In 1897, J.J. Thomson discovered the electron. Within a decade, the "electron sea" model of metallic bonding emerged: a lattice of positive metal ions surrounded by freely flowing valence electrons. This model — over 125 years old — still explains every strange property of metals, from why copper wire bends without breaking to why mercury is liquid at room temperature.
Core Content
When metal atoms come together, their valence electrons are released from individual atoms and become shared across the entire structure — they are delocalised. The result is a regular lattice of positive metal cations (now missing their valence electrons) immersed in a mobile "sea" of delocalised electrons. The metallic bond is the electrostatic attraction between this cation lattice and the electron sea.
| Property | Explanation using the electron sea model |
|---|---|
| Electrical conductivity | Delocalised electrons are always mobile — applying a voltage causes them to flow as current. No lattice disruption needed. Conductivity in solid AND liquid state. |
| Thermal conductivity | Mobile electrons absorb kinetic energy at the heated end and rapidly transfer it throughout the lattice — much faster than vibration alone in ionic/covalent lattices. |
| Malleability / ductility | When force is applied, cation layers slide past each other. The electron sea redistributes around the new arrangement, maintaining bonding. No localised bonds break → no fracture. |
| Metallic lustre | Delocalised electrons on the surface absorb and re-emit light across all visible wavelengths → shiny appearance (characteristic metallic shine). |
| Variable melting point | Depends on number of delocalised electrons per atom and ion charge. More delocalised electrons + higher charge = stronger electron-cation attraction = higher MP. (e.g. W 3422°C vs Cs 29°C) |
Insert diagram: rows of close-packed metal cations (labelled M⁺, shown as circles) with a background of tiny dots representing delocalised electrons throughout. Label: metal cation (M⁺), delocalised electron sea, metallic bond (electrostatic attraction between cations and electron sea). Show layered arrangement to illustrate malleability — one layer slightly shifted with arrows showing direction of force.
Not all metals behave identically. The strength of metallic bonding — and therefore properties like MP and hardness — varies depending on:
| Metal | Group | Approx. MP (°C) | Delocalised e⁻ per atom | Trend explanation |
|---|---|---|---|---|
| Caesium (Cs) | 1 | 29 | 1 | Very low MP — weak metallic bonding (1 delocalised e⁻, large ion) |
| Sodium (Na) | 1 | 98 | 1 | Low MP — 1 delocalised e⁻, smaller than Cs |
| Magnesium (Mg) | 2 | 650 | 2 | Moderate MP — 2 delocalised e⁻ |
| Iron (Fe) | Transition | 1538 | 2–3+ | High MP — multiple d electrons contribute to bonding |
| Tungsten (W) | Transition | 3422 | 6+ | Highest MP of all metals — very strong metallic bonding |
A pure metal has a regular lattice of identical-sized cations. This regularity means layers slide easily — pure metals are often too soft or too corrosion-prone for engineering applications. An alloy introduces atoms of different sizes into the lattice, disrupting the regular arrangement and making it harder for layers to slide.
| Alloy | Base metal | Added elements | Key property improvement | Application |
|---|---|---|---|---|
| Steel | Iron (Fe) | Carbon (C, 0.2–2%) | Harder, stronger than pure Fe | Construction, tools |
| Stainless steel | Iron | Chromium (Cr, ~18%), Ni | Corrosion resistant, harder | Cutlery, surgical tools |
| Bronze | Copper (Cu) | Tin (Sn, ~10–12%) | Harder, stronger than pure Cu | Bearings, medals, instruments |
| Brass | Copper | Zinc (Zn, 20–45%) | Stronger, corrosion resistant, golden colour | Pipes, musical instruments |
| Duralumin | Aluminium (Al) | Cu (~4%), Mg, Mn | Much stronger than pure Al, low density | Aircraft bodies |
Insert side-by-side diagrams: (left) pure metal — rows of identical-sized circles, neat layers, arrow showing easy layer sliding; (right) alloy — rows disrupted by a few larger/smaller foreign atoms, arrow showing layer sliding blocked at disruption points. Label: pure metal lattice, foreign atom (different size), lattice distortion, blocked sliding plane.
Worked Examples
Activities
Predict which metal has the higher melting point. Explain using ionic charge and number of delocalised electrons.
Compare the hardness and malleability of pure iron versus steel. Explain using the alloy structure model.
An engineer needs a metal for high-temperature jet engine turbine blades. She is considering tungsten (W, MP 3422°C) or aluminium (Al, MP 660°C). Using the electron sea model, explain which is more suitable for this application and why W has such a dramatically higher MP than Al.
A jeweller argues that 24-carat gold (pure gold) makes the best jewellery because it is "the purest and strongest." A chemist disagrees. Evaluate the chemist's position using your knowledge of alloy structure. Would 18-carat gold (75% Au, 25% other metals) be stronger or weaker than pure gold? Explain.
Multiple Choice
Click to check. One attempt only.
1. The primary reason metals are malleable is that:
2. Which trend in metallic bonding strength is correctly described?
3. Why is bronze (Cu + Sn) harder than pure copper?
4. A metal has the following properties: excellent electrical conductivity, high melting point, hard but not brittle, malleable. Which structural model best explains ALL of these properties simultaneously?
5. An engineer wants to maximise both strength and corrosion resistance in a metal used for outdoor structures. Which material would be most appropriate?
Short Answer
6. Using the electron sea model, explain why metals are good conductors of both electricity and heat. Clearly distinguish the mechanisms for each type of conductivity. 3 MARKS
7. Explain why adding carbon atoms to iron produces steel that is harder and less malleable than pure iron. Refer specifically to the effect on the metallic lattice structure. 3 MARKS
8. Tungsten (W, Group 6 transition metal, MP 3422°C) has one of the highest melting points of all metals, while caesium (Cs, Group 1, MP 29°C) has one of the lowest. Using the electron sea model, explain this large difference in melting points in terms of the metallic bonding in each metal. 4 MARKS
A: Calcium has the higher MP. K (Group 1) contributes 1 valence electron and forms K⁺ (charge +1). Ca (Group 2) contributes 2 valence electrons and forms Ca²⁺ (charge +2). Ca²⁺ has a higher ionic charge and contributes more electrons to the electron sea — both factors increase the electrostatic attraction between cations and the electron sea → stronger metallic bonding → higher MP (Ca: 842°C vs K: 63°C).
B: Pure Fe has a regular lattice of identical Fe cations — layers slide relatively smoothly, making it malleable (though less so than Group 1 metals due to stronger bonding). When 0.5% carbon is added, the smaller C atoms occupy interstitial spaces in the Fe lattice, distorting the regular arrangement. These distortions act as obstacles to layer sliding — more force is required to deform the steel. Result: steel is harder and less malleable than pure iron, which is why raw iron is rarely used in structural applications.
Novel Context 1: Tungsten (W) is far more suitable for turbine blades — its MP of 3422°C means it remains solid at operating temperatures of jet engines (~1500°C). Aluminium (MP 660°C) would melt immediately. W has such a dramatically higher MP because it is a transition metal that contributes ~6 valence electrons per atom into the electron sea (compared to Al's 3). Additionally, W⁶⁺ carries a much higher charge than Al³⁺. The combination of many delocalised electrons and high cation charge produces extremely strong metallic bonding requiring enormous energy (very high temperature) to overcome.
Novel Context 2: The chemist is correct — pure 24-carat gold is actually the weakest and softest form of gold jewellery, not the strongest. 18-carat gold (an alloy with 25% other metals such as Ag, Cu, or Pd) is significantly harder and stronger than pure gold. The added atoms have different sizes from Au and disrupt the regular gold lattice, creating distortions that prevent smooth layer sliding — more force is required to scratch or deform the alloy. Pure gold is so soft that rings made from it will deform under normal wear. The trade-off is reduced purity, but the alloy is far more practical for everyday jewellery.
1. B — Non-directional bonding allows layer sliding while the electron sea maintains cohesion. A, C, D are all incorrect descriptions of metallic structure.
2. C — More delocalised electrons + higher charge → stronger bonding → higher MP. This is the correct general principle.
3. D — Different-sized Sn atoms distort the regular Cu lattice → impede layer sliding → harder. Not about electron numbers, ionic bonds, or covalent bonds.
4. A — Only the electron sea model simultaneously explains conductivity (mobile electrons), malleability (non-directional bonding, layer sliding), and high MP (strong cation–electron attraction).
5. B — Stainless steel combines alloying strength with Cr's passive oxide layer for corrosion resistance. Pure iron corrodes easily; pure Al is weak structurally; bronze is for bearings, not structural applications.
Q6 (3 marks): Metals conduct electricity because their delocalised valence electrons are free to move throughout the lattice at all times. When a voltage (potential difference) is applied, electrons flow from the negative terminal toward the positive terminal — this directed electron movement constitutes an electric current (1 mark). Metals conduct heat because mobile delocalised electrons can absorb kinetic energy at the hot end of the metal and rapidly transfer this energy through collisions with other electrons and cations throughout the lattice — much faster than vibration-mediated heat transfer in non-metallic solids (1 mark). The mechanisms differ: electrical conductivity is directed electron movement driven by a voltage gradient; thermal conductivity is kinetic energy transfer by electrons moving randomly but carrying energy from hot to cool regions — one is electrical, the other is thermal (1 mark).
Q7 (3 marks): Pure iron has a regular lattice of Fe cations of uniform size, allowing layers to slide past each other relatively easily under applied force — the electron sea redistributes and maintains bonding as layers shift (1 mark). Carbon atoms are much smaller than Fe atoms. When added, they occupy interstitial spaces in the Fe lattice, distorting the regular cubic arrangement at those sites (1 mark). These distortions act as obstacles — when a shear force is applied, layers cannot slide smoothly past the sites where C atoms sit, because the C atom's size difference blocks dislocation movement. Greater force is required to deform the steel → harder; reduced ability to slide → less malleable (1 mark).
Q8 (4 marks): Caesium is a Group 1 metal — each Cs atom contributes only 1 valence electron to the electron sea, and forms a Cs⁺ cation with charge +1 (1 mark). The attraction between Cs⁺ (low charge, very large ion) and the sparse electron sea (1 electron per atom) is very weak → very low lattice energy → low MP of 29°C (1 mark). Tungsten is a Group 6 transition metal — each W atom can contribute up to ~6 valence electrons to the electron sea, and the cation carries a much higher effective charge (1 mark). The electrostatic attraction between the highly charged W cation and the very dense electron sea (~6× more electrons per atom than Cs) is enormously strong → very high lattice energy → highest MP of any metal at 3422°C. The 3393°C difference in melting point reflects this ~6× difference in the number of delocalised electrons and the dramatic difference in cation charge (1 mark).
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