Metallic bonding — practice questions

1. Electrical Wiring: Copper is an excellent electrical conductor. This is because its metallic structure consists of a lattice of Cu$^{2+}$ ions surrounded by a sea of delocalised electrons. [1 mark] These electrons are mobile and free to move throughout the structure to carry charge when a potential difference is applied. [1 mark]
2. Plumbing (and being drawn into wires): Copper is ductile, meaning it can be drawn into wires, and malleable, meaning it can be shaped into pipes. [1 mark] This is because the layers of positive copper ions can slide over each other when a force is applied. The delocalised electrons move with the ions, so the strong metallic bonds are maintained and the structure does not break. [1 mark]

The strength of the metallic bond increases from Na to Mg to Al, which explains the general increase in melting points.

1. Number of Delocalised Electrons: Sodium contributes 1 outer electron per atom to the delocalised sea, Magnesium contributes 2, and Aluminium contributes 3. [1 mark]
2. Ionic Charge: The resulting cations in the lattice are Na$^{+}$, Mg$^{2+}$, and Al$^{3+}$. The charge on the cation increases across the period. [1 mark]
3. Ionic Radius: The ionic radius decreases across the period (Na$^{+}$ > Mg$^{2+}$ > Al$^{3+}$). This increases the charge density of the cation. [1 mark]

Conclusion: The combination of an increasing number of delocalised electrons and an increasing charge density of the cation leads to a much stronger electrostatic attraction between the cations and the electron sea. Therefore, more energy is required to overcome these forces, resulting in higher melting points from Na to Mg to Al. [1 mark]