Question 4
Nitrogen has a valency of 3. What is the formula of the compound it forms with hydrogen?
The right answer is the one marked above.
NH3 — ammonia. Hydrogen has a valency of 1, so three hydrogen atoms combine with one nitrogen atom.
The same reasoning gives the others: oxygen's valency of 2 makes H2O, and carbon's valency of 4 makes CH4. NH4+ exists, but as the ammonium ion — a polyatomic ion with a charge, not a neutral compound.