Properties of metals and alloys — AQA GCSE Combined Science
Test yourself on Properties of metals and alloys with AQA GCSE practice questions.
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Properties of metals and alloys explained
Metals are giant structures in which positive metal ions are arranged in a regular lattice and are surrounded by a sea of delocalised electrons.
Read the full explanation
The electrostatic attraction between the positive ions and the shared delocalised electrons is metallic bonding, and it acts in all directions throughout the structure. Because this bonding is strong and extends across the whole giant lattice, a large amount of energy is needed to overcome it, so most metals have high melting and boiling points. For example, magnesium melts at about 650 °C and iron at about 1538 °C. Melting point trends can be linked to the strength of metallic bonding: metals with more delocalised electrons per atom and smaller ions often have stronger bonding and higher melting points.
In pure metals, atoms are arranged in layers, which allows metals to be bent and shaped. Pure metals are too soft for many uses and so are mixed with other metals to make alloys which are harder.
Pure metals contain atoms of one element only, packed in regular layers. Because all atoms are the same size, a layer can slide over the one below when a force is applied, so the metal bends or is shaped rather than cracking. This explains malleability and ductility. However, the same easy sliding makes pure metals soft, so they wear or dent in demanding uses. Mixing a pure metal with other metals produces an alloy: the different-sized atoms disrupt the regular layers, so layers no longer slide easily and the alloy is harder. For example, pure iron is soft, but adding carbon and other elements makes steel, used for tools, bridges and car bodies.
Students should be able to explain why alloys are harder than pure metals in terms of distortion of the layers of atoms in the structure of a pure metal.
In a pure metal, all atoms are the same size and sit in regular layers. When a force is applied, whole layers can slide over one another, so the metal bends easily and feels soft. In an alloy, atoms of a different element are mixed in. These atoms have a different size from the metal atoms, so they distort the regular layers. The distorted layers cannot slide over one another as easily, so the alloy resists bending and is harder than the pure metal. For example, adding carbon atoms to iron distorts the iron layers and produces steel, which is harder than pure iron and suitable for tools and structures.
Your focus
- Describe the arrangement of positive ions and delocalised electrons in a metal.
- Explain why most metals have high melting and boiling points in terms of strong metallic bonding.
- Apply the model of metallic bonding to compare the melting points of different metals or to contrast metals with simple molecular substances.
Show all 9 objectives
- Describe the layered arrangement of atoms in a pure metal and link it to bending and shaping.
- Explain why pure metals are often too soft for particular uses.
- Explain how forming an alloy changes the structure and makes the material harder.
- Describe the regular layered structure of a pure metal.
- Explain how different-sized atoms in an alloy distort the layers.
- Relate the distortion of layers to the increased hardness of alloys compared with pure metals.
Properties of metals and alloys exam tips
Marking Points
- Describe a metal as a giant lattice of positive ions with delocalised electrons throughout the structure.
- Explain metallic bonding as the strong electrostatic attraction between positive metal ions and delocalised electrons.
- Link the strength and extent of metallic bonding to the large energy needed to melt or boil the metal.
- State that most metals therefore have high melting and boiling points, and use a named example such as iron or magnesium.
- Compare metals with simple molecular substances, where weak intermolecular forces mean low melting and boiling points.
- Recognise that the same delocalised electrons explain why metals conduct electricity and heat.
- Pure metals contain atoms of a single element arranged in regular layers.
- Layers of identical atoms can slide over one another when a force is applied, so the metal can be bent and shaped.
- This sliding explains why pure metals are malleable and ductile.
- Pure metals are too soft for many uses because the layers slide too easily.
- An alloy is a mixture of a metal with other elements, often other metals.
- In an alloy, atoms of different sizes disrupt the regular layers, so the layers cannot slide as easily.
- Disrupted sliding makes the alloy harder than the pure metal.
- A named example, such as steel from iron with carbon, can illustrate the idea.
- Pure metals have atoms of the same size arranged in regular layers.
- In a pure metal, layers can slide over one another when a force is applied.
- An alloy contains atoms of a different element mixed into the metal structure.
- The different-sized atoms distort the regular layers of the pure metal.
- Distorted layers cannot slide over one another as easily.
- Less easy sliding means the alloy is harder than the pure metal.
- A named example, such as carbon in iron to make steel, can support the explanation.
Examiner Tips
- 💡Always name the two charged or electron parts involved in metallic bonding, then state that the attraction is strong and acts throughout the giant structure.
- 💡When explaining a high melting point, refer to energy needed to overcome the strong metallic bonds, not to breaking individual atoms.
- 💡Use a specific metal and its approximate melting point as evidence, and keep the link between bonding strength and energy explicit.
- 💡Use the phrase 'layers of atoms slide over each other' when explaining bending or shaping of a pure metal.
- 💡When comparing hardness, link the alloy's different-sized atoms directly to the disruption of the layers.
- 💡Include one named alloy and its use, such as steel for tools or bridges, to support your explanation.
- 💡Start by describing the regular layers in the pure metal before introducing the alloy atoms.
- 💡Use the word 'distort' when explaining how different-sized atoms affect the layers.
- 💡Finish by linking the distorted layers to reduced sliding and increased hardness.
Common Mistakes
- Saying metallic bonding is between atoms only; correction: describe positive metal ions in a lattice with delocalised electrons, and the attraction between them.
- Claiming all metals have equally high melting points; correction: most are high, but the strength of metallic bonding varies, so values differ, for example mercury is liquid at room temperature.
- Confusing delocalised electrons with shared pairs in covalent bonds; correction: in metallic bonding the electrons are delocalised across the whole giant structure, not shared between two specific atoms.
- Saying that atoms in a pure metal are randomly arranged: correct this by stating that pure metal atoms form regular layers.
- Claiming that alloy atoms are larger than metal atoms in every case: correct this by saying the added atoms are different in size, which may be larger or smaller.
- Stating that alloys are harder because they contain stronger bonds: correct this by explaining that different-sized atoms distort the layers and prevent them sliding.
- Saying that alloy atoms make the layers stronger by forming stronger bonds: correct this by explaining that the different-sized atoms distort the layers and stop them sliding.
- Describing alloy atoms as fitting neatly into the layers without changing them: correct this by stating that the different-sized atoms distort the regular arrangement.
- Explaining hardness only by saying the alloy is denser: correct this by linking hardness to the disruption of layer sliding.