2024 O Level Chem P1 Q9 — Ionic Bonding / Dot-and-Cross Diagrams Solution

Answer: C. Magnesium has electronic configuration 2,8,2 — it loses both outer electrons to form \(\text{Mg}^{2+}\) (2,8). Oxygen has configuration 2,6 — it gains both electrons to form \(\text{O}^{2-}\) (2,8). One Mg gives both electrons to one O; MgO has a 1:1 ratio.

Diagram C shows exactly this: Mg with 2 outer dots transferring both electrons (double arrow) to a single O atom which then has 8 outer crosses.

A shows Mg transferring electrons to TWO separate O atoms — wrong stoichiometry. B shows finished products only, with no transfer arrows. D has the arrow reversed, implying electrons flow from O to Mg, which is the wrong direction.