Bonding and structure

WCH11Topic 1.33 questions

Ionic, covalent and metallic bonding, shapes of molecules, electronegativity and giant structures.

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Question 14 marks
State the shape of a water molecule and its $\text{H}{-}\text{O}{-}\text{H}$ bond angle. Explain your answer using electron-pair repulsion theory.
Mark scheme
- Oxygen has two bonding pairs and two lone pairs of electrons (1)
- Electron pairs repel each other and move as far apart as possible to minimise repulsion (1)
- Lone pairs repel more strongly than bonding pairs, so the bond angle is reduced below $109.5^\circ$ (1)
- Bent (V-shaped), bond angle $104.5^\circ$ (accept $104$ – $105^\circ$ ) (1)
Question 23 marks
Explain why magnesium has a much higher melting temperature than sodium.
Mark scheme
- Magnesium ions are $\text{Mg}^{2+}$ and each atom contributes two delocalised electrons, compared with $\text{Na}^+$ and one electron (1)
- ${Mg}^{}$ ions are smaller, so have a greater charge density (1)

2

+

\text{Mg}^{2+}

Mg^{2 +}

So the electrostatic attraction between the metal ions and the delocalised electrons is stronger and more energy is needed to overcome it (1)