Q4. Calcium melts at 842 °C and potassium at 64 °C. Explain.
Calcium gives two electrons per atom to the electron sea and forms a smaller Ca2+ core; potassium gives only one and forms a larger K+ core. The metallic bonding in calcium is therefore much stronger.
Q5. Vanadium (3d3 4s2) melts at 1910 °C but zinc (3d10 4s2) at only 420 °C. Suggest why.
In vanadium the unpaired 3d electrons join the 4s electrons in the electron sea, so the metallic bonding is strong. Zinc’s 3d subshell is full and takes almost no part; only the two 4s electrons are delocalised, so the bonding is much weaker.
Q6. Why does an alloy such as brass usually conduct electricity less well than pure copper?
In an alloy the ion cores are of different sizes, so the lattice is irregular. The moving electrons are scattered more often by the irregularities, which lowers the conductivity.
Q7. Mercury is a liquid at room temperature (melting point −39 °C). What does this suggest about its metallic bonding?
Its low melting point is consistent with relatively weak cohesion, although melting also depends on entropy and structure. Mercury has a closed-shell 5d6s atomic configuration; relativistic effects strongly contract and stabilise 6s electrons and are essential to explaining its unusually low melting point. The electron-sea model alone is not a quantitative explanation.
Q8. Which of these are explained by the electron sea model: (a) lustre; (b) brittleness of metals; (c) conduction of electricity by solid metals; (d) malleability?
(a), (c) and (d). Metals are generally not brittle; brittleness is typical of ionic solids, where sliding layers bring like charges together.