How does metallic bonding explain why metals conduct and bend, and why are alloys harder than pure metals?
Describe metallic bonding as positive ions in a sea of delocalised electrons, relate it to metal properties, and explain why alloys are harder than pure metals.
A focused answer to WJEC GCSE Chemistry topic 2.1, 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 topic is asking
WJEC topic 2.1 wants you to describe metallic bonding as a lattice of positive ions in a sea of delocalised electrons, to use this model to explain the typical properties of metals (conductivity, malleability), and to explain why alloys are harder than pure metals.
How metallic bonding works
When metal atoms pack together, each atom loses its outer electrons into a shared "sea". The atoms become positive ions held in fixed positions, and the delocalised electrons move freely between them. The attraction between the positive ions and the negative electron sea holds the metal together and is strong, which is why most metals have high melting points.
Explaining metal properties
The metallic bonding model explains the everyday properties of metals:
Alloys
A pure metal is often too soft for many uses, because its atoms are all the same size and arranged in regular layers that can slide easily.
The added atoms are usually a different size from the host metal atoms.
Why alloys are harder
This is why steel (iron with carbon and other elements) is much harder and more useful than pure iron, and why most metals we use in everyday life are alloys.
Alloys are designed to give the best mix of properties for a job. Brass (copper and zinc) is harder than pure copper and used for door fittings and instruments; bronze (copper and tin) is hard and resists corrosion; steel is strong and used for buildings, bridges and cars. The trade-off is that alloys are usually slightly poorer conductors than the pure metal, because the irregular arrangement of atoms scatters the moving electrons, but for structural uses the extra hardness is far more important than a small loss of conductivity.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC sample4 marksDescribe metallic bonding and use it to explain why metals are good conductors of electricity and can be bent into shape.Show worked answer →
A Unit 2.1 structured question. Reward: a metal consists of a regular arrangement of positive metal ions surrounded by a sea of delocalised electrons (the outer electrons that are free to move). 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 and carry charge. Metals can be bent and shaped (malleable) because the layers of ions can slide over each other while the delocalised electrons hold the structure together. Markers credit positive ions, a sea of delocalised electrons, free electrons carrying charge, and sliding layers for malleability. A common slip is to say the ions move.
WJEC sample3 marksExplain why an alloy such as steel is harder than pure iron.Show worked answer →
A Unit 2.1 explanation question. Reward: an alloy contains atoms of different sizes mixed in. The different-sized atoms distort the regular layers of metal ions, so the layers can no longer slide over each other easily. This makes the alloy harder than the pure metal. Markers credit the idea that different-sized atoms disrupt the regular arrangement and stop the layers sliding, making the alloy harder. A common error is to say the alloy is harder simply because it contains two metals, without explaining the layers.
Related dot points
- Describe ionic bonding as electron transfer, draw dot-and-cross diagrams for simple ionic compounds, and relate the giant ionic lattice to the properties of ionic compounds.
A focused answer to WJEC GCSE Chemistry topic 2.1, covering ionic bonding as the transfer of electrons between metals and non-metals, drawing dot-and-cross diagrams, the giant ionic lattice, and how the structure explains high melting points and conduction when molten or dissolved.
- Describe covalent bonding as shared pairs of electrons, draw dot-and-cross diagrams for simple molecules, and relate simple molecular structure to low melting points and poor conduction.
A focused answer to WJEC GCSE Chemistry topic 2.1, covering covalent bonding as the sharing of electron pairs between non-metal atoms, drawing dot-and-cross diagrams for simple molecules, and explaining why simple molecular substances have low melting points and do not conduct.
- Describe giant covalent structures including diamond and graphite, and relate their bonding and structure to their very different properties.
A focused answer to WJEC GCSE Chemistry topic 2.1, covering giant covalent structures, the bonding and structure of diamond and graphite, and how these explain their hardness, melting points and electrical conductivity.
- Use the properties of a substance (melting point, conductivity, state) to deduce its type of bonding and structure.
A focused answer to WJEC GCSE Chemistry topic 2.1, bringing ionic, simple molecular, giant covalent and metallic structures together and showing how to deduce the bonding in a substance from its melting point, electrical conductivity and state.
Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) — WJEC (2016)