Metallic Bonding

SPEC 4.2.1.5 METALLIC

Positive metal ions in a sea of delocalised electrons. The model that explains conductivity, malleability, and why alloys are harder than pure metals — plus the four AQA word-traps that decide whether you score the marks.

What Is a Metallic Bond?

Metallic bonding is the third type of strong chemical bond. It forms in metals and alloys — never in non-metals. The bonding is fundamentally different from ionic and covalent: nothing is transferred to a single ion; nothing is shared between just two atoms.

Metallic bond

The (electrostatic) attraction between positive metal ions and a sea of delocalised electrons shared across the whole structure.

The principle

Each metal atom loses its outer-shell electron(s). Those released electrons become delocalised — they aren't tied to any specific atom anymore but are free to move through the whole structure. What's left behind is a regular lattice of positive metal ions. The strong electrostatic attraction between those positive ions and the negative electron sea is the metallic bond.

Three verbs — the third one

Ionic = electrons transferred. Covalent = electrons shared. Metallic = electrons delocalised. If a question asks about bonding in a metal, the verb the mark scheme wants is delocalised (or "free"). Mark schemes accept "free electrons" as an alternative for delocalised (P1 Jun22 MS).

The Metallic Bonding Model

The conventional diagram: regular layers of positive ions surrounded by a sea of delocalised electrons.

Reading the model

The "+" symbols are the metal ions (with their outer electrons stripped away). The "e−" symbols are the delocalised electrons floating between the ions. The arrangement is regular — ions sit in repeating layers, like stacked oranges. The whole structure is held together by the electrostatic attraction between every positive ion and the surrounding electron sea.

Four Properties Metallic Bonding Explains

The same model explains all the properties of metals you need to know. Each one comes back to either the delocalised electrons or the regular layers of ions.

1. Electrical conductivity

Delocalised electrons move through the structure when a potential difference is applied, carrying charge.

2. Thermal conductivity

Delocalised electrons gain kinetic energy at the hot end and transfer thermal energy as they move through the metal.

3. Malleability

Layers of positive ions slide over each other; the electron sea moves with them so the bond isn't broken.

4. High melting point

Strong electrostatic attraction between ions and electrons requires a lot of energy to overcome.

The pattern

Every property answer mentions delocalised electrons (or "free electrons"). When you write the answer, name the right mechanism: electrons carry charge (electrical), transfer thermal energy (thermal), flow with sliding layers (malleability), or are strongly attracted to ions (high mp).

01Worked Example: Electrical Conductivity 3 MARKS

Aluminium is a metal. Describe how metals conduct electricity. Answer in terms of electrons.

(Real exam question: AQA 8462/1H Jun 2022, Q01.5)

M1

Metals contain delocalised electrons (mark scheme also accepts "free electrons").

M2

The electrons move through the structure (the metal / aluminium).

M3

The electrons carry (electrical) charge.

Mark scheme exact wording (Jun22 MS): "delocalised electrons" + "(the electrons) carry (electrical) charge" + "(the electrons move) through the metal / aluminium / structure". Note the explicit instruction: "ignore throughout for through" — the word "throughout" doesn't earn the mark on its own, only "through" implies directional flow under a potential difference. "Ignore current/electricity for charge" — you must say charge.

Worked Example: Thermal Conductivity 3 MARKS02

Explain why copper is a good conductor of thermal energy.

M1

Copper contains delocalised electrons that can move freely through the metal.

M2

When one end is heated, the delocalised electrons gain kinetic energy.

M3

The electrons transfer thermal energy through the metal as they move.

The trap (P1 Jun24 Q07.1): the examiner reported that most students described electrical conductivity when asked about thermal. Both use delocalised electrons, but the mechanism is different — thermal involves kinetic energy transfer, not charge. Read the question word ("thermal" vs "electrical") and match your answer to it.

Malleability — Why Metals Bend

Metals are malleable: they can be hammered or shaped into different forms without shattering. This comes directly from the regular layered structure.

Mark scheme answer (2 marks)

"Layers of (positive) ions can slide over each other; the delocalised electrons (electron sea) move with them so the metallic bond isn't broken."

Don't say "bonds break"

Mark schemes do not accept "bonds break" for malleability. The bonds don't break — the layers just slide while the electron sea flows with them, maintaining the metallic bonding throughout.

Why Metals Have High Melting Points

Most metals are solid at room temperature with relatively high melting points. Iron melts at 1538 °C, copper at 1085 °C, aluminium at 660 °C. The reason is the strength of metallic bonding.

The mechanism

The metallic bond is a strong electrostatic attraction between every positive metal ion and the surrounding sea of delocalised electrons. To melt the metal, you must give the ions enough kinetic energy to break free from their fixed positions in the lattice. That requires a lot of energy — hence the high melting point.

Mark scheme structure (3 marks)

1 mark = identify strong electrostatic attraction between positive ions and delocalised electrons. 1 mark = state that a lot of energy is required to overcome it. 1 mark = link to high melting point. The same logic explains why metals are solid at room temperature — thermal energy at 25 °C isn't enough to overcome the metallic bonding.

Mercury (Hg) is the exception

Mercury is the only metal that's liquid at room temperature (mp −39 °C). It still has metallic bonding, but the attraction is weaker than in most metals. You won't be asked to explain mercury's melting point in detail at GCSE, but be aware it's the standard "exception" answer if asked which metal is liquid at 25 °C.

Alloys — Mixtures with Modified Properties

Pure metals are often too soft for practical use. To make them harder and stronger, we mix them with other elements to make alloys.

Alloy

A mixture of a metal with one or more other elements (usually metals).

AlloyCompositionUse

SteelIron + carbon (and sometimes other metals)Construction, tools, vehicles

BrassCopper + zincDoor fittings, instruments

BronzeCopper + tinStatues, ornaments

SolderTin + lead (or modern lead-free mixes)Joining electrical components

Key distinction

An alloy is a mixture, not a compound — there's no fixed ratio of elements (e.g., steel can have 0.05–2.1% carbon). The atoms are still held together by metallic bonding throughout the structure, but the proportion of each element can vary. Steel is iron with carbon atoms scattered throughout the iron lattice. The mixing is what changes the properties.

Why Alloys Are Harder Than Pure Metals

This is the most-tested alloy question in AQA. The mark scheme has very specific language — learn it word for word.

Mark scheme chain (3 marks)

(1) Atoms of different sizes in the alloy (2) the differently-sized atoms distort the regular layers(3) distorted layers can't slide easily over each other — making the alloy harder than the pure metal.

Three traps to avoid (P1 Jun22 Q07.2, P1 Nov21 Q06.2)

Don't say "different metals" — say "atoms of different sizes".

Don't say "different shaped atoms" — atoms are spherical; they differ in size, not shape.

Don't mention "intermolecular forces" — metals contain no molecules.

Alloys: Poorer Conductors Than Pure Metals Higher Tier focus

Here's the synoptic point that almost no GCSE student makes — and the examiner specifically called this out (P1 Jun23 Q06.3): alloys conduct electricity less well than the pure metal they're based on.

Why?

The same distorted layers that make alloys harder also impede the flow of delocalised electrons through the structure. In a pure metal, electrons flow smoothly along regular layers; in an alloy, the differently-sized atoms create obstacles that the electron flow has to work around. So the alloy is a slightly worse electrical conductor.

P1 Jun23 Q06 context

The exam question presented data showing that pure copper conducts electricity better than its alloys. Most students stated that alloys are poorer conductors but couldn't explain why. The 2-mark answer needs both: (1) distorted layers (from differently-sized atoms) (2) impede the movement of delocalised electrons.

Real-world consequence

This is why high-voltage power cables use pure copper or pure aluminium — conductivity matters more than strength. Alloys like steel are great for structural parts but never the conductor in a circuit.

Four AQA Word-Traps

The metallic bonding topic has very specific mark-scheme language. Each of the four below appears in multiple examiner reports.

Trap 1 — "throughout" vs "through" (P1 Jun22, P1 Jun23)

Writing "electrons move throughout the structure" suggests random motion in all directions. Mark scheme: "ignore throughout for through ". Use "through" — implying directional flow under a potential difference.

Trap 2 — thermal vs electrical (P1 Jun24 Q07.1)

When the question says thermal conductivity, you must talk about thermal energy / kinetic energy transfer, NOT charge. When the question says electrical conductivity, you must talk about charge, not thermal energy. Both use delocalised electrons; the mechanism named must match the question.

Trap 3 — alloys: "different SIZED atoms" (P1 Jun22 Q07.2, P1 Nov21 Q06.2)

"Different metals" — wrong. "Different shaped atoms" — wrong. The exact phrase needed: "atoms of different sizes" distort the regular layers.

Trap 4 — "intermolecular forces in metals" — never (cross-cutting)

Metals contain no molecules. There are no intermolecular forces in metals or alloys. The forces are metallic bonds — electrostatic attraction between positive ions and delocalised electrons.

Weak answer

"Aluminium has free electrons that move throughout the metal carrying current."

"Throughout" loses M2; "current" loses M3 (the mark scheme requires "charge").

Strong answer

"Aluminium contains delocalised electrons that move through the structure; the electrons carry charge."

All three mark points hit, mark-scheme phrasing throughout.

Common Errors

Students often writeCorrect versionWhy it matters

"Electrons move throughout the structure""Electrons move through the structure""Through" implies directional flow (Jun22 MS)

"Electrons carry current""Electrons carry charge "Mark scheme says "ignore current/electricity for charge"

"Electrons flow and carry charge" (when asked about thermal)"Electrons gain kinetic energy and transfer thermal energy "Match mechanism to question type (Jun24 Q07.1)

"Different metals" / "different shaped atoms" (alloys)"Atoms of different sizes distort the layers"Atoms differ in size, not shape (Jun22 Q07.2)

"Bonds break when metals bend""Layers slide ; electron sea moves with them; bonds preserved"Malleability does not break bonds

"Intermolecular forces hold the metal together"" Metallic bonds hold the metal together — no molecules"Metals contain no molecules

Practice Question 1

Iron is a metal. Describe how iron conducts electricity. Answer in terms of electrons.3 MARKS

M1

Iron contains delocalised electrons (or "free electrons" — mark scheme accepts both).

M2

These electrons move through the structure (the metal / iron). Use "through" not "throughout".

M3

The electrons carry (electrical) charge. Use "charge" not "current".

Common slips: writing "electrons move throughout" (loses M2 on its own); writing "carry current" (loses M3); describing electron transfer between atoms (that's ionic, not metallic).

Practice Question 2

Steel is an alloy of iron and carbon. Explain why steel is harder than pure iron.3 MARKS

M1

Steel contains atoms of different sizes (iron atoms and smaller carbon atoms).

M2

The differently-sized atoms distort the regular layers of the iron lattice.

M3

The distorted layers can't slide easily over each other — making steel harder.

Common slips: "different metals" or "different shaped atoms" instead of "different-sized atoms"; mentioning intermolecular forces (no molecules in metals); skipping the "distorted layers" step (that's the M2 mark point).

Practice Question 3

For each substance below, identify whether the bonding is ionic , covalent , or metallic . Then state which type of bonding describes "electrons transferred", "electrons shared", and "electrons delocalised".5 MARKS

1

Sodium chloride (NaCl)

2

Methane (CH₄)

3

Aluminium (Al)

4

Brass (Cu + Zn)

1

NaCl = ionic. Metal (Na) + non-metal (Cl).

2

CH₄ = covalent. Two non-metals (C, H).

3

Al = metallic. Pure metal.

4

Brass = metallic. Alloy is still metallic bonding (mixture of two metals; same model of positive ions + delocalised electrons).

5

Verbs: transferred = ionicshared = covalentdelocalised = metallic. (Mark scheme also accepts "free" for delocalised.)

Summary

Definition

Metallic bond = the (electrostatic) attraction between positive metal ions and a sea of delocalised electrons. The verb is delocalised (or "free") — not "transferred", not "shared".

The four properties metallic bonding explains

Electrical conductivity: delocalised electrons move through the structure, carry charge

Thermal conductivity: delocalised electrons gain kinetic energy and transfer thermal energy

Malleability: layers of positive ions slide; electron sea moves with them; bonds preserved

High melting points: strong attraction between ions and electrons requires lots of energy

Alloys

Mixture of a metal with one or more other elements. Atoms of different sizes distort the regular layers; distorted layers can't slide easily, so alloys are harder than pure metals. The same distortion impedes electron flow — so alloys are poorer conductors than pure metals (HT synoptic point).

Four word-traps

(1) "through" not "throughout" (2) "charge" not "current" (3) "different sizes" not "different metals/shapes" (4) never say "intermolecular forces in metals" — metals have no molecules.

Ready for 2.05

Next up: The Three States of Matter and State Symbols — particle theory, state changes, and the case-sensitive formula traps that catch students out.