An alloy is a mixture of a metal with one or more other elements. Because the added atoms are a different size, they disturb the regular layers, so an alloy is usually harder and stronger than the pure metal.
This lesson belongs to metals and reactivity and uses the bonding ideas from bonding and structure.
How do the layers in a pure metal behave?
A pure metal has atoms of one kind, all the same size, packed in regular layers. A sea of delocalised electrons holds the positive ions together.
When you push or hit the metal, layers can slide over each other and the metal changes shape without breaking. That is why pure metals are malleable and ductile, and also why they can be too soft.
What changes in an alloy?
In an alloy, atoms of a different size sit among the atoms of the main metal. They break up the regular arrangement, so the layers cannot slide so easily.
The alloy is therefore harder and stronger. It is still a metallic structure, so it still conducts. Brass is copper with zinc, and steel is iron with a small amount of carbon.
How to compare an alloy with a pure metal, step by step
- Name the property being compared: hardness, strength, malleability.
- Describe the pure metal’s structure: atoms of one size in regular layers.
- Describe the alloy’s structure: atoms of different sizes, layers disrupted.
- Link structure to behaviour: layers slide less easily in the alloy.
- State the conclusion using the words “harder”, “stronger” or “less malleable” for the named property.
Worked example
Brass is an alloy of 70% copper and 30% zinc by mass. Compare it with pure copper, and find the mass of each metal in 250 g of brass.
Comparison: pure copper has identical atoms in regular layers that slide easily, so it is soft. In brass, zinc atoms are a different size from copper atoms. They disrupt the layers, which slide less easily, so brass is harder than copper.
Mass of copper: 70% of 250 g = 0.70 × 250 = 175 g.
Mass of zinc: 30% of 250 g = 0.30 × 250 = 75 g.
Check: 175 g + 75 g = 250 g, so the masses add up to the whole.
Answer: brass is harder because the layers are disrupted; 250 g contains 175 g copper and 75 g zinc.
The mistake to watch for
A common slip is to say an alloy is stronger without mentioning the structure, or to call it a compound.
Mistaken answer: “An alloy is harder because the atoms are bonded more strongly in a new compound.”
The student treated the alloy as a compound. The real reason is that different-sized atoms disrupt the layers.
The correction is to use the word “mixture” and to describe the layers. An alloy has no fixed formula, and its atoms are not bonded in a fixed ratio. The hardness comes from the irregular arrangement, not from a new substance.
Check yourself
1. A sample of an invented alloy is 85% metal X and 15% metal Y by mass. Find the mass of each in 480 g.
Show answer
X: 0.85 × 480 = 408 g. Y: 0.15 × 480 = 72 g. Check: 408 + 72 = 480. 408 g of X and 72 g of Y.
2. Draw or describe layers in pure iron and in steel, and say which is harder.
Show answer
Pure iron has same-sized atoms in regular layers that slide easily. Steel has small carbon atoms among the iron atoms, which disrupt the layers. Steel is harder.
3. True or false: an alloy conducts electricity because it is a compound.
Show answer
False. An alloy is a mixture with a metallic structure, so it conducts because of delocalised electrons. It is not a compound.
Where this leads next
The last lesson in this module asks you to weigh a decision rather than describe a structure: interpreting a recycling trade-off. You can test everything so far with the metals and reactivity practice set.
Students who know that alloys are harder but cannot phrase the reason in two clear sentences often gain from online one-to-one Chemistry tuition, where a teacher can work on the wording with you.