Metallic Bonding, Alloys and Properties of Metals
Metallic bonding at IGCSE extends the model of a lattice of positive ions in a sea of delocalised electrons to explain why alloys are harder than the pure metals they are made from, and requires naming and describing the composition and uses of specific everyday alloys, including bronze, brass and different types of steel.
Before you start
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Method
- Recap the metallic bonding model: a giant lattice of positive metal ions arranged in regular layers, surrounded by a 'sea' of delocalised electrons (from the outer shell of every metal atom) that are free to move throughout the whole structure, holding the ions together by strong electrostatic attraction.
- Explain malleability and ductility: because the layers of ions in a pure metal are arranged regularly, one layer can slide over another when a force is applied, without breaking any specific bond, since the delocalised electrons hold the structure together throughout, allowing pure metals to be bent, hammered and drawn into wires.
- Explain electrical and thermal conductivity: the delocalised electrons are free to move throughout the structure, carrying electrical charge (electrical conductivity) and transferring kinetic energy rapidly between particles (thermal conductivity).
- Define an alloy as a mixture of a metal with one or more other elements, often another metal or carbon, and explain why alloys are harder than the pure metal: atoms of the added element are usually a different size to the atoms of the main metal, which distorts the regular layers of the lattice, so the layers can no longer slide over each other as easily, making the alloy harder and less malleable.
- Learn the composition and a use of key alloys: bronze (copper and tin, used for statues and medals), brass (copper and zinc, used for musical instruments and taps), mild steel (iron with a small amount of carbon, used for car bodies and machinery), and stainless steel (iron with chromium and nickel, resistant to corrosion, used for cutlery and kitchen sinks).
- When answering a question about why an alloy is used instead of a pure metal, always link the property required for the job (hardness, corrosion resistance, appearance) back to the specific difference in structure between the alloy and the pure metal.
Worked example
Pure copper is a soft metal. Explain why bronze, an alloy of copper and tin, is harder than pure copper.
- Recall that in pure copper, all the atoms are the same size, so they are arranged in regular layers that can slide easily over each other.
- Identify that bronze contains tin atoms mixed in with the copper atoms, and that tin atoms are a different size to copper atoms.
- Explain that these differently sized tin atoms distort the regular layers of copper atoms, disrupting the orderly arrangement.
- State that this distortion means the layers can no longer slide over each other as easily as they can in pure copper.
- Final answer: bronze is harder than pure copper because the tin atoms disrupt the regular layers of the metal lattice, making it more difficult for the layers to slide over one another.
Practice questions
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Q1Name the two elements that make up brass.Show answer
Answer: Copper and zinc.
Q2Name the two elements that make up bronze.Show answer
Answer: Copper and tin.
Q3Give the two elements that make up mild steel.Show answer
Answer: Iron and carbon (a small amount of carbon).
Q4Name the two additional elements added to iron to make stainless steel.Show answer
Answer: Chromium and nickel.
Q5State why stainless steel, rather than mild steel, is used to make kitchen sinks.Show answer
Answer: Stainless steel is much more resistant to corrosion (rusting) than mild steel, because of the chromium (and nickel) it contains.
Q6Explain, in terms of structure, why pure metals are malleable (can be hammered into shape).Show answer
Answer: The regular layers of identically sized metal ions can slide over each other when a force is applied, without breaking the metallic bonding, since the delocalised electrons hold the whole structure together, so the metal changes shape rather than shattering.
Q7Explain why alloys are generally harder than the pure metals they are made from.Show answer
Answer: Atoms of the added element(s) are usually a different size from the atoms of the main metal, which disrupts/distorts the regular layers in the lattice, so the layers cannot slide over each other as easily as in the pure metal.
Exam-style questions
Written in the style of a IGCSE Science exam paper, with a full mark scheme.
Give the name of the alloy made from copper and tin, and one use of this alloy.
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Pure iron is too soft to be used for most structural purposes. (a) Name the alloy formed by adding a small amount of carbon to iron. (b) Explain, in terms of structure, why adding carbon makes the alloy harder than pure iron.
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A manufacturer is choosing a material to make a set of cutlery (knives, forks and spoons). They are considering pure iron, mild steel (iron with a small amount of carbon) and stainless steel (iron with chromium and nickel). Evaluate which material would be the most suitable choice, referring to structure, bonding and properties in your answer.
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Free printable worksheet
Want more practice on paper? Download the metallic bonding, alloys and properties of metals worksheet pack - 6 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 16 of IGCSE Science Workbook, the whole course as one free printable PDF.
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