Section 1 of 5Pure Metals and Their Structure
The properties of pure metals come from their regular arrangement of atoms in layers.
Metallic structure
- Metallic structure Atoms in a pure metal are arranged in regular layers that can slide over one another when a force is applied.
Softness
- Softness Because the layers slide easily, pure metals are often soft, malleable and ductile, which is useful for shaping but limits strength.
Pure gold
- Pure gold Pure gold is too soft for everyday jewellery, which is why it is mixed with other metals.
Pure iron
- Pure iron Pure iron is soft and easily deformed, so it is rarely used on its own in engineering structures.
Pure aluminium
- Pure aluminium Pure aluminium is light but weak, so it is usually alloyed to make it strong enough for structural use.
Section 2 of 5How an Alloy Works
An alloy is made by adding atoms of a different element to a metal, which changes how the layers move.
1
An alloy is a mixture of a metal with at least one other element, usually another metal or a non-metal such as carbon.
2
The atoms of the added element are a different size from the metal atoms, so they distort the regular layers of the structure.
3
The distorted layers find it harder to slide past one another, so the alloy is harder and stronger than the pure metal.
4
Changing the proportions of the elements in an alloy changes its properties, so alloys can be designed for particular uses.
5
Alloys still contain metallic bonding, so they conduct electricity, although they usually conduct less well than the pure metal.
Section 3 of 5Choosing and Using Alloys
Engineers choose an alloy by matching its properties to the job it has to do.
Choosing and Using Alloys
Strength
- Strength Alloys are chosen when a material must resist bending, breaking or scratching while in service.
Density
- Density A lighter alloy, such as an aluminium-based alloy, reduces the fuel needed to move aircraft and vehicles.
Corrosion resistance
- Corrosion resistance Stainless steel contains chromium, which forms a protective layer that resists rusting.
Cost
- Cost Expensive alloying metals must be justified by better performance, so cheaper alloys are used wherever they are good enough.
| Alloy | Main elements | Useful property | Typical use |
|---|---|---|---|
| Steel | Iron and carbon | Strong and tough | Bridges and car bodies |
| Brass | Copper and zinc | Hard and easy to shape | Musical instruments and electrical fittings |
| Duralumin (aluminium alloy) | Aluminium with small amounts of copper, magnesium and manganese | Strong and low density | Aircraft frames |
| 18 carat gold | Gold, with copper and silver | Harder and more durable than pure gold | Jewellery |
Section 4 of 5Worked Example
Question
- An 18 carat gold ring has a mass of 9.0 g.
- Calculate the mass of pure gold it contains.
- 18 carat means 18 parts of gold out of 24 equal parts of the alloy.
- Proportion of gold = 18 ÷ 24 = 0.75, or 75%.
- Mass of gold = 0.75 × 9.0 g = 6.75 g.
Answer The ring contains 6.75 g of gold, and the remaining 2.25 g is made of other metals.
Section 5 of 5Exam Checklist & Tips
10.3.2 – Key Points for the Exam
- Alloy A mixture of a metal with at least one other element, with properties that differ from the pure metal.
- Malleable Able to be hammered or pressed into shape without breaking.
- Layer structure The arrangement of metal atoms in layers that can slide over one another.
- Carat A measure of gold purity, in which 24 carat is pure gold.
- Stainless steel An alloy of iron, carbon and chromium that resists corrosion.
Common Mistakes
- Describing an alloy as a compound;
- an alloy is a mixture of elements, not a chemically bonded substance with a fixed formula.
- Saying that pure metals are harder than alloys; the added atoms make alloys harder.
- Stating that the layers in an alloy are removed; they are distorted, not removed.
- Thinking carat measures parts out of 100; it measures parts out of 24.