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S2.4

From models to materials

4 SL · +1 HL (SL 2.4.1-2.4.5) (HL 2.4.6)

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Parent topic: Models of bonding and structure

Guiding question

Guiding questionWhat role do bonding and structure have in the design of materials?

The one big idea

Bonding is a continuum, not three discrete categories. The bonding triangle places compounds by their ionic, covalent and metallic character, and from those positions you can predict material properties — including alloys and polymers.

What this rests on

Our editorial judgement.The dependencies in this section are this site’s own assessment of what a sub-topic rests on. They have notbeen verified by a chemistry teacher and are not part of the official IB guide. By contrast, section 8 (Linking questions) reproduces the IB’s own wording verbatim.

What rests on this

No dependents recorded.

Core concepts that must be mastered

  • The bonding continuum(2.4.1)Bonding is a continuum between ionic, covalent and metallic models, not three discrete boxes. A compound's position is representable by a bonding triangle (in the data booklet): each corner is a pure bonding type and every real compound sits somewhere inside, determined by the relative contributions of the three bonding types. The bonding triangle lets you explain why some compounds have properties intermediate between the pure models — a compound with polar covalent bonding may share some properties with ionic compounds (solubility in polar solvents, high-ish melting point) without being fully ionic. Use bonding models to explain material properties: the position in the triangle predicts whether a substance will be brittle or malleable, soluble in water or in organic solvents, conducting or insulating.
  • Position from electronegativity(2.4.2)The position in the bonding triangle is set by the relative contributions of the three bonding types. For binary compounds, you determine the position from electronegativity data: the electronegativity difference Δχ indicates the degree of ionic character, while the average electronegativity χ̄ indicates whether the compound is more covalent or metallic in character. High Δχ → ionic corner; low Δχ and high χ̄ → covalent corner; low Δχ and low χ̄ → metallic corner. Only binary compounds are considered, and calculations of percentage ionic character are not required — qualitative positioning from the data is sufficient. From the position, predict properties: compounds near the ionic corner have high melting points and conduct when molten; those near the covalent corner may be molecular (low melting point) or network (high melting point).
  • Alloys(2.4.3)Alloys are mixtures of a metal with other metals or non-metals, with enhanced properties compared to the pure metal. Bronze (copper + tin), brass (copper + zinc) and stainless steel (iron + chromium + carbon) are examples — specific alloys need not be memorised. Alloys work because metallic bonding is non-directional: different-sized atoms can sit in the lattice and distort it, making it harder for layers to slide past one another. This is why alloys are typically harder and stronger than pure metals. The property enhancement is explained in terms of non-directional bonding: the electron sea adjusts to accommodate the foreign atoms without breaking the metallic bond.
  • Polymers and addition polymerisation(2.4.4, 2.4.5)Polymers are macromolecules made from repeating monomer subunits. Both natural (proteins, cellulose, DNA) and synthetic (polythene, PVC, nylon) polymers exist. Common properties of plastics — flexibility, low density, electrical insulation, low melting point (relative to ceramics or metals) — are explained by their structure: long chains of covalently bonded carbon atoms held together by intermolecular forces (London forces and sometimes hydrogen bonding). Addition polymers form by breaking the C=C double bond in each monomer: the π bond opens and the monomers link through new σ bonds. Alkene polymerisations are the classic example. You must be able to represent the repeating unit from a given monomer structure, and work backwards from a polymer to the monomer. The repeating unit is drawn with bonds extending beyond the brackets, showing where it connects to the next unit.
  • HL: Condensation polymers(2.4.6)At HL, condensation polymers form by reaction between functional groups with the release of a small molecule — usually water. The two types you must handle are polyamides (from a diamine + a dicarboxylic acid, or from amino acids) and polyesters (from a diol + a dicarboxylic acid, or from hydroxy acids). You must be able to represent their repeating units, showing the amide linkage (–CO–NH–) or the ester linkage (–CO–O–). All biological macromolecules — proteins, polysaccharides, nucleic acids — form by condensation and break down by hydrolysis (water is added back to split the chain). This is a unifying principle: the same chemistry that makes nylon makes proteins.
  • How this sub-topic connectsStructure 2.4 is the capstone of the bonding strand. The bonding triangle unifies S2.1 (ionic), S2.2 (covalent) and S2.3 (metallic) into a single framework, and the periodic trends in S3.1 (metallic → non-metallic continuum, oxide acidity) are the periodic-table expression of this continuum. Polymers connect to organic chemistry (S3.2: functional groups determine which monomers can polymerise) and to green chemistry (R2.1: addition polymerisation has 100% atom economy). At HL, condensation polymers connect to the amide and ester functional groups in S3.2 and to biological macromolecules.

Quantitative non-negotiables

  • Use the bonding triangle (data booklet) to determine position from electronegativity data and predict properties.
  • Represent the repeating unit of an addition polymer from a given monomer structure.
  • At HL: represent repeating units of polyamides and polyesters.HL
  • Calculations of percentage ionic character are not required — qualitative positioning from electronegativity data is sufficient.

Common failure modes

  • M-11 — A compound is either ionic or covalent.

    Confidence: verified

    Why it’s wrong: Bonding is not a binary classification.

    Correction: Bonding is a continuum; use Δχ and χ̄ and the bonding triangle. Most bonds are polar covalent to some degree.

What "HL standard" actually looks like

HLHL adds condensation polymers — polyamides and polyesters — and the principle that all biological macromolecules form by condensation and break down by hydrolysis.

See the command terms reference →

Linking questions

Official IB.The questions in this section are the IB’s own linking questions, reproduced verbatim from the guide. They are not this site’s editorial judgement — see section 3 for that distinction.

  1. Official IB

    Why are some substances solid while others are fluid under standard conditions?

    Structure 1.1.2 → S2.4 From models to materials

  2. Official IB

    What are the features of metallic bonding that make it possible for metals to form alloys?

    Structure 2.3.2 → S2.4 From models to materials

  3. Official IB

    How do the trends in properties of period 3 oxides reflect the trend in their bonding?

    Structure 2.4.1 → S3.1 The periodic table: Classification of elements

  4. Official IB

    What are the limitations of discrete bonding categories?

    Structure 2.4.1 → S2.1 The ionic model

  5. Official IB

    Why do composites like reinforced concretes, which are made from ionic and covalently bonded components and steel bars, have unique properties?

    Structure 2.4.2 → S2.1 The ionic model

  6. Official IB

    Why are alloys more correctly described as mixtures rather than as compounds?

    Structure 2.4.3 → S1.1 Introduction to the particulate nature of matter

  7. Official IB

    What are the structural features of some plastics that make them biodegradable?

    Structure 2.4.4 → S3.2 Functional groups: Classification of organic compounds

  8. Official IB

    What functional groups in molecules can enable them to act as monomers for addition reactions?

    Structure 2.4.5 → S3.2 Functional groups: Classification of organic compounds

  9. Official IB

    Why is the atom economy 100% for an addition polymerization reaction?

    Structure 2.4.5 → R2.1 How much? The amount of chemical change

  10. Official IB

    What functional groups in molecules can enable them to act as monomers for condensation reactions?

    Structure 2.4.6 → S3.2 Functional groups: Classification of organic compounds

  11. Official IB

    What is the nature of the reaction that occurs when two amino acids form a dipeptide?

    Structure 3.2.2 → S2.4 From models to materials

  12. Official IB

    The atom economy and the percentage yield both give important information about the "efficiency" of a chemical process. What other factors should be considered in this assessment?

    Reactivity 2.1.5 → S2.4 From models to materials

  13. Official IB

    Why are alkenes sometimes known as "starting molecules" in industry?

    Reactivity 3.4.5 → S2.4 From models to materials

Bridge: GCSE → IB HL

Derived. These are the bridge items tagged to this sub-topic— places where the GCSE model gets redefined, genuinely new territory, or carry-over strengths. The tagging is this site’s editorial judgement.

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