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.

Common traps

Saying metals conduct because they contain ions. They contain cations, not free ions in motion. Conductivity is carried by the delocalised electrons, not by the cations.

Calling metallic bonding "weak". It is comparable in strength to ionic bonding for many metals; transition metals exceed most ionic compounds in melting point.

Confusing "delocalised" with "loose". Delocalised does not mean weakly held; it means not localised between two specific atoms. The electron sea is held tightly to the lattice overall.

Missing the contrast with ionic brittleness. The malleability of metals and brittleness of ionic compounds share the same diagnostic test (apply a stress, observe the response) but for opposite reasons. State the non-directionality of the metallic bond explicitly.

Treating melting point as set by cation charge alone. Cation size and number of contributed electrons also matter. Quote at least two factors when comparing.

In one sentence

A metal is a 3D lattice of positive cation cores held together by the electrostatic attraction of a sea of delocalised valence electrons, and this non-directional bonding with mobile electrons accounts for electrical and thermal conductivity (free electrons), malleability and ductility (lattice can shift without breaking), lustre, and the wide variation in melting point (set by cation charge, size and number of electrons contributed).