Describe metallic bonding as the electrostatic attraction between a lattice of metal cations and a sea of delocalised valence electrons, and explain the characteristic properties of metals (electrical and thermal conductivity, malleability, ductility, lustre, variable melting point) in terms of this model

What this dot point is asking

QCAA wants you to describe the structure of a metal at the atomic level (cation cores plus delocalised electron sea), explain the characteristic properties of metals in terms of that model, and account for differences in melting point and hardness between specific metals (Group 1 versus Group 2, or transition metals versus alkali metals) using charge density and electron count.

The answer

A metal consists of a regular three-dimensional lattice of positive metal ion cores embedded in a sea of delocalised valence electrons. The electrostatic attraction between the cations and the shared electrons is the metallic bond. Because the bonding is non-directional and the electrons are mobile, metals show a distinctive set of physical properties.

The cation-and-electron-sea model

Each metal atom donates its valence electrons to a shared pool. What remains is a cation core (the nucleus plus inner-shell electrons). The cores arrange in a regular close-packed lattice. The valence electrons are not localised to any one bond or atom; they roam throughout the lattice, free to move under an applied field.

Key points:

• The cations are held in fixed positions in the lattice.
• The electrons are delocalised: free to move in any direction.
• The bond is the electrostatic attraction between the cation lattice and the electron sea. It is non-directional, unlike a covalent bond.

Strength of the metallic bond

Two factors govern the strength of the attraction:

1. Charge on the cation. Higher charge (Mg^2+, Al^3+) gives stronger attraction to the electron sea than +1 (Na+, K+).
2. Number of valence electrons donated. More electrons per atom means a denser electron sea, increasing the attraction. Mg donates 2 electrons per atom; Na donates 1.

A third factor, cation size, plays a smaller role: smaller cations sit closer to the electron sea and bond more strongly. So Na (large +1 cation, 1 valence electron) is soft and low-melting; Mg (smaller +2 cation, 2 valence electrons) is harder and higher-melting; transition metals (variable cations with d-electrons contributing to the sea) tend to be the hardest and highest-melting.

Transition metals can also use partially filled d-orbitals to contribute to the bonding, which is why they tend to be the hardest and highest-melting metals.

Properties from the model

Electrical conductivity

The delocalised electrons move freely under an applied potential difference, carrying current through the metal. Conductivity is high in both the solid and molten states, because the electron sea persists. (This contrasts with ionic compounds, which conduct only when molten or dissolved.)

Thermal conductivity

Delocalised electrons also transport kinetic energy: a hot region of the lattice transfers energy to the electrons, which carry it rapidly to cooler regions. Metals therefore conduct heat as well as charge.

Malleability and ductility

Under stress, planes of cations can slide over one another. The non-directional electron sea immediately readjusts to maintain bonding between the cations in their new positions. So a metal can be hammered into thin sheets (malleable) or drawn into wires (ductile) without fracturing. This is the key contrast with ionic solids: a similar shift in NaCl puts like charges adjacent and the lattice shatters.

Lustre

The delocalised electrons absorb and re-emit photons across a broad range of visible wavelengths, giving a freshly cut metal surface its characteristic shiny appearance.

High density

Cation cores pack closely in characteristic structures (face-centred cubic, body-centred cubic, hexagonal close-packed), so most metals are dense compared with molecular solids.

Variable melting point

As shown in the table above, melting points vary by orders of magnitude depending on charge, electron count and ion size.

Insolubility in water (typically)

Metals do not dissolve in water in the same sense as ionic compounds (the electron sea is not solvable). Some metals react with water (Group 1 vigorously; Group 2 modestly), but this is reaction, not dissolution.

Alloys (brief)

An alloy is a mixture of two or more elements, at least one of which is a metal, in which the components are not chemically bonded but share the same lattice. Substitutional alloys (similar-sized atoms swap into the lattice; e.g. brass = Cu + Zn) and interstitial alloys (smaller atoms fill gaps; e.g. steel = Fe + C) usually have modified properties (harder, less ductile) than the pure metal. Mentioning alloys is not strictly required for this dot point but is examined in IA contexts where materials are compared.

Contrast with ionic and covalent bonding

The four-category comparison is a standard QCAA EA Paper 1 short response.

Examples in context

Example 1. Copper transmission lines from Mount Isa to Townsville. Powerlink Queensland uses high-conductivity copper wire produced at Glencore's Townsville refinery from Mount Isa concentrate. Copper's metallic lattice consists of $\text{Cu}^+$ cations in a sea of one delocalised valence electron per atom. The electrons are not bound to any single nucleus, so an applied potential difference drifts them through the lattice: copper's electrical conductivity is $5.96 \times 10^7 \, \text{S m}^{-1}$. The same delocalised electrons absorb and re-emit visible photons, giving lustrous appearance. Layers of $\text{Cu}^+$ also slide past each other under stress without breaking bonds, allowing the metal to be drawn into kilometre-long wires.

Example 2. Steel-reinforced concrete in Brisbane bridges. Brisbane's Story Bridge uses mild steel: an alloy of iron with about $0.2\%$ carbon. Pure iron's $\text{Fe}^{2+}$ cations sit in a regular lattice with two delocalised electrons each, but layer slip lets it deform under load. Adding small carbon atoms into interstitial sites distorts the lattice, pinning layer movement and roughly tripling the tensile strength. The alloy retains conductivity (still metallic bonding) but trades some ductility for hardness. Higher carbon ($0.6\%$+) gives tool steel, which is still harder but brittle: too much lattice strain. QCAA application questions often test these trade-offs.

Try this

Q1. Explain, using the metallic bonding model, why copper is ductile. [3 marks]

• Cue. Layers of $\text{Cu}^+$ slide past each other; delocalised electrons re-adapt; non-directional bonds maintained.

Q2. Melting points: $\text{Na}\,98^{\circ}\text{C}$, $\text{Mg}\,650^{\circ}\text{C}$, $\text{Al}\,660^{\circ}\text{C}$. Account for the increase across period 3. [3 marks]

• Cue. More delocalised electrons per atom (1, 2, 3) and higher cation charge increase electrostatic attraction to electron sea.

Q3. Stainless steel is iron alloyed with chromium and nickel. (a) Define an alloy. (b) Explain why stainless steel is harder than pure iron. (c) Suggest why stainless steel still conducts electricity. [2+3+2 marks]

• Cue. (a) Solid solution of two or more elements, one a metal. (b) Different-sized atoms disrupt slip planes. (c) Metallic bonding retained: delocalised electrons mobile. ISMG application + communication.