How does metallic bonding explain why metals conduct, bend and stay solid at high temperatures, and why alloys are harder than pure metals?
Describe metallic bonding as a lattice of positive ions in a sea of delocalised electrons, relate it to the properties of metals, and explain why alloys are harder than pure metals
A focused answer to the O-Level Chemistry outcome on metallic bonding. The lattice of positive ions in a sea of delocalised electrons, how it explains conduction, malleability and high melting points, and why alloys are harder than pure metals.
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What this dot point is asking
SEAB wants you to describe metallic bonding as a lattice of positive metal ions held together by a sea of delocalised electrons, use this model to explain the characteristic properties of metals (electrical and thermal conduction, malleability and ductility, high melting points), and explain why alloys are harder than the pure metals they are made from. This completes the three bonding types and links to the metals topic.
The answer
The metallic bonding model
In a metal, the atoms are packed closely in a regular giant lattice. Each metal atom loses its outer-shell electrons, which become delocalised: free to move throughout the whole structure rather than belonging to any one atom. This leaves a lattice of positive metal ions sitting in a sea of delocalised electrons.
The metallic bond is the strong electrostatic attraction between the positive ions and the surrounding sea of negative electrons. It acts in all directions and holds the structure firmly together.
Explaining the properties of metals
The model explains every typical metal property:
- Good electrical conductor: the delocalised electrons are free to move through the lattice and carry charge, in the solid and when molten.
- Good thermal conductor: the mobile electrons also carry energy quickly through the metal.
- Malleable and ductile (can be hammered into sheets and drawn into wires): when a force is applied, layers of positive ions slide over one another into new positions, while the sea of electrons moves with them and keeps holding the structure together, so the metal changes shape without shattering.
- High melting and boiling points: the strong attraction between the ions and the electron sea needs a lot of energy to overcome.
Why alloys are harder
A pure metal has identical atoms arranged in regular layers, so the layers slide over one another easily, which is why pure metals are relatively soft.
An alloy is a mixture of a metal with one or more other elements (often another metal). The added atoms are usually a different size, so they disrupt the regular layers. The distorted layers cannot slide over one another as easily, so the alloy is harder and stronger than the pure metal. This is why brass (copper and zinc), bronze (copper and tin) and steel (iron and carbon) are used in preference to the pure metals.
Examples in context
Example 1. Why copper is used for wiring and steel for buildings. Copper's delocalised electrons make it one of the best conductors, so it carries current in cables, while steel (an iron alloy) uses the hardness from carbon atoms disrupting the iron lattice to bear heavy loads. Each use is chosen from the property the bonding model predicts.
Example 2. Gold jewellery is an alloy. Pure gold is too soft to keep its shape, so it is alloyed with metals such as copper to make it harder and more hard-wearing. The added atoms disrupt the gold layers, demonstrating the same alloy-hardening idea in an everyday object.
Try this
Q1. State what is meant by the sea of delocalised electrons in a metal. [1 mark]
- Cue. The outer-shell electrons that have left the atoms and are free to move throughout the whole metal lattice.
Q2. Explain why metals are good conductors of electricity. [2 marks]
- Cue. The delocalised electrons are free to move through the lattice and carry electric charge.
Q3. Explain why steel is harder than pure iron. [2 marks]
- Cue. Steel is an alloy; carbon atoms of different size disrupt the regular layers of iron atoms, so the layers cannot slide easily, making it harder.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksCopper is a metal used in electrical wiring. (a) Describe the structure and bonding in copper. (b) Use this to explain why copper conducts electricity and can be drawn into wires.Show worked answer →
(a) Copper consists of a giant lattice of positive metal ions arranged regularly, surrounded by a sea of delocalised (free) electrons that came from the outer shells of the atoms. The metallic bond is the strong attraction between the positive ions and the sea of electrons.
(b) It conducts electricity because the delocalised electrons are free to move through the lattice and carry charge. It can be drawn into wires (it is ductile) because the layers of positive ions can slide over one another when a force is applied, while the sea of electrons holds the structure together, so it changes shape without breaking.
Markers reward the lattice of positive ions in a sea of delocalised electrons, conduction from the mobile electrons, and ductility from layers of ions sliding while the electron sea holds them together.
Original4 marksPure copper is soft, but adding a small amount of tin to make bronze produces a harder material. (a) Explain why pure copper is relatively soft. (b) Explain why bronze is harder than pure copper.Show worked answer →
(a) In pure copper the atoms (ions) are the same size and arranged in regular layers, so the layers can easily slide over one another when a force is applied, making it soft and easy to shape.
(b) Bronze is an alloy: the different-sized tin atoms disrupt the regular layers of copper atoms. This makes it harder for the layers to slide over one another, so the alloy is harder and stronger than the pure metal.
Markers reward soft pure metal explained by regular layers sliding easily, and the harder alloy explained by different-sized atoms disrupting the layers so they cannot slide as easily.
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