2023 H2 Chem P1 Q4 — Chemical bonding Solution

The answer is D (1, 2 and 3).
Shared electrons are the electrons in the covalent bonds (circled in the diagram).
• Statement 1, correct. The ammonium ion, \(\text{NH}_4^+\), has four N-H single bonds (one of them dative), giving \(4 \times 2 = 8\) shared electrons.
• Statement 2, correct. Carbon dioxide, O=C=O, has two C=O double bonds, giving \(2 \times 4 = 8\) shared electrons.
• Statement 3, correct. Methanal, HCHO, has two C-H single bonds and one C=O double bond, giving \((2 \times 2) + 4 = 8\) shared electrons.
• A is wrong: it leaves out statement 3, but methanal also has 8 shared electrons.
• B is wrong: it leaves out statement 2, but carbon dioxide also has 8 shared electrons.
• C is wrong: it leaves out statement 1, but the ammonium ion also has 8 shared electrons (the dative bond counts as a shared pair).