The big idea: We can improve a material's properties by mixing or joining particles in new ways.
- An alloy is a mixture of a metal with one or more other elements (usually metals). It is not a compound — the components are not chemically bonded in a fixed ratio. - An addition polymer is a very long molecule made by joining many small alkene monomers together, with no atoms lost.
An alloy is a mixture of a metal with one or more other elements. The different-sized atoms (two colours) disrupt the regular layers, so they cannot slide past each other as easily — the alloy is harder than the pure metal.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Two ways to build a better material: - Alloy → physically mix metal atoms of different sizes to make the metal harder/stronger. - Polymer → chemically join many small molecules (monomers) into one giant chain.
In a pure metal, the atoms are all the same size and pack into regular layers. When a force is applied, these layers can slide over one another, so a pure metal is fairly soft and easily bent.
An alloy adds atoms of a different size. These different-sized atoms distort the regular layers so they can no longer slide past each other easily. The result is a material that is harder and stronger than the pure metal.
Why an alloy is harder: Different-sized atoms disrupt the orderly layers, so the layers cannot slide over each other as easily. More force is needed to deform the metal → the alloy is harder and stronger.
Everyday alloys: - Brass — copper + zinc (taps, instruments). - Steel — iron + a little carbon (stronger, harder than pure iron). - Bronze — copper + tin.
Each keeps the metallic bonding (a sea of delocalised electrons), so alloys still conduct electricity — they are just harder than the pure metal.
Spot an alloy from the data booklet: A Paper 1A 'identify the alloy' question gives average electronegativity and electronegativity difference values. An alloy is metallic, so it sits in the bottom-left of the bonding triangle: a low average electronegativity and a small electronegativity difference.
Study smarter, not longer
Most students waste 40% of study time on topics they already know. Our AI tracks your progress and optimizes every minute.
A monomer is a small molecule that joins to many others to make a polymer (a giant molecule). Addition polymers are made from alkene monomers — molecules with a reactive carbon–carbon double bond (C=C).
In addition polymerisation, the double bond opens up. One bond of the C=C stays as a single bond; the other is used to join to the next monomer. Many monomers link into one long chain, and no atoms are lost (nothing else is made).
| Structure | What changes | |
|---|---|---|
| Monomer (ethene) | CH2=CH2 | has a C=C double bond |
| Repeating unit | –CH2–CH2– | double bond becomes a single bond; two new bonds join neighbouring units |
| Polymer (poly(ethene)) | (–CH2–CH2–)n | n repeat units joined in a long chain |
The repeating unit: The repeating unit is the part of the chain that repeats over and over. Get it from the monomer by opening the C=C (double bond → single bond) and drawing bonds out of each end:
- Monomer: CH2=CH2 → Repeat unit: –CH_{2}–CH_{2}–
The polymer is written as (–CH_{2}–CH_{2}–)_{n}, where n is a large number.
Worked example — repeat unit of poly(propene)
Propene is CH2=CH(CH3). Deduce the repeating unit of the addition polymer it forms.
Solution
- Find the C=C. Propene's double bond is between the first two carbons; the CH3 is a side group.
- Open the double bond. Change C=C to a single C–C bond, and draw a bond out of each end carbon.
- Write the unit. Keep the CH3 side group on its carbon.
Final answer
–CH2–CH(CH3)– (the polymer is (–CH2–CH(CH3)–)n).
How this is tested: S2.4.2 shows up two ways:
- Paper 1A (MCQ): given a repeating unit, identify the alkene monomer (put the C=C back); or use electronegativity data to identify which substance is an alloy. - Paper 2: deduce the monomer from a shown polymer, then describe a property that makes the polymer a useful material.
Monomer ⇌ repeat unit: Don't mix them up. The monomer has the C=C double bond; the repeat unit has a single C–C bond with a bond drawn out of each end. Going from polymer back to monomer = put the double bond back.
IB-style question — deduce the monomer (a)
An addition polymer has the repeating unit (–CH2–CHCl–)n. (a) Deduce the structure of the alkene monomer from which it is made. [1]
How to score the marks
- Take one repeating unit and remove the bonds drawn out of each end: –CH2–CHCl–.
- Put the C=C double bond back between the two carbons to recreate the alkene.
Final answer
CH2=CHCl (chloroethene).
IB-style question — useful property (b)
(b) Describe one chemical property of the polymer that makes it a useful material. [1]
How to score the marks
- Name a useful chemical property. The C–C and C–H bonds are strong and non-polar, so the polymer is unreactive / chemically inert — it does not corrode or react with acids, water or air.
- (Any one valid property scores: e.g. unreactive/inert, waterproof/insoluble in water, an electrical insulator.)
Final answer
It is chemically unreactive (inert), so it resists corrosion and does not react with water or acids — useful for containers and packaging.