HSC Exam Practice

Sample response. Metallic bonding is the electrostatic attraction between a regular lattice of positive metal cations and a sea of mobile, delocalised valence electrons. The two structural components are: (1) the cation lattice (metal ions that have released their valence electrons) and (2) the electron sea (delocalised valence electrons free to move throughout the entire structure). The bonding force is electrostatic — the attraction between opposite charges (positive cations and negative electrons).

Marking notes. 1 mark for defining metallic bonding as electrostatic attraction; 1 mark for correctly identifying the cation lattice as one component; 1 mark for correctly identifying the electron sea (delocalised electrons) as the other component. Accept “positive metal ions” for cation lattice; must say “delocalised” or “mobile” for electrons.

Sample response. Metals are malleable because metallic bonding is non-directional — the electrostatic attraction between the cation lattice and the electron sea operates equally in all directions. When a force causes a layer of metal ions to shift sideways, the electron sea redistributes to maintain the attraction around the new positions, and no bonds are broken. In diamond, every carbon atom is covalently bonded to four others in a rigid 3D tetrahedral network; these bonds have fixed, preferred directions. A shear force breaks these directional covalent bonds rather than allowing them to flex, so the crystal shatters instead of deforming.

Marking notes. 1 mark for correctly identifying metallic bonding as non-directional, allowing layers to slide; 1 mark for stating that the electron sea redistributes, maintaining cohesion; 1 mark for correctly stating that covalent bonds in diamond are directional, so force breaks bonds rather than allowing deformation.

Sample response. Metals conduct electricity because their delocalised electrons are free to move throughout the lattice. When a potential difference (voltage) is applied across the metal, electrons flow in a directed manner from the negative terminal toward the positive terminal; this directed movement of charge constitutes an electric current. Metals conduct heat because delocalised electrons at the hot end of the metal absorb kinetic energy and rapidly transport this energy through the lattice by collisions with other electrons and with cations; this electron-mediated energy transfer is rapid because the mobile electron sea can carry energy throughout the entire lattice. The mechanisms differ: electrical conduction is directed electron flow driven by a voltage gradient; thermal conduction is kinetic energy transfer by electrons moving randomly but carrying heat from hot to cold regions.

Marking notes. 1 mark for correctly explaining electrical conductivity (directed electron flow under applied voltage). 1 mark for correctly explaining thermal conductivity (kinetic energy transfer by mobile electrons). 1 mark for clearly distinguishing the two mechanisms (directed vs energy transfer). 1 mark for specifying the role of delocalised electrons in both processes.

Sample response. Sodium (Na, Group 1) releases 1 valence electron per atom, forming Na⁺ (charge +1) and contributing 1 electron per atom to the electron sea. Magnesium (Mg, Group 2) releases 2 valence electrons per atom, forming Mg²⁺ (charge +2) and contributing 2 electrons per atom to the electron sea. The electrostatic attraction between Mg²⁺ and the denser electron sea is much stronger than for Na⁺ (higher cation charge, more electrons). More energy is therefore needed to overcome the metallic bonding in Mg — hence Mg has a significantly higher melting point (650°C vs 98°C).

Marking notes. 1 mark for correctly stating that Na contributes 1 and Mg contributes 2 delocalised electrons per atom; 1 mark for correctly comparing cation charges (Na⁺ vs Mg²⁺); 1 mark for correctly concluding that stronger attraction in Mg produces stronger metallic bonding and higher melting point.

Sample response. Pure iron has a regular lattice of Fe cations of uniform size, allowing layers to slide past each other relatively easily under an applied force. When carbon is added, the smaller C atoms occupy interstitial spaces between the Fe ions, distorting the regular lattice at those sites. These distortions act as obstacles to layer sliding: when a shear force is applied, ion layers cannot move smoothly past the sites where C atoms sit, because the size mismatch blocks dislocation movement. Greater force is required to deform the steel (harder), and the ability to deform without fracturing is reduced (less malleable).

Marking notes. 1 mark for stating that pure iron has a regular lattice allowing easy layer sliding; 1 mark for stating that carbon atoms occupy interstitial sites and distort the lattice; 1 mark for explaining that distortions impede layer sliding, requiring greater force — hence harder and less malleable.

Sample response. The student is incorrect: purity does not equate to strength. 24-carat gold (pure gold) is actually the softest form of gold jewellery because it has a regular lattice of identical-sized Au ions, allowing layers to slide very easily — it deforms under light loads. 18-carat gold is an alloy containing ~25% other metals (e.g. copper and silver), whose atoms are different sizes from Au. These foreign atoms distort the regular Au lattice, preventing smooth layer sliding and making the 18-carat gold significantly harder and stronger than the pure metal. The property that is highest in 24-carat gold is malleability (and ductility — it can be worked easily). The property that is highest in 18-carat gold alloy is hardness and tensile/yield strength.

Marking notes. 1 mark for correctly identifying that 24-carat gold is softer/more malleable, not stronger; 1 mark for explaining why 18-carat gold is harder (alloy foreign atoms distort lattice, impede layer sliding); 1 mark for correctly identifying the property superior in each: 24-carat (malleability/ductility) and 18-carat (hardness/strength).

Sample response (a). The graph shows a positive, approximately linear trend: as the percentage of tin increases from 0% (pure Cu, ~40 HV) to 20% (bronze, ~160 HV), the Vickers hardness increases steadily, roughly quadrupling over the range. Using the electron sea and alloy structure models: pure copper has a regular lattice of identical-sized Cu²⁺ ions; layers slide easily, giving low hardness (~40 HV). As tin (Sn) atoms are added — Sn is larger than Cu — they displace Cu ions in the lattice and create increasing numbers of distortions. Each additional Sn atom creates a local size mismatch that obstructs dislocation movement. The more Sn added, the more distortions present, the harder it becomes to slide layers past each other, and the higher the hardness.

Sample response (b). At 10% Sn the hardness is ~105 HV; at 15% Sn it is ~133 HV. Linearly interpolating: at 12% Sn (which is 40% of the way from 10% to 15%), hardness ≈ 105 + 0.4 × (133 − 105) = 105 + 11.2 ≈ 116 HV. Accept any estimate in the range 110–120 HV with reasonable working. Assumption: the relationship between Sn% and hardness is approximately linear in the 10–15% range (which the graph supports).

Sample response (c). One trade-off is that increasing tin percentage beyond ~15% significantly reduces malleability and ductility. As more Sn atoms are added, the lattice becomes more distorted and brittle — the alloy becomes harder but also more prone to fracture rather than deforming under impact. For a sculptor, a very hard bronze (high Sn) would be difficult to work with tools (chisels, hammers), crack easily during finishing or chasing, and might not survive transport or installation without fracturing.

Marking notes. Part (a): 1 mark for correctly describing the trend (hardness increases with Sn%); 1 mark for explaining in terms of Cu lattice distortions caused by different-sized Sn atoms; 1 mark for linking more distortions to greater impedance of layer sliding → higher hardness. Part (b): 1 mark for correctly reading values at 10% and 15%; 1 mark for correctly interpolating an estimate in range 110–120 HV; 1 mark for identifying the assumption (linear interpolation valid in this range). Part (c): 1 mark for identifying a valid trade-off (reduced malleability/ductility OR increased brittleness); 1 mark for explaining it using alloy structure (greater lattice distortion → less deformation before fracture).

Sample response. The electron sea model — a lattice of positive metal cations immersed in a sea of delocalised valence electrons held together by electrostatic attraction — is a highly useful and elegant model that accounts for four key metallic properties with a single structural concept. First, electrical conductivity: the delocalised electrons are perpetually mobile throughout the lattice, so when a voltage is applied they flow as a directed electric current; no other bonding model predicts this conductivity from structure alone as clearly. Second, thermal conductivity: the same mobile electrons carry kinetic energy rapidly through the lattice. Third, malleability and ductility: because metallic bonding is non-directional (the electrostatic attraction operates in all directions), layers of cations can slide past each other under force without breaking bonding — the electron sea redistributes. This explains why copper, for example, can be drawn into fine wire or hammered into sheets. Fourth, melting point trends: the model correctly predicts that metals with more delocalised electrons per atom and higher-charge cations (e.g. transition metals like tungsten, W, MP 3422°C) have much stronger bonding and higher melting points than Group 1 metals like sodium (Na, MP 98°C) with only 1 delocalised electron and a +1 cation. However, the model has a significant limitation: it cannot fully account for differences in melting point between metals with similar electron counts but different structures (e.g. aluminium vs iron, both ~3 electrons per atom but very different MPs). The simple model treats the electron sea as uniform and does not fully account for the varying contribution of d electrons in transition metals (as shown by the Al vs Fe anomaly in the lesson table), nor does it account for differences in crystal structure packing that affect how closely cations approach the electron sea. The alloy structure concept extends the model usefully: when a foreign atom of different size is introduced into the lattice (e.g. adding carbon to iron to make steel, or tin to copper to make bronze), it creates lattice distortions that impede layer sliding, increasing hardness and strength while reducing malleability. The data on bronze confirm this: hardness rises from ~40 HV (pure Cu) to ~160 HV at 20% Sn as more lattice distortions accumulate. This extension explains why alloys are deliberately engineered — the disruption of lattice regularity is exploited to tailor mechanical properties for specific applications (stainless steel for corrosion resistance, duralumin for light aircraft frames). In summary, the electron sea model is highly effective for explaining the qualitative behaviour of metals and their trends, provides a single structural origin for multiple diverse properties, and is readily extended to alloys; its main limitation is quantitative accuracy, particularly for predicting exact melting point values and for transition metals where d-orbital contributions are significant.

Marking criteria (7 marks). 1 = correctly explains at least two properties of metals using the electron sea model (conductivity, malleability, lustre, MP trends — any two with correct mechanism). 1 = explains how non-directional bonding specifically accounts for malleability (layers slide, electron sea redistributes). 1 = explains melting point trend with reference to electron count and/or cation charge, with at least one specific named metal example. 1 = identifies a valid limitation of the simple electron sea model (e.g. cannot explain precise quantitative differences, ignores d-orbital electrons, ignores crystal structure). 1 = correctly explains how the alloy structure concept (foreign atoms, lattice distortions, impeded layer sliding) extends the electron sea model. 1 = names at least two specific metals or alloys correctly (any valid pair: Na, Mg, Fe, W, Cu, Al, bronze, steel, stainless steel, duralumin, etc.) with accurate statements about them. 1 = reaches an explicit evaluative judgement about the overall usefulness of the model, integrating strengths and the identified limitation.