IB DP Chemistry Structure 2.4: From models to materials – Revision Notes

These notes condense Structure 2.4, From models to materials, as listed in the IB Diploma Programme Chemistry guide (published February 2023; first assessment 2025). They cover understandings Structure 2.4.1 to 2.4.6. Understandings 2.4.1 to 2.4.5 are for SL and HL; 2.4.6, condensation polymers, is HL only. They apply to the course as first assessed in 2025.

The full study guide for this unit explains each idea in depth; the matching question set lets you apply it. To see every unit, open the course page or the syllabus checklist. A free diagnostic can point you at weak spots.

Key definitions

Term	Meaning
Bonding continuum	The idea that real bonding lies somewhere between the ionic, covalent and metallic models, not in one box
Bonding triangle	A plot of electronegativity difference against average electronegativity for binary compounds; its corners are the three pure models
Polar covalent	Covalent bonding with some ionic character, from unequal sharing
Alloy	A metal combined physically with other elements (metals or non-metals), giving enhanced properties
Non-directional bonding	Bonding with no fixed direction: delocalized electrons attract cations equally all round
Polymer (macromolecule)	A very large molecule built from many repeating subunits
Monomer	The small molecule that joins up to make a polymer
Repeating unit	The smallest section of a polymer chain that repeats
Addition polymer	Made by breaking a C=C double bond in each monomer; no by-product
Condensation polymer (HL only)	Made by reaction between functional groups on monomers, releasing a small molecule such as water
Hydrolysis (HL only)	Breaking a link by reaction with water; the reverse of condensation

Structure 2.4.1: the three models side by side

Model	Typical melting behaviour	Electrical conduction	Response to force
Ionic lattice	High melting	Only when molten or dissolved	Brittle
Molecular covalent	Low melting	None	Soft or brittle
Covalent network	Very high melting	None (graphite excepted)	Very hard
Metallic	Mostly high melting	As solid and liquid	Malleable, ductile

Method: explaining a property from a model

1. Name the bonding model that best fits the material.
2. Name the particles and what holds them together.
3. Say whether those particles or electrons can move, and how strong the attraction is.
4. Link that directly to the property asked about.

Remember that most substances show mixed character. Discrete categories are a simplification. For each model in detail, see the ionic model and the metallic model.

Structure 2.4.2: the bonding triangle

Quantity	Formula	Axis
Electronegativity difference	Δχ = larger χ − smaller χ	Vertical
Average electronegativity	(χA + χB) / 2	Horizontal

According to the guide, the data booklet provides a triangular bonding diagram and electronegativity data. You are not asked to calculate percentage ionic character, and only compounds of two elements are considered.

Method: placing a compound

1. Look up χ for the two elements.
2. Subtract to get Δχ (always positive).
3. Add and halve to get the average. Ignore subscripts in the formula.
4. Plot the point and name the region.

Where things land

• Top of the triangle: ionic.
• Bottom left (low average, small Δχ): metallic.
• Bottom right (high average, small Δχ): covalent.
• In between: mixed character, for example polar covalent.

Worked reminder. Use Pauling values Na 0.93, Br 2.96, P 2.19, Cl 3.16, B 2.04, N 3.04.

Compound	Δχ	Average	Prediction
NaBr	2.03	1.945	Ionic: high melting point, conducts when molten
PCl₃	0.97	2.675	Covalent, polar bonds: molecular, low boiling point, no conduction
BN	1.00	2.54	Covalent: a covalent network with a very high melting point

Must-know distinction. Δχ alone does not decide the region. Two compounds with the same small Δχ can be metallic (low average) or covalent (high average). Always use both coordinates.

Structure 2.4.3: alloys

• An alloy is a mixture: its composition can vary, and it has no fixed formula.
• Metallic bonding is non-directional, so atoms of a different element can fit into the lattice.
• Different-sized atoms distort the layers, which then resist sliding: the alloy is harder and stronger than the pure metal.
• The delocalized electrons remain, so alloys still conduct.

Alloy	Made of	Example improvement
Bronze	Copper and tin	Harder, more corrosion resistant than copper
Brass	Copper and zinc	Harder than copper
Stainless steel	Iron, chromium, often nickel, some carbon	Resists rusting

Specific alloys do not have to be learned. The explanation does.

Structure 2.4.4: polymers and plastics

Natural polymers: proteins, starch, cellulose, DNA, natural rubber. Synthetic polymers: poly(ethene), PVC, nylons, polyesters.

Plastic property	Because
Insulator	No mobile ions or delocalized electrons
Low density	Light atoms, loosely packed chains
Mouldable on heating (most)	Only intermolecular forces between chains are overcome
Strong	Long tangled chains with many intermolecular forces
Slow to break down	Strong, unreactive C–C and C–H bonds

Longer, less branched chains pack more closely: stronger and denser. Cross-linked chains do not soften on heating.

Structure 2.4.5: addition polymers

Method: monomer to repeating unit

1. Change C=C to C–C.
2. Keep all four groups on those two carbons where they were.
3. Draw continuation bonds through square brackets; write n outside.

Method: polymer to monomer

1. Find the two-carbon section of backbone that repeats.
2. Put C=C between those carbons and remove the continuation bonds.

Worked reminder. 3-Chloropropene is CH₂=CH–CH₂Cl. Its repeating unit is –[–CH₂–CH(CH₂Cl)–]–ₙ. The CH₂Cl group is a side chain; only the two C=C carbons form the backbone.

Atom economy. No atoms are lost to a by-product, so addition polymerization has 100% atom economy. The molar mass of the repeating unit equals the molar mass of the monomer.

Structure 2.4.6: condensation polymers (HL only)

Polymer type	Groups that react	Link formed	Small molecule released
Polyester	–COOH and –OH	Ester, –COO–	H₂O
Polyamide	–COOH and –NH₂	Amide, –CONH–	H₂O

Method: drawing a condensation repeating unit

1. Identify the two reacting groups on each monomer.
2. Remove H from one group and OH from the other at every link.
3. Join the remaining atoms to form the ester or amide link.
4. For two different monomers, the repeating unit contains one of each and two links; for one monomer with both groups, it contains one residue and one link.

Worked reminder. 6-Hydroxyhexanoic acid, HO(CH₂)₅COOH (M = 132.18 g mol⁻¹), forms a polyester with repeating unit –[–O(CH₂)₅CO–]–ₙ (M = 114.16 g mol⁻¹). The difference, 18.02 g mol⁻¹, is the water lost per link.

Biological macromolecules. All form by condensation and break down by hydrolysis.

• Proteins: 2-amino acids joined by amide (peptide) links.
• Polysaccharides (starch, cellulose): glucose units joined with loss of water.
• Nucleic acids: nucleotides joined by condensation.

A chain of N monomer units has N − 1 links, so N − 1 water molecules are released when it forms, and N − 1 are needed to hydrolyse it completely.

Quick self-test

1. State what is plotted on each axis of the bonding triangle.
2. Use Li 0.98 and Al 1.61 to find Δχ and the average for LiAl. Name the bonding type.
3. Explain why Δχ alone cannot place a compound in the triangle.
4. Explain why an alloy is described as a mixture rather than a compound.
5. Explain, in terms of structure, why bronze is harder than copper.
6. Name two natural polymers and two synthetic polymers.
7. Explain why most plastics are electrical insulators.
8. Draw the repeating unit of the polymer from hexafluoropropene, CF₂=CF–CF₃.
9. A polymer has the repeating unit –CH₂–CH(OCH₃)–. Give the monomer’s formula.
10. Explain why addition polymerization has 100% atom economy.
11. (HL only) State the link and the small molecule formed when an amino group reacts with a carboxyl group.
12. (HL only) How many water molecules are released when 60 amino acids form one polypeptide chain?

Answers

1. Vertical: electronegativity difference, Δχ. Horizontal: average electronegativity of the two elements.
2. Δχ = 1.61 − 0.98 = 0.63; average = (0.98 + 1.61) / 2 = 1.295. Low on the left: metallic.
3. Small Δχ can mean metallic or covalent; the average electronegativity separates them.
4. Its composition can vary; it has no fixed formula and its components are not chemically combined in fixed ratios.
5. Tin atoms, being a different size, disrupt the regular copper layers, making it harder for them to slide.
6. Natural: any two of proteins, starch, cellulose, DNA, natural rubber. Synthetic: any two of poly(ethene), PVC, nylon, polyester.
7. Electrons are held in localized covalent bonds; there are no mobile ions or delocalized electrons to carry charge.
8. –[–CF₂–CF(CF₃)–]–ₙ
9. CH₂=CH–OCH₃ (methoxyethene).
10. The only product is the polymer; every atom of every monomer ends up in it.
11. An amide link, –CONH–, and water.
12. 59

Where marks are usually lost

• Calculating Δχ for a compound such as AlCl₃ by multiplying the chlorine value by three.
• Placing a compound using Δχ only and ignoring the average electronegativity.
• Calling the bonding in a high-Δχ compound “100% ionic”: the triangle shows degrees of character.
• Writing that alloys are harder because “the metallic bonds are stronger”, with no mention of layers being unable to slide.
• Describing an alloy as a compound.
• Explaining that plastics melt because covalent bonds break, instead of intermolecular forces being overcome.
• Drawing an addition repeating unit with the side group in the chain, or with a C=C still present.
• Leaving off the square brackets, continuation bonds or n.
• (HL only) Leaving an extra –OH or –H at the ends of a condensation repeating unit.
• (HL only) Saying hydrolysis “removes water” rather than adds it.

Official syllabus

Document followed: Chemistry guide, Diploma Programme, International Baccalaureate Organization (February 2023; first examined 2025). Scope here: Structure 2.4, From models to materials; 2.4.1 to 2.4.5 for SL and HL, 2.4.6 additional higher level.