How are metals bonded, and why are alloys harder than pure metals?
Metallic bonding as positive ions in a sea of delocalised electrons, how this explains the properties of metals, and why alloys are harder than the pure metal.
A CCEA GCSE Chemistry answer on metallic bonding, covering the model of positive ions in a sea of delocalised electrons, how it explains conductivity, malleability and high melting points, and why alloys are harder than the pure metal.
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What this dot point is asking
CCEA wants you to describe the metallic bonding model of positive ions in a sea of delocalised electrons, use it to explain the typical properties of metals, and explain why alloys are harder than the pure metal.
The metallic bonding model
The delocalised electrons are not attached to any one ion; they move freely. The attraction between the positive ions and this negative electron sea holds the metal together and is strong, which is why metals are usually solids with high melting points.
Explaining the properties of metals
Because the same electron sea explains conduction and malleability, a good exam answer always names the delocalised electrons and the sliding layers of ions.
Alloys
This is why alloys are so widely used: steel is far harder and stronger than pure iron, making it better for building and tools, even though it is mostly iron with a small amount of carbon.
Worked example
Examples in context
Example 1. Copper for wiring. Copper is used in electrical cables because its delocalised electrons make it an excellent conductor, and its malleability lets it be drawn into thin wires. Both properties come straight from the metallic bonding model.
Example 2. Gold jewellery alloys. Pure gold is too soft to keep its shape, so jewellers alloy it with metals such as copper. The different-sized atoms harden the gold by stopping the layers sliding, which is why most "gold" jewellery is actually an alloy.
Try this
Q1. State what is meant by an alloy. [1 mark]
- Cue. A mixture of a metal with one or more other elements.
Q2. Explain why metals conduct electricity. [2 marks]
- Cue. Delocalised electrons are free to move and carry charge through the metal.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20184 marksDescribe the structure and bonding in a metal, and use it to explain why metals conduct electricity and can be bent into shape.Show worked answer →
Markers want the model described, then each property linked to it.
A metal consists of a regular arrangement of positive metal ions surrounded by a sea of delocalised electrons. The strong electrostatic attraction between the positive ions and the delocalised electrons is the metallic bond.
Metals conduct electricity because the delocalised electrons are free to move through the structure and carry charge.
Metals can be bent and shaped (malleable) because the layers of ions can slide over each other while the delocalised electrons keep holding the structure together, so it does not shatter.
Markers reward positive ions in a sea of delocalised electrons, free electrons carrying charge for conduction, and sliding layers held by the electron sea for malleability.
CCEA 20213 marksExplain why an alloy such as brass is harder than the pure copper it is made from.Show worked answer →
The marks are for the disrupted layers.
Pure copper has layers of identical ions that can slide over each other easily, so it is relatively soft.
An alloy such as brass contains atoms of a different size (zinc mixed with copper). These different-sized atoms distort the regular layers, so the layers can no longer slide over each other easily.
Because the layers cannot slide, the alloy is harder than the pure metal.
Markers reward the idea that different-sized atoms disrupt the layers and stop them sliding, making the alloy harder.
Related dot points
- Ionic bonding as the transfer of electrons to form charged ions, drawing dot-and-cross diagrams, the giant ionic lattice, and how the structure explains the properties of ionic compounds.
A CCEA GCSE Chemistry answer on ionic bonding, covering how electrons transfer from metals to non-metals to form ions, dot-and-cross diagrams, the giant ionic lattice, and how this structure explains the high melting points, conductivity and solubility of ionic compounds.
- Giant covalent structures, the structures and properties of diamond, graphite and silicon dioxide, and how bonding explains hardness, melting point and electrical conductivity.
A CCEA GCSE Chemistry answer on giant covalent structures, covering the structures of diamond, graphite and silicon dioxide, and how their covalent bonding explains very high melting points, hardness, and why graphite conducts electricity while diamond does not.
- Covalent bonding as the sharing of electron pairs between non-metal atoms, drawing dot-and-cross diagrams for simple molecules, and the properties of simple molecular substances.
A CCEA GCSE Chemistry answer on covalent bonding, covering how non-metal atoms share pairs of electrons to fill their outer shells, dot-and-cross diagrams for molecules such as hydrogen, water, ammonia and methane, and why simple molecular substances have low melting points and do not conduct.
- The development and modern organisation of the Periodic Table by atomic number into periods and groups, the position of metals and non-metals, and how the table predicts properties.
A CCEA GCSE Chemistry answer on the Periodic Table, covering how Mendeleev's table developed into the modern one, how elements are arranged by atomic number into periods and groups, where metals and non-metals sit, and how the layout lets us predict the properties of an element.
- How the reactivity of a metal determines its extraction method, the extraction of iron by reduction with carbon in the blast furnace, and why reactive metals are extracted by electrolysis.
A CCEA GCSE Chemistry answer on the extraction of metals, covering how a metal's position in the reactivity series sets its extraction method, the reduction of iron oxide with carbon in the blast furnace, and why metals above carbon are extracted by electrolysis.
Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)