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The Metallic Model · Paper 2 — Free Chemistry SL Practice Questions

What this topic covers

Every property question IB throws at you — why does this melt higher, why does that conduct here but not there — traces back to one of three bonding models: ionic, covalent, or metallic. This topic asks you to build each model from its electrostatic basics, then use it to predict and explain real behaviour: melting point, conductivity in different states, shape, solubility, malleability. The exam almost never tests the model in isolation — it tests whether you can name the right structural detail (coordination number, electron domain count, number of delocalized electrons) and explain why that detail produces the observed property. Vocabulary without mechanism gets you nowhere on the mark scheme.

Read the full Structure: Models of Bonding and Structure lesson notes →

1ChallengeSAQ-LMetallic bonding and electron sea model7 marksPaper 2~11 min

Data

- Electrical resistivity of pure aluminium

= 2.65 \times 10^{-8}\,\Omega\,\text{m}

298\,\text{K}

- Electrical resistivity of Duralumin

= 3.45 \times 10^{-8}\,\Omega\,\text{m}

298\,\text{K}

- Melting point of pure aluminium

= 933\,\text{K}

- Melting point of Duralumin

\approx 775\,\text{K}

- Atomic radii:

\text{Al} = 143\,\text{pm}

;

\text{Cu} = 128\,\text{pm}

;

\text{Mg} = 160\,\text{pm}

;

\text{Mn} = 127\,\text{pm}

Note: Resistivity is not in the Data Booklet; use the values above as experimental data. Percentage change $= \dfrac{\text{new} - \text{original}}{\text{original}} \times 100\%$

The aerospace alloy Duralumin (approximately

94\%

aluminium,

4\%

copper,

1\%

magnesium,

1\%

manganese) is stronger than pure aluminium yet maintains good electrical conductivity.

(a)

Calculate the percentage increase in electrical resistivity when aluminium is alloyed to form Duralumin. [1 mark]

(b)

Using the electron sea model and the atomic radii data, explain why Duralumin has a higher electrical resistivity than pure aluminium. [2 marks]

(c)

Using the electron sea model and the atomic radii data, explain why Duralumin has a lower melting point than pure aluminium. [2 marks]

(d)

Evaluate the electron sea model as an explanation for the properties of Duralumin. Identify one specific limitation and suggest how a more sophisticated model would address it. [2 marks]

Solutions

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