IB Chemistry SL S2.3 The metallic model questions

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Question 1

Multiple choiceMedium1 mark

In the metallic model, sodium forms Na⁺ ions and magnesium forms Mg²⁺ ions; Mg²⁺ has a smaller radius than Na⁺. Which row predicts the metal with stronger metallic bonding and the metal with the higher melting point, respectively?

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Answer: C. Mg²⁺ has a greater charge and smaller radius than Na⁺, so it attracts delocalized electrons more strongly. Stronger metallic bonding requires more energy to overcome during melting, so Mg has the higher melting point.

A: treats the larger radius of Na⁺ as strengthening the attraction and carries that reversal through to melting point.
B: identifies Mg as more strongly bonded but reverses the relationship between bonding strength and melting point.
D: assigns the higher melting point to Mg but incorrectly assigns stronger metallic bonding to Na.

Remember: greater cation charge and smaller radius strengthen metallic bonding; stronger bonding generally corresponds to a higher melting point.

Question 2

Paper 2Hard12 marks

Three 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.

sodium chloridepotassium chloridemagnesium chloride[Na][Cl]−cation[K][Cl]−cation[Cl]−[Mg][Cl]−cation
MetalCation radius adopted in the model / pmMelting point / °C
K13863.4
Na10297.8
Mg72650
(a)

Use Figure 1 to deduce the charge of the metal ion in each of the three compounds.

[3]
(b)

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.

[4]
(c)

Compare magnesium with sodium to explain their difference in melting points. Refer to both cation charge and radius.

[2]
(d)

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.

[3]

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