Chemical Bonding (Ionic, Covalent and Metallic): Question 3

Worked solution

Part (a): Formation of the ions

Magnesium is in Group II, so a magnesium atom has 2 electrons in its outer shell. To reach a stable, noble-gas-like electron arrangement, each magnesium atom loses both of these electrons:

$\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^-$

Chlorine is in Group VII, so a chlorine atom has 7 electrons in its outer shell and needs just 1 more to become stable. Each chlorine atom gains 1 electron:

$\text{Cl} + e^- \rightarrow \text{Cl}^-$

Since one magnesium atom releases 2 electrons, but each chlorine atom can only accept 1, two chlorine atoms are needed for every magnesium atom. This matches the formula $\text{MgCl}_2$: one $\text{Mg}^{2+}$ ion for every two $\text{Cl}^-$ ions.

Part (b): Structure of solid magnesium chloride

Solid magnesium chloride is a giant ionic lattice: a regular, repeating three-dimensional arrangement of $\text{Mg}^{2+}$ ions and $\text{Cl}^-$ ions, alternating throughout the structure. The ions are held together by strong electrostatic forces of attraction between the oppositely charged ions, acting in all directions through the lattice.

Part (c): Explaining electrical conductivity

Solid: the $\text{Mg}^{2+}$ and $\text{Cl}^-$ ions are held rigidly in fixed positions within the lattice by strong electrostatic forces. Since the ions cannot move from their positions, they cannot carry a charge from place to place, so solid magnesium chloride does not conduct electricity.

Molten: heating melts the solid, breaking down the rigid lattice structure. The $\text{Mg}^{2+}$ and $\text{Cl}^-$ ions are now free to move throughout the liquid. Since these charged ions can move and carry charge, molten magnesium chloride does conduct electricity.

Final answers

• (a) $\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^-$; $2\text{Cl} + 2e^- \rightarrow 2\text{Cl}^-$; the 1 : 2 ratio of ions matches the formula $\text{MgCl}_2$.
• (b) A giant ionic lattice of alternating $\text{Mg}^{2+}$ and $\text{Cl}^-$ ions held together by strong electrostatic forces of attraction.
• (c) Solid: ions fixed in place, cannot move, no conduction. Molten: ions free to move, so the compound conducts electricity.