Polymers, Giant Covalent Structures, Properties of Metals and Alloys

SPEC 4.2.2.5–4.2.2.8 • PAPER 1

Polymers, Giant Covalent, Metals & Alloys

Three more structure types to complete the set. The most-failed precision question in this topic: never call a polymer a giant structure — it is a very large molecule.

Why This Lesson Matters

This lesson consolidates the structural landscape of GCSE Chemistry. By the end, you should be able to match structure type to properties in either direction — given a property, identify the structure; given a structure, predict the properties.

FIVE STRUCTURE TYPES IN TOTAL

Lesson 2.06 covered giant ionic lattices and simple molecules. This lesson adds the remaining three: polymers, giant covalent structures, and metals/alloys. Every substance you meet at GCSE belongs to one of these five categories.

EXAMINER PRIORITY POINTS

Polymer is a (very large) molecule — never a giant structure

Diamond high mp — many strong covalent bonds need to be broken

Alloys harder than pure metals — atoms of different sizes distort the layers

Metals conduct — delocalised electrons carry both electrical charge and thermal energy

Key Definitions

Three terms underpin every answer in this lesson. Memorise them word for word.

POLYMER

A very large molecule made of many repeating units (monomers) joined by covalent bonds.

GIANT COVALENT STRUCTURE

A structure containing many atoms joined by strong covalent bonds in a continuous 3D network — there are no individual molecules.

ALLOY

A mixture of a metal with one or more other elements.

Polymers — Structure

Polymers like poly(ethene) and poly(propene) are very large molecules built from many small repeating units (monomers).

TWO TYPES OF INTERACTION

Within each chain → strong covalent bonds linking all the atoms. Between chains → intermolecular forces. Because chains are very long, there are many points of contact between neighbouring chains, so the intermolecular forces between polymer chains are relatively strong — strong enough to make polymers solid at room temperature.

Polymer Properties

Three key properties — every one follows from the structure (very large molecules + relatively strong intermolecular forces).

1Solid at room temperature — IMFs strong enough due to chain length

2Soften on heating — IMFs weaken; chains slide past each other

3Do not conduct electricity — no free charged particles

4Higher mp than small molecules; lower than giant covalent

WHEN A POLYMER MELTS / SOFTENS

It is the intermolecular forces between chains that are overcome — not the covalent bonds along the chain. The chains stay intact; they just slide past each other. Same principle as melting simple molecules — only the intermolecular forces are overcome, never the covalent bonds.

Examiner Alert — Polymer ≠ Giant Structure

This is the most repeated polymer error in Paper 1 examiner reports.

EXAMINER ALERT — P1 Jun 2023 Q04.5 (4 marks)

From the examiner report on the methane vs poly(ethene) question: "This question proved to be difficult, with few students giving fully correct answers to gain all 4 marks and with a third of students gaining no credit."

"Many students did not mention molecules at all, seeming to believe that the polymer is a giant structure. There was much confusion between bonds and intermolecular forces. Students often thought intermolecular forces had to be broken in methane but bonds had to be broken in poly(ethene)."

FIX THE LANGUAGE

✓ "Poly(ethene) consists of very large molecules…"

✓ "Stronger intermolecular forces between the larger molecules…"

✗ "Poly(ethene) is a giant structure" — this is wrong.

✗ "Bonds break when poly(ethene) melts" — this is wrong.

QUICK CHECK

If a question gives you a long polymer chain in a diagram, ask: does it have discrete chain ends (even if very long)? If yes → polymer (large molecule). If every atom is bonded into a continuous network with no ends → giant covalent.

Giant Covalent — Structure

A continuous 3D network of atoms bonded by covalent bonds. There are no individual molecules — every atom is connected to its neighbours by strong covalent bonds across the whole crystal.

EXAMPLES IN THE SPEC

Diamond and graphite (forms of carbon) and silicon dioxide, SiO₂ (also called silica). Detail of diamond, graphite, graphene and fullerenes is covered in Lesson 2.08.

Giant Covalent — Properties

Two properties to know — both follow directly from the continuous 3D network of strong covalent bonds.

1Many atoms joined by strong covalent bonds in 3D

2Many bonds must be broken to melt

3Very high melting points — solids at RT

CONDUCTIVITY

Most giant covalent structures (e.g. diamond, SiO₂) do not conduct electricity. All outer electrons are used in the covalent bonds, so there are no free charged particles to carry charge.

EXCEPTION

Graphite is the one giant covalent that does conduct electricity. Its structure (covered in Lesson 2.08) leaves one electron per carbon delocalised. Don't worry about the detail here — just know diamond and SiO₂ are non-conductors.

PEE — Diamond's Very High mp

The 3-mark question from P1 Jun22 Q03.2. Here is the verbatim mark-scheme answer.

Q: Explain why diamond has a very high melting point. 3 MARKS

P

Diamond is a giant covalent structure in which the (covalent) bonds are strong.

E

To melt diamond, many of these strong covalent bonds must be broken.

E

A lot of energy is required to break that many strong bonds — so diamond has a very high melting point.

VERBATIM MARK SCHEME

Mark 1: "(covalent) bonds are strong"

Mark 2: "(and many covalent) bonds must be broken"

Mark 3: "(so) a lot of energy is required"

Common Error — Diamond & IMFs

Students sometimes write "strong intermolecular forces in diamond" — wrong. Diamond has no intermolecular forces, because there are no separate molecules. Every atom is bonded directly. Saying IMFs in a giant covalent answer cancels marks.

Metals — Bonding

The metallic structure that drives every property in the spec. Three things to know: positive metal ions, in regular layers, surrounded by a sea of delocalised electrons.

METALLIC BONDING — IN ONE SENTENCE

A giant structure of positive metal ions arranged in regular layers, surrounded by a sea of delocalised electrons. The strong electrostatic attraction between the positive ions and the negative delocalised electrons is the metallic bond.

Metals — Three Properties From One Structure

Spec 4.2.2.7 and 4.2.2.8 require three properties: high mp, malleability, and conductivity (electrical and thermal). Each follows directly from the metallic structure.

PropertyWhy?

High melting and boiling pointsStrong metallic bonding (electrostatic attraction between positive ions and delocalised electrons). A lot of energy needed to overcome the bonds.

Malleable and ductile (can be bent and shaped)Atoms are arranged in layers that can slide over each other when a force is applied.

Good conductor of electricityDelocalised electrons in the metal are free to move and carry electrical charge through the metal.

Good conductor of thermal energyDelocalised electrons transfer thermal energy through the metal as they move.

SPEC LANGUAGE (4.2.2.8)

The exact spec wording: "Metals are good conductors of electricity because the delocalised electrons in the metal carry electrical charge through the metal. Metals are good conductors of thermal energy because energy is transferred by the delocalised electrons."

EXAMINER ALERT — P1 Jun 2024 Q07.1 (thermal conduction, 2 marks)

"Only a quarter of students gained both marks, and a third did not score." Many described how solids in general conduct, or described electrical conduction instead of thermal. The mark scheme is just two phrases: "(thermal) energy is transferred" + "by delocalised electrons". Examiner: "The idea of the delocalised electrons transferring thermal energy through the metal was not at all well known."

Alloys — Why Harder Than Pure Metals?

Pure metals are too soft for many uses, so they are mixed with other elements to make alloys (mixtures). The alloy is harder than the pure metal — and the mechanism appears in mark schemes word for word.

1Alloy contains atoms of different sizes

2Layers of atoms become distorted

3Layers cannot slide easily → harder

Examiner Alert — Alloy Language Traps

Two examiner reports give us a complete catalogue of the language errors that lose marks on alloy questions.

EXAMINER ALERT — P1 Nov 2021 Q06.2 (3 marks)

Verbatim from the examiner report: "Many instances of incorrect use of scientific terms. Some referred to molecules instead of atoms, different shapes instead of different sizes, and rows instead of layers. Many did not refer to the layers in a metal at all… Even those who did refer to layers rarely said those layers were distorted. There were many incorrect answers in terms of an incorrect type of bonding, and many thought that alloys are compounds ."

EXAMINER ALERT — P1 Jun 2024 Q07.2 (3 marks)

Verbatim from the examiner report: "Common errors often involved an attempt to describe some sort of stronger bonding (sometimes covalent or ionic) in the alloy . Some lost credit because of poor use of language; it is not 'metals' which are different sizes, but their atoms (or positive ions) . A common error was to refer to different shapes of atoms. 'Particles' is also insufficient as it could refer to other types of particle than atoms. 'Intermolecular forces' were often referred to, although they are not present in either a pure metal or an alloy ."

RIGHT VS WRONG — FIVE LANGUAGE ERRORS

Wrong (no mark)Right (scores)

"Atoms of different metals""Atoms of different sizes "

"Different shaped atoms""Different sized atoms"

"Different sized particles""Different sized atoms " (or ions)

"Stronger bonds in alloy" / "covalent bonds in alloy""Layers distorted — cannot slide easily"

"Intermolecular forces in alloy"(no IMFs — metals/alloys have metallic bonding only)

"Alloy is a compound""Alloy is a mixture "

WHY ATOMS & SIZES?

Atoms are spherical — they differ in diameter, not in shape. "Particles" is too vague (could mean ions, molecules, or anything). And alloys are mixtures (not chemically combined): if they were compounds, you couldn't vary the proportions of components.

Alloys Are Worse Conductors

A higher-mark synoptic question that catches almost everyone — the same "distorted layers" idea also explains why alloys conduct electricity less well than pure metals.

EXAMINER ALERT — P1 Jun 2023 Q06.3 (2 marks, AO3 synoptic)

"Nearly a third of students scored at least one mark on this high demand question, which required the linking of two different areas of the specification." Verbatim mark scheme: "in alloys different sized atoms distort the layers / structure (so) the movement of (delocalised) electrons is restricted." Spec ref 4.2.2.7 + 4.2.2.8 — the same "distorted layers" idea applied to conductivity.

1Alloys contain different-sized atoms

2Layers are distorted

3Movement of delocalised electrons restricted → poorer conductor

SAME MECHANISM, TWO QUESTIONS

"Distorted layers" explains both hardness and reduced conductivity. The same structural feature (different-sized atoms breaking up the regular layer arrangement) impedes both the sliding of layers (→ harder) and the free flow of delocalised electrons (→ worse conductor).

PEE — Alloy Hardness

Real exam question from P1 Jun24 Q07.2 — also appeared in similar form in Nov21 Q06.2. Both have the same 3-mark scheme.

Q: Explain why an alloy is harder than a pure metal. 3 MARKS

P

The alloy contains atoms of different sizes compared with the pure metal.

E

The different-sized atoms distort the regular layers of metal atoms.

E

The distorted layers cannot slide over each other easily, so the alloy is harder than the pure metal.

VERBATIM MARK SCHEME (P1 JUN24 Q07.2)

Mark 1: "(the alloy / mixture has) different sized atoms" (allow "ions")

Mark 2: "(so the) layers are distorted"

Mark 3: "(so the) layers cannot easily slide" (allow "atoms cannot slide over each other")

PEE — Polymer vs Small Molecule

A classic 4-mark comparison from P1 Jun23 Q04.5. Tests whether you can apply the structure-to-property mechanism to TWO substances at once.

Q: Methane is a gas at room temperature but poly(ethene) is a solid at room temperature. Explain why methane and poly(ethene) exist in different states at room temperature. 4 MARKS

P

Methane consists of much smaller molecules than poly(ethene) — and poly(ethene) consists of much larger molecules.

E

Smaller molecules (methane) have weaker intermolecular forces between them; the larger molecules (poly(ethene)) have stronger intermolecular forces.

E

The intermolecular forces between methane molecules need less energy to overcome, so methane has a low boiling point and is a gas.

E

The stronger intermolecular forces between poly(ethene) chains need more energy to overcome, so poly(ethene) has a higher melting point and is a solid.

Watch for the trap

The mark scheme rejects answers that confuse covalent bonds with intermolecular forces. Both methane and poly(ethene) have strong covalent bonds within them — those are not what's overcome on melting/boiling. Always say "intermolecular forces" — never "bonds" — when explaining states of matter.

Five Structure Types — Master Table

The big-picture comparison covering Lessons 2.06 and 2.07 together. Memorise this table.

StructureBonding withinm.p./b.p.Conducts?

Giant ionic lattice (NaCl)Strong electrostatic forces between ionsHighOnly when molten/dissolved (ions free to move)

Simple molecular (CO₂, CH₄)Weak IMF between moleculesLowNo — molecules have no overall charge

Polymer (poly(ethene))Relatively strong IMF between long chainsHigher than small moleculesNo — no free charged particles

Giant covalent (diamond, SiO₂)Many strong covalent bonds in 3DVery highNo (graphite is the exception — see 2.08)

Metal / alloy (Cu, steel)Metallic — ions + delocalised electronsHigh (variable)Yes — delocalised electrons carry charge

QUICK-FIRE EXAM RULE

Identify the structure first, then state the property. Every "explain why X has property Y" answer follows the same pattern: structure type → bonding/forces involved → consequence for the property.

Practice Question

Pure copper is too soft for many uses, so it is mixed with tin to make bronze.

(a) Explain why bronze is harder than pure copper. 3 MARKS

(b) Predict whether bronze conducts electricity better than, worse than, or the same as pure copper. Give a reason. 2 MARKS

(a) Alloy hardness — 3 marks:

P

Bronze contains atoms of different sizes (tin atoms are a different size to copper atoms).

E

The different-sized atoms distort the regular layers of atoms.

E

The distorted layers cannot slide easily over each other, so bronze is harder.

(b) Conductivity — 2 marks:

P

Bronze conducts electricity worse than pure copper.

E

The distorted layers restrict the movement of the delocalised electrons, so the alloy is a poorer conductor.

MARK ALLOCATION

Part (a) uses the standard 3-mark mark scheme (P1 Jun24 Q07.2): different sizes → distorted layers → can't slide. Part (b) is the Jun23 Q06.3 synoptic point: distorted layers also restrict delocalised electron movement → worse conductor.

Summary

Three New Structures Covered

Polymer = very large molecule made of repeating units. Strong covalent bonds within chains, relatively strong intermolecular forces between chains. Solid at RT, softens on heating, doesn't conduct.

Giant covalent = continuous 3D network of atoms held by many strong covalent bonds. Very high mp (many bonds to break). Doesn't conduct (no free electrons) — except graphite.

Metal / alloy = giant structure of positive ions in a sea of delocalised electrons. High mp, malleable, conducts heat & electricity. Alloys harder + worse conductors than pure metals because different-sized atoms distort layers.

DIAMOND HIGH MP (3 MARKS)

(1) Strong covalent bonds; (2) many bonds must be broken; (3) lots of energy required.

ALLOY HARDNESS (3 MARKS)

(1) Atoms of different sizes; (2) layers distorted; (3) layers cannot slide easily.

EXAM TIP — THE THREE LANGUAGE TRAPS

(1) Polymer = very large molecule, never a "giant structure". (2) Alloys have atoms of different sizes, never "different shapes" or "different metals". (3) Diamond has no intermolecular forces — never mention IMFs in a giant covalent answer.