Properties of metals and alloys — AQA GCSE Chemistry
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Properties of metals and alloys explained
Properties of metals and alloys covers 5 specification points, including Metals have giant structures of atoms with… and This means that most metals have high melting….
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Metals have giant structures of atoms with strong metallic bonding.
Metals are giant structures because their atoms are packed together in enormous numbers. Each atom contributes its outer electrons to a shared pool of delocalised electrons. The positive metal ions sit in a regular lattice and are held to this electron sea by strong electrostatic attraction, known as metallic bonding. Because the bonding extends throughout the whole piece of metal, there are no weak edges or small molecules to pull apart. For example, in a piece of copper, countless Cu atoms are arranged in a lattice with delocalised electrons moving between them. The strength of the bonding explains why metals are generally strong, while the delocalised electrons allow them to conduct electricity.
This means that most metals have high melting and boiling points.
Melting and boiling require energy to overcome the forces holding particles together. In a metal, the particles are positive ions held by strong metallic bonds to a sea of delocalised electrons. Because these bonds act throughout a giant lattice, a large amount of energy is needed to break them. This is why most metals have high melting and boiling points. For example, magnesium melts at about 650 °C and iron at about 1538 °C, both much higher than the melting point of a simple molecular substance such as iodine. The stronger the metallic bonding and the more electrons donated per atom, the higher the melting point tends to be. Metals with weaker metallic bonding, such as mercury, are liquid at room temperature.
In pure metals, atoms are arranged in layers, which allows metals to be bent and shaped.
In a pure metal, the positive ions are arranged in regular layers, like sheets stacked on top of one another. When a force is applied, these layers can slide over each other without the metallic bonding breaking completely, because the delocalised electrons move with the ions and continue to hold the structure together. This means the metal can be bent, hammered or pressed into shape without cracking. This property is described as being malleable, and metals can also be drawn into wires, which is ductility. For example, copper can be hammered into thin sheets and gold can be beaten into very thin leaf. The regular layered arrangement is the key reason pure metals are malleable and ductile.
Pure metals are too soft for many uses and so are mixed with other metals to make alloys which are harder.
Pure metals have regular layers that slide easily, so they are often too soft for practical uses such as tools, aircraft parts or coins. To make a metal harder, it is mixed with one or more other metals to form an alloy. The atoms of the different metals have different sizes, so they disrupt the regular layered arrangement. This prevents the layers from sliding over each other as easily, making the alloy harder than the pure metal. For example, pure gold is very soft, but gold jewellery is usually alloyed with copper or silver to make it harder and more durable. Steel is an alloy of iron with carbon and sometimes other elements, and it is much harder than pure iron.
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.
Pure metals have a regular giant lattice in which identical atoms are arranged in layers. Because the atoms are all the same size, the layers can slide over one another when a force is applied, so the metal is soft and can be bent or shaped. An alloy is a mixture of a metal with other elements, often other metals. The atoms of the added element are a different size from the atoms of the main metal. When these differently sized atoms sit in the lattice, they disrupt the regular arrangement and distort the layers. The distorted layers cannot slide over one another as easily, so the alloy is harder and stronger than the pure metal. For example, pure iron is relatively soft, but adding carbon and other elements produces steel, which is harder and more suitable for tools, bridges and car bodies.
Your focus
- Describe the structure of a metal as a giant lattice of positive ions in a sea of delocalised electrons.
- Explain metallic bonding as the strong electrostatic attraction between positive metal ions and delocalised electrons.
- Relate the giant nature of the metallic structure to at least one physical property of metals.
Show all 15 objectives
- Explain why most metals have high melting and boiling points in terms of strong metallic bonding.
- Use data to compare the melting points of metals with those of simple molecular substances.
- Describe how the strength of metallic bonding varies between different metals.
- Describe the layered arrangement of atoms in a pure metal.
- Explain how the sliding of layers allows pure metals to be bent and shaped.
- Distinguish between malleable and ductile properties using correct terminology.
- Define an alloy as a mixture of a metal with other elements.
- Explain why alloys are harder than pure metals in terms of different-sized atoms disrupting layers.
- Give examples of alloys and explain why they are used instead of pure metals.
- Describe the layered arrangement of atoms in 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.
Properties of metals and alloys exam tips
Marking Points
- Metals form giant structures, meaning very large numbers of atoms are joined together in one continuous lattice rather than in small molecules.
- Each metal atom releases one or more outer electrons, producing a sea of delocalised electrons and positive metal ions.
- Metallic bonding is the strong electrostatic attraction between the positive metal ions and the delocalised electrons.
- The bonding acts in all directions throughout the giant structure, so there are no weak intermolecular forces between separate metal molecules.
- A correct example, such as copper or magnesium, can be used to show a lattice of positive ions surrounded by delocalised electrons.
- Melting and boiling involve overcoming the strong metallic bonds between positive metal ions and delocalised electrons.
- The giant lattice means many strong bonds must be broken, so a large amount of energy is required.
- Most metals therefore have high melting and boiling points compared with simple molecular substances.
- The strength of metallic bonding varies between metals, so melting points vary; for example, mercury is liquid at room temperature while iron has a very high melting point.
- A valid comparison, such as magnesium melting at about 650 °C versus iodine melting at about 114 °C, supports the explanation.
- In a pure metal, the positive ions are arranged in regular layers or rows.
- When a force is applied, layers of ions can slide over one another.
- The metallic bonding is not broken by this sliding because the delocalised electrons move with the ions and maintain the attraction.
- This explains why pure metals are malleable, meaning they can be bent or hammered into shape, and ductile, meaning they can be drawn into wires.
- A correct example, such as copper being hammered into sheets or gold being beaten into leaf, illustrates the property.
- Pure metals are often too soft for many uses because their regular layers slide easily.
- An alloy is a mixture of a metal with one or more other elements, usually other metals.
- The atoms of the added element have a different size from the atoms of the main metal, which distorts the regular layers.
- Distorted layers cannot slide over each other as easily, so the alloy is harder than the pure metal.
- A correct example, such as steel being harder than pure iron or gold jewellery being alloyed for strength, supports the explanation.
- Pure metals contain layers of identical atoms that can slide over one another, making the metal soft.
- An alloy contains atoms of at least one different element, and these atoms have a different size from the atoms of the main metal.
- The differently sized atoms distort the regular layers in the metal lattice.
- Distorted layers cannot slide over one another as easily, so the alloy is harder than the pure metal.
- A named example, such as steel being harder than pure iron, can support the explanation.
Examiner Tips
- 💡Use the phrase delocalised electrons rather than free electrons when explaining metallic bonding.
- 💡Link the giant structure directly to a property, such as a high melting point or electrical conductivity, to show understanding.
- 💡When drawing a diagram, label the positive metal ions and the delocalised electrons clearly so the examiner can see both parts of the model.
- 💡Always name the force being overcome when explaining a melting or boiling point.
- 💡Use a comparison with a simple molecular substance to make the high value meaningful.
- 💡If a data table is given, quote a specific melting point to support your explanation rather than saying it is high.
- 💡Use the words layers and slide when explaining why pure metals can be shaped.
- 💡Mention that the delocalised electrons move with the ions so the bonding is maintained during sliding.
- 💡Give a named example of a metal being shaped to make the explanation concrete.
- 💡Define an alloy as a mixture before explaining how it changes properties.
- 💡Use the idea of different-sized atoms disrupting layers as the key explanation for increased hardness.
- 💡Give a named alloy and its use, such as steel in tools or gold alloy in jewellery, to show the link between property and purpose.
- 💡Use the phrase layers of atoms and explain that they slide in a pure metal before contrasting this with an alloy.
- 💡Refer explicitly to atoms of a different size causing distortion, rather than saying the structure is just different.
- 💡Link the distortion directly to the consequence: the layers cannot slide, so the alloy is harder.
Common Mistakes
- Saying that metal atoms share pairs of electrons in the same way as covalent bonding; the correction is that outer electrons become delocalised and are shared across the whole lattice.
- Describing metallic bonding as positive ions attracting negative ions; the correction is that the attraction is between positive metal ions and delocalised electrons.
- Treating a metal as a collection of separate molecules with weak forces between them; the correction is that the whole piece is one giant structure with strong bonding throughout.
- Saying that melting breaks covalent bonds inside metal molecules; the correction is that metallic bonds between ions and delocalised electrons are overcome.
- Claiming that all metals have equally high melting points; the correction is that the strength of metallic bonding differs, so values range widely.
- Stating that melting point depends on the number of delocalised electrons only; the correction is that ion charge, ion size and lattice arrangement also affect bond strength.
- Saying that layers of atoms break apart when a metal is bent; the correction is that the layers slide while the metallic bonding continues to hold the structure together.
- Confusing malleable with ductile; the correction is that malleable means can be hammered or bent into shape, while ductile means can be drawn into wires.
- Stating that delocalised electrons stop the layers from moving; the correction is that the electrons move with the ions and allow the layers to slide while maintaining bonding.
- Saying that alloys are compounds of metals; the correction is that alloys are mixtures, so the elements are not chemically combined in fixed ratios.
- Stating that alloying makes a metal softer; the correction is that different-sized atoms disrupt the layers and make the alloy harder.
- Claiming that alloy atoms fit perfectly into the layers; the correction is that their different sizes distort the regular arrangement and prevent easy sliding.
- Saying that alloys are harder because the added atoms are heavier; the key point is the difference in atomic size causing distortion, not mass.
- Stating that alloys have stronger bonds between atoms; the explanation concerns the disruption of layer sliding, not a change in bond strength.
- Describing the layers as melting or breaking; the layers are distorted and prevented from sliding, not destroyed.