What is metallic bonding, and how does it explain the properties of metals and alloys?
Metallic bonding as a lattice of positive ions in a sea of delocalised electrons, the link to the properties of metals (conduction, malleability, high melting points), and why alloys are harder than pure metals.
A focused answer to OCR Gateway GCSE Chemistry A topic C2.1 on metallic bonding, covering the lattice of positive ions in a sea of delocalised electrons, how this explains electrical and thermal conduction, malleability and high melting points, and why alloys are harder than pure metals.
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What this dot point is asking
OCR wants you to describe metallic bonding as a lattice of positive ions in a sea of delocalised electrons, and use this model to explain the properties of metals: electrical and thermal conduction, malleability (being bent and shaped), and high melting points. You also need to explain why alloys are harder than pure metals. This is the third of the three bond types.
The metallic bonding model
Because the electrons are shared across the whole structure (not between particular atoms), the bonding acts in all directions and is strong. Metals are giant structures with a regular arrangement of positive ions.
Explaining the properties of metals
The metallic bonding model explains all the typical properties of metals:
Alloys
Alloys are harder than the pure metal they are based on. In a pure metal, all the atoms are the same size and arranged in regular layers that can slide over each other easily, so the pure metal is relatively soft. In an alloy, the added atoms are a different size, which distorts the layers of metal ions. This makes it harder for the layers to slide over one another, so the alloy is harder and stronger than the pure metal.
Why metals are malleable but ionic solids are brittle
A useful contrast: metals are malleable because the delocalised electrons let the layers slide without breaking the bonding. Ionic solids are brittle because, if a layer slides, ions of the same charge line up and repel, shattering the crystal. This difference comes straight from the bonding model.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksDescribe the structure and bonding in a metal, and explain why metals are good conductors of electricity and can be bent into shape.Show worked answer →
A C2.1 structured question. Reward: a metal is a giant structure of positive metal ions arranged in a regular lattice, surrounded by a sea of delocalised (free) electrons that have come from the outer shells of the atoms. The bonding is the strong electrostatic attraction between the positive ions and the delocalised electrons. Metals conduct electricity because the delocalised electrons are free to move through the structure and carry charge. Metals can be bent or hammered into shape (they are malleable) because the layers of ions can slide over each other while the delocalised electrons hold the structure together. Markers credit positive ions in a sea of delocalised electrons, the moving electrons carrying charge for conduction, and the sliding layers for malleability.
OCR 20224 marksPure copper is soft, but the alloy bronze (copper mixed with tin) is harder. Explain, in terms of structure, why alloys such as bronze are harder than the pure metal.Show worked answer →
A Higher tier question on alloys. Reward: in a pure metal the atoms (ions) are the same size and arranged in regular layers that can easily slide over one another, which is why pure copper is soft. An alloy is a mixture of a metal with one or more other elements; the added atoms are a different size, so they distort the regular layers. This makes it harder for the layers to slide over each other, so more force is needed to change the shape and the alloy is harder. Markers credit the regular sliding layers in the pure metal, the different-sized atoms in the alloy distorting the layers, and the layers being unable to slide as easily, making the alloy harder.
Related dot points
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A focused answer to OCR Gateway GCSE Chemistry A topic C2.1 on ionic bonding, covering electron transfer between metals and non-metals, the formation of ions, dot and cross diagrams, working out ionic formulae, and the giant ionic lattice structure.
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