Question 1
Paper 2Hard12 marksThree metals are compared using the simplified metallic model. Figure 1 shows formula-unit representations of their chlorides. The chloride-ion charges are shown, but the metal-ion charges are omitted. Table 1 gives the cation radii adopted for the model and the melting points of the pure metals at standard pressure. The students use melting point as evidence of the relative strength of metallic bonding.
| Metal | Cation radius adopted in the model / pm | Melting point / °C |
|---|---|---|
| K | 138 | 63.4 |
| Na | 102 | 97.8 |
| Mg | 72 | 650 |
Use Figure 1 to deduce the charge of the metal ion in each of the three compounds.
A student wants to examine the effect of cation radius on melting point while keeping cation charge constant. Identify the appropriate pair of metals, calculate the difference in their cation radii and melting points, and use the results to explain which metal has stronger metallic bonding according to the model.
Compare magnesium with sodium to explain their difference in melting points. Refer to both cation charge and radius.
A student concludes that the rise in melting point from sodium to magnesium is caused only by the increase in cation charge. Assess whether this conclusion is justified, using evidence from Figure 1 and Table 1.