Bonding continuum and materials (polymers)

IB Chemistry SL· Structure 3.2.12-3.2.15· 12 min read

1. The Bonding Continuum Fundamentals★★☆☆☆SL only⏱ 3 min

Traditional teaching often frames bonding as 3 discrete separate categories, but real chemical bonding exists on a smooth continuum determined by the difference in electronegativity between the two bonded atoms. There are no hard cutoffs between regimes, only gradual shifts in properties.

📘 Definition

Bonding Continuum

$Δ χ$

A spectrum of bonding behaviour ordered by electronegativity difference between bonded atoms, ranging from fully ionic (electron transfer) to fully covalent (equal electron sharing) to metallic (delocalised electron sea).

\Delta \chi = \chi_{\text{more electronegative atom}} - \chi_{\text{less electronegative atom}}

$Δ χ > 1.8$ : Dominantly ionic bonding
$0.5 < Δ χ < 1.8$ : Polar covalent bonding
$Δ χ < 0.5$ : Non-polar covalent bonding

📐 Worked Example

Classify the bonding in HCl, KF and C (diamond) using the bonding continuum, given electronegativity values H=2.2, Cl=3.2, K=0.8, F=4.0, C=2.6.

1
Step 1: Calculate electronegativity difference for each pair
2
$\Delta \chi_{HCl} = 3.2 - 2.2 = 1.0$
3
$\Delta \chi_{KF} = 4.0 - 0.8 = 3.2$
4
$\Delta \chi_{C-C} = 2.6 - 2.6 = 0.0$
5
Step 2: Assign each to the correct continuum regime
6
HCl is polar covalent, KF is ionic, diamond is non-polar covalent (covalent network)

✓ Quick check

Confirm your understanding of the continuum before moving on:

Which of the following has the most polar covalent bonding?
- H-H
- O-H
- Na-Cl
- C-H
Reveal answer
O-H —
Oxygen and hydrogen have an electronegativity difference of 1.4, which falls in the middle of the polar covalent range.

2. Polymer Structure Classification★★★☆☆SL only⏱ 4 min

Polymers are long chain molecules made from repeating monomer units, and their properties are directly determined by the type of covalent bonds within the chain and the intermolecular forces between adjacent chains. They are split into two core classes: addition polymers and condensation polymers.

📘 Definition

Addition Polymer

A polymer formed when alkene monomers break their double bond and link together to form a long saturated carbon chain, with no small byproduct molecules released during reaction.

Polymer Type	Monomer Feature	Byproduct Formed	Example
Addition	C=C double bond	None	Polyethene, PVC
Condensation	Two reactive functional groups	H2O or HCl	Nylon, PET

📐 Worked Example

Draw the repeat unit for the condensation polymer formed from ethane-1,2-diol and ethanedioic acid.

1
Step 1: Identify the functional groups on each monomer: diol has two -OH groups, diacid has two -COOH groups
2
Step 2: Remove one H from the diol -OH and one -OH from the diacid -COOH to form one H2O molecule per link
3
Step 3: Join the remaining -O- from the diol and -CO- from the diacid to form the ester link, creating a repeat unit of -O-CH2CH2-O-CO-CO-

3. Bonding Continuum and Polymer Physical Properties★★★☆☆SL only⏱ 3 min

The bonding continuum lets you predict polymer properties without memorising every material. For non-polar addition polymers like polyethene, only weak London forces act between chains, so lower molecular weight samples are flexible and have low melting ranges. Adding polar side groups like Cl in PVC introduces stronger dipole-dipole forces, increasing hardness and melting range.

Methods compared

The difference between thermoplastics and thermosets comes entirely from cross-linking behaviour:

Thermoplastics

No covalent cross-links between chains, only intermolecular forces hold adjacent chains together

+ Pros: Can be melted and reshaped, easy to recycle

− Cons: Low maximum operating temperature

Thermosets

Permanent covalent cross-links connect all polymer chains into a single giant lattice

+ Pros: Very high heat and chemical resistance

− Cons: Cannot be melted, cannot be recycled easily

📐 Worked Example

Explain why high density polyethene (HDPE) has a higher tensile strength than low density polyethene (LDPE).

1
Step 1: Note HDPE has almost no side branching, so polymer chains can pack very closely together
2
Step 2: Close packing increases the total surface area of contact between adjacent chains, strengthening London dispersion forces
3
Step 3: Stronger intermolecular forces require more force to separate chains, giving HDPE higher tensile strength than branched, loosely packed LDPE

4. Exam Command Term Practice★★★★☆⏱ 2 min

5. Common Pitfalls

Wrong move:

Classifying all bonding as discrete ionic/covalent/metallic with no gradations

Why:

The IB SL syllabus explicitly requires you to understand the continuous nature of bonding, not just 3 separate categories

Correct move:

Always reference electronegativity difference to place a substance on the smooth bonding continuum

Wrong move:

Stating polymers have a single exact melting point

Why:

Polymer samples contain a distribution of different chain lengths, so intermolecular force strength is not uniform across all molecules

Correct move:

Refer to a polymer melting range rather than a single melting point value in all exam answers

Wrong move:

Drawing condensation polymer repeat units that retain the full H2O molecule

Why:

Condensation polymerisation eliminates one small byproduct molecule (usually H2O) per link between monomers

Correct move:

Remove one H from one monomer functional group and one OH from the other to form the linking group

Wrong move:

Claiming thermoplastics have covalent cross-links between chains

Why:

Thermoplastics only have intermolecular forces between chains, no permanent covalent cross-links

Correct move:

Reserve covalent cross-link descriptions exclusively for thermoset polymer materials

Wrong move:

Placing metallic bonding at the far non-polar end of the covalent continuum

Why:

Metallic bonding relies on a delocalised electron sea, not shared localised electron pairs, so it is a distinct adjacent regime

Correct move:

Position metallic bonding as a separate end of the continuum, not a subset of covalent bonding

6. Quick Reference Cheatsheet

Bonding Regime	Electronegativity Difference Range	Key Property	Polymer Example
Ionic	$Δ χ > 1.8$	High molten conductivity	Ionomer polymers
Polar Covalent	$0.5 < Δ χ < 1.8$	Dipole-dipole intermolecular forces	Polyvinyl chloride
Non-polar Covalent	$Δ χ < 0.5$	London forces only	Polyethene
Metallic	N/A	Delocalised electron conductivity	Metal-filled polymer composites
Covalent Network	$Δ χ = 0$	Giant covalent lattice	Cross-linked epoxy resin

7. Frequently Asked

Why do polymers not have a sharp melting point?

Polymer samples contain a wide distribution of different chain lengths, so there is no uniform strength of intermolecular forces across all molecules. This means chains overcome intermolecular attractions over a range of temperatures, producing a melting range rather than a single exact melting point.

Where do covalent network solids sit on the bonding continuum?

Covalent network solids sit at the extreme non-polar covalent end of the continuum, where electronegativity difference between bonded atoms is near zero, and covalent bonds extend across the entire giant lattice structure.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · Paper 2
Compare polymer tensile strength values
2022 · Paper 1
Bonding continuum classification task
2021 · Paper 2
Polyamide structure deduction

What's Next

Mastering the bonding continuum and polymer properties gives you a huge advantage on IB SL Paper 2 long answer questions, where 3-5 mark structure-property links are almost always tested. This framework directly supports your understanding of organic reaction mechanisms for addition and condensation polymerisation, and you will be able to apply this continuum logic to other material classes including nanomaterials and semiconductors later in the course. This content is also extremely useful if you choose a polymer-related practical investigation for your internal assessment, as you can use the bonding continuum to explain unexpected results in your writeup.