2020 O Level Chem P1 Q32 — Chemical Bonding / Covalent Bonds Solution

Answer: C (4 electrons and 2 electrons). In carbon dioxide the structure is O=C=O, so each carbon-to-oxygen link is a double bond. A double bond is two shared pairs of electrons, which is 4 electrons. So the bonding between carbon and one of the oxygens in CO₂ involves 4 electrons.

In ethanol, CH₃CH₂OH, the carbon-to-oxygen link is a single bond. A single bond is one shared pair, which is 2 electrons. So the C–O bond in ethanol involves 2 electrons.

The correct pairing is therefore 4 electrons for CO₂ and 2 electrons for ethanol, which is row C. It helps to remember that each shared pair always contains two electrons, so a double bond holds four electrons and a single bond holds two. Common error: counting the number of shared pairs (2 and 1) instead of the number of electrons (4 and 2), which gives the wrong row. The answer is C.