** explains the reason:

“As the size of the elements F, Cl, Br, I increases down the group, the bond strength of HF, HCl, HBr and HI decreases and so the acid strength increases.”

The logic is exactly the one described above: larger atomic size → weaker H–X bond → stronger acid. Hence Statement II is also correct.

**Conclusion**

Both statements accurately describe the relationship between atomic size, bond strength, and acid strength for the hydrogen halides. Therefore the correct option is:

Both Statement I and Statement II are true.