S2.4Structure 2 — Models of bonding and structure

From models to materials — IB Chemistry HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 4 of 4 1 quiz

📚 IB Chemistry HL Hub / Topical Revision

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Lesson summary

S2.4 From models to materials — IB CHEMISTRY HL notes: Place substances on the bonding triangle, explain how alloys differ from pure metals, and describe addition and condensation polymers.Three types — ionic, covalent and metallic bonding help explain chemical and physical prope…

Real bonding
Data booklet
Ionic compounds
Polar covalent compounds
Interpreting only
Sodium chloride

Part 1 of 8Bonding models and the continuum

Bonding models and the continuum

Three types

Three types ionic, covalent and metallic bonding help explain chemical and physical properties.

Real bonding

Real bonding it is more complex than three types suggest;
- AlCl₃ melts at 192 °C, lower than a purely ionic compound, because of covalent character.

Continuum

Continuum bonding is better shown on a bonding triangle between ionic, covalent and metallic.

Data booklet

Data booklet the bonding triangle is in Section 17 and electronegativity values are in Section 9.

Part 2 of 8Using the bonding triangle

x-axis

x-axis average electronegativity, (χ_A + χ_B) / 2.

y-axis

y-axis difference in electronegativity, Δχ = χ_A − χ_B.

Elements

Elements have Δχ = 0 and lie on the x-axis: metals near the metallic corner (bottom left) and non-metals at the covalent corner (bottom right).

Ionic compounds

Ionic compounds large Δχ, near the apex.

Polar covalent compounds

Polar covalent compounds between the ionic and covalent regions.

Interpreting only

Interpreting only the triangle need not be memorised, and exact percentages are not calculated in exams.

Part 3 of 8Bonding and properties

Sodium chloride

Sodium chloride average electronegativity 2.05 and difference 2.3, near the apex with about 75 % ionic character, explaining a high melting point and conduction when molten.

Aluminium chloride

Aluminium chloride about 50 % ionic, melting at 192 °C because of weaker covalent interactions.

Aluminium oxide

Aluminium oxide about 60 % ionic, melting at 2072 °C because of stronger ionic bonding.

Predictions

Predictions the triangle helps predict melting point, solubility and electrical conductivity.

Part 4 of 8Alloys

Alloys

Alloy

Alloy a mixture of metals, or a metal mixed with a non-metal such as carbon.

Structure

Structure atoms of different elements are spread through the metallic lattice, held by delocalised electrons and not chemically bonded.

Non-directional bonds

Non-directional bonds they let atoms of different sizes sit in the lattice.

Pure metal

Pure metal regular layers slide past each other easily, so it is malleable.

Alloy

Alloy different-sized atoms disrupt the pattern, so layers slide less easily.

Effect

Effect alloys are generally harder and stronger, and may resist corrosion or extreme temperatures.

Part 5 of 8Common alloys

Alloy	Components	Properties	Uses
Brass	Copper and zinc	Strong, corrosion resistant	Door handles, hinges, instruments
Steel	Iron with carbon (often Cr, V, Mo)	Very strong	Construction, bridges, cars
Stainless steel	Iron, chromium, nickel, carbon	Corrosion resistant	Cutlery, surgical instruments, cookware
Solder	Lead and tin	Low melting point	Joining metals in circuits
Bronze	Copper and tin	Hard, corrosion resistant	Medals, sculptures, ship fittings

Not required

Not required specific alloy names, compositions and uses need not be memorised.

Required

Required compare an alloy with its pure metal and explain greater hardness by atomic size and lattice disruption.

Part 6 of 8Polymers

1Polymer

Polymer a large molecule made by chemically linking many small molecules called monomers.

2Size

Size at least 50 repeating units joined by strong covalent bonds; also called macromolecules.

3Single monomer

Single monomer poly(ethene) and poly(chloroethene) (PVC).

4Copolymers

Copolymers made from two or more monomers, such as nylon and proteins.

5Properties

Properties low density, unreactive, water resistant and strong.

6Natural polymers

Natural polymers proteins, starch and DNA, made by living organisms from monomers such as amino acids and nucleotides.

7Synthetic polymers

Synthetic polymers plastics such as poly(ethene) and nylon, durable and unreactive.

8Environment

Environment synthetic polymers are mostly non-biodegradable, so plastic accumulates as long-lasting pollution.

Part 7 of 8Addition polymers

Addition polymerisation

Addition polymerisation monomers with at least one C=C join to make a long chain with no by-product.

Mechanism

Mechanism the π bond in each C=C breaks and monomers link by new C–C single bonds.

Examples

Examples poly(ethene) from ethene and poly(chloroethene) from chloroethene.

Repeat unit

Repeat unit the smallest group of atoms that repeats, shown in square brackets with n outside.

Link to monomer

Link to monomer the repeat unit is the monomer with C=C changed to C–C.

Deduce the monomer

Deduce the monomer the repeat unit of an addition polymer has two carbons in the main chain.

Part 8 of 8Condensation polymers (HL)

Condensation polymers (HL)

Condensation polymerisation

Condensation polymerisation monomers join with the release of a small molecule such as water or HCl.

Polyesters

Polyesters ester links –COO– from a carboxylic acid and an alcohol, formed from diols with dicarboxylic acids, or from hydroxycarboxylic acids.

Polyamides

Polyamides amide (peptide) links –CONH– from a carboxylic acid and an amine, formed from diamines with dicarboxylic acids or dioyl dichlorides,
- or from amino acids.

Examples

PET, nylon-6,6 and proteins.
- (Terylene, Dacron)

Hydrolysis

Hydrolysis ester and amide links can be broken by water into the original monomers, so these polymers are more biodegradable.

Addition polymers

Addition polymers they cannot be hydrolysed and persist in the environment.

Comparison

Comparison addition uses C=C monomers with no by-product;
- condensation uses monomers with two functional groups and releases water.

Key terms

Term	Meaning
Bonding triangle	A diagram placing substances between ionic, covalent and metallic bonding.
Alloy	A mixture of a metal with another metal or non-metal.
Polymer	A large molecule built from repeating monomer units.
Monomer	A small molecule that joins to form a polymer.
Repeat unit	The smallest repeating group of atoms in a polymer.
Condensation polymerisation	Polymerisation that releases a small molecule.
Hydrolysis	Breaking down by reaction with water.

Avoid these mistakes

Saying a compound of a metal and non-metal is always purely ionic — electronegativity data show covalent character.

Saying alloys are chemically bonded compounds — they are mixtures held by metallic bonding.

Confusing the repeat unit with the monomer — the monomer has C=C and the repeat unit has C–C.

Forgetting the small molecule released in condensation polymerisation.

Last updated: 14 October 2026