How does metallic bonding explain the properties of metals and alloys?
Metallic bonding; positive ions in a sea of delocalised electrons; the properties of metals; why alloys are harder than pure metals.
A focused answer to AQA GCSE Chemistry 4.2.1 and 4.2.2, covering metallic bonding as positive ions in a sea of delocalised electrons, how this explains conductivity and malleability, and why alloys are harder than pure metals.
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What this dot point is asking
AQA wants you to describe metallic bonding as positive ions in a sea of delocalised electrons, use this model to explain why metals conduct electricity and heat and are malleable, and explain why alloys are usually harder than the pure metals they are made from. Every metal property in this topic traces back to two features: the free-moving delocalised electrons and the regular sliding layers of ions.
The metallic bonding model
The strong electrostatic attraction between the positive ions and the delocalised electrons is what holds the metal together and gives it a high melting point. Because each atom has contributed its outer electrons to the shared sea, what remains in the lattice are positively charged ions.
Explaining metal properties
Alloys
An alloy is a mixture of a metal with one or more other elements. Alloys are usually harder than the pure metal, which is why most metals used in everyday objects (steel, brass, bronze) are alloys rather than pure metals.
The atoms of the added element are a different size, so they distort the regular layers of ions. This makes it harder for the layers to slide over each other, so the alloy is harder and stronger than the pure metal. Common examples include steel (iron with carbon and sometimes other metals), brass (copper and zinc) and bronze (copper and tin), all chosen because the alloy is harder, stronger or more corrosion-resistant than the pure metal would be.
Why metals have these properties together
It is worth noticing that the single metallic bonding model explains every typical metal property at once. The delocalised electrons account for both electrical and thermal conductivity, because the same mobile electrons that carry charge also transfer energy quickly through the structure. The regular layers of ions held by non-directional attraction (the attraction acts equally in all directions to the electron sea) account for malleability and ductility, because the layers can shift without the bonding being destroyed. And the strength of that attraction across a giant structure accounts for the high melting and boiling points. A pure metal therefore behaves predictably, and an alloy modifies that behaviour mainly by disrupting the neat layers.
Try this
Q1. Explain why metals conduct electricity. [2 marks]
- Cue. Delocalised electrons are free to move and carry charge.
Q2. Explain why an alloy is harder than the pure metal. [2 marks]
- Cue. Different-sized atoms distort the layers so they cannot slide easily.
Q3. State why metals have high melting points. [1 mark]
- Cue. Strong attraction between the positive ions and the delocalised electrons needs a lot of energy to overcome.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksDescribe the structure and bonding in a pure metal, and use this model to explain why metals are good conductors of electricity and are malleable.Show worked answer →
A 4-mark Paper 1 metallic-bonding question.
Structure and bonding (2 marks): a metal is a giant structure of positive metal ions arranged in regular layers, held together by a sea of delocalised electrons (from the outer shells) that are free to move throughout; there is strong electrostatic attraction between the positive ions and the delocalised electrons. Conduction (1 mark): the delocalised electrons are free to move and carry electrical charge (and thermal energy) through the metal. Malleable (1 mark): the layers of ions can slide over each other without breaking the bonding, so the metal can be bent and shaped.
Markers reward the delocalised-electrons idea for conduction and the sliding-layers idea for malleability.
AQA 20213 marksGold is often mixed with copper to make jewellery. Explain why this alloy is harder than pure gold, and describe what an alloy is.Show worked answer →
A 3-mark question on alloys.
Alloy definition (1 mark): a mixture of a metal with one or more other elements. Why harder (2 marks): the copper atoms are a different size from the gold atoms, so they distort the regular layers of metal ions; this makes it harder for the layers to slide over each other, so the alloy is harder than the pure metal.
Markers want the different-sized atoms and disrupted-layers explanation, not "the bonds are stronger".
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)