To determine which of the given compounds is the most volatile, we need to analyze the volatility in relation to bond strength among the hydrogen halides (HF, HCl, HBr, HI). Volatility is inversely related to bond strength; the weaker the bond, the more volatile the compound.
### Step-by-Step Solution:
1. **Identify the Compounds**: The compounds we are considering are HF (hydrogen fluoride), HCl (hydrogen chloride), HBr (hydrogen bromide), and HI (hydrogen iodide).
2. **Understand the Trend in Group 17**: As we move down the group from fluorine to iodine, the size of the halogen atoms increases. This affects the bond lengths and bond strengths of the hydrogen halides.
3. **Analyze Bond Strength**:
- HF has the strongest bond due to the small size of fluorine, leading to a strong overlap of orbitals.
- HCl has a weaker bond than HF, but stronger than HBr.
- HBr has an even weaker bond than HCl.
- HI has the weakest bond among these compounds due to the larger size of iodine, which leads to less effective overlap of orbitals.
4. **Determine Volatility**: Since volatility is related to bond strength, the compound with the weakest bond will be the most volatile. Therefore, HI, having the weakest bond, will be the most volatile.
5. **Conclusion**: The most volatile compound among HF, HCl, HBr, and HI is HI (hydrogen iodide).
### Final Answer:
The most volatile compound is HI (hydrogen iodide).
