In which of the following will hydrogen form ionic compound?

Analysis

The question asks us to identify which of the given compounds contains hydrogen in an ionic bond. To solve this, we must look at the electronegativity difference between hydrogen and the element it is bonded to.

Concept Explanation

• Ionic Compounds: These are formed when there is a large difference in electronegativity between two atoms, usually a metal and a non-metal. The metal loses an electron to become a cation, and the non-metal gains that electron to become an anion.
• Hydrogen's Behavior: Hydrogen is unique. It can form covalent bonds with non-metals (by sharing electrons) or ionic bonds with highly electropositive metals (Group 1 and Group 2 elements). When hydrogen bonds with a metal, it gains an electron to form a hydride ion ($H^-$), resulting in an ionic compound.

Step-by-Step Solution

1. Analyze Option A (HCl): Hydrogen and Chlorine are both non-metals. They share electrons to form a covalent bond.
2. Analyze Option B (NaH): Sodium (Na) is a Group 1 alkali metal (highly electropositive). Hydrogen is a non-metal. When they react, Sodium transfers its valence electron to Hydrogen. This creates $Na^+$ and $H^-$ ions held together by electrostatic forces. This is an ionic hydride.
3. Analyze Option C (NH$_3$): Nitrogen and Hydrogen are both non-metals. They share electrons to form polar covalent bonds.
4. Analyze Option D (CH$_4$): Carbon and Hydrogen are both non-metals. They share electrons to form non-polar covalent bonds.

Conclusion

Since Sodium is a metal and Hydrogen acts as the anion in this structure, Sodium Hydride (NaH) is the only ionic compound listed.

Exam Tip: Whenever you see a hydride of a Group 1 or Group 2 metal (like LiH, NaH, KH, or $CaH_2$), always remember they are ionic. If hydrogen is bonded to a non-metal (like C, N, O, or Cl), it is almost certainly a covalent compound.