In which of the following compounds does hydrogen form ionic compounds?

Analyze the question: We need the compound in which hydrogen is part of an ionic bond, meaning hydrogen takes an electron and becomes the hydride ion (H⁻).

Concept: Ionic bonds form between a metal, which loses electrons, and a non-metal, which gains them. Hydrogen is unusual because it can behave in two ways:

• With non-metals (C, Cl, N) it shares electrons, forming covalent compounds.
• With very reactive metals (Group 1 and 2, such as Na, K, Ca) it gains an electron to form H⁻, giving ionic (saline) hydrides.

Solution, option by option:

• A. CH₄: Carbon and hydrogen share electrons, so this is covalent.
• B. HCl: The bond is polar covalent. It only ionises when dissolved in water, so it is not an ionic compound in itself.
• C. NH₃: Nitrogen and hydrogen share electrons, so this is covalent (with a lone pair on N).
• D. NaH: Sodium is a reactive metal and gives up its electron to hydrogen: Na → Na⁺ + e⁻ and H + e⁻ → H⁻. This gives Na⁺H⁻, an ionic lattice (sodium hydride).

Answer: D. NaH.

Tip: If hydrogen is bonded to a metal, it is a hydride (H⁻) and the compound is ionic. If hydrogen is bonded to a non-metal, it is covalent. Remember that "metal + H = ionic hydride."