Describe the general physical properties of metals, explain what an alloy is, and explain why alloys are often harder and more useful than pure metals

What this dot point is asking

The syllabus wants you to describe the general physical properties of metals, to explain what an alloy is, and to explain why alloys are often harder and more useful than pure metals. The key idea is that the atoms in a metal are arranged in regular layers, and disrupting those layers with different atoms makes the metal harder. This connects everyday materials such as steel and brass to a simple picture of atoms.

The answer

General properties of metals

Most metals share a set of physical properties:

• good conductors of electricity and heat,
• shiny (lustrous) when polished,
• malleable, meaning they can be hammered or bent into shape,
• ductile, meaning they can be drawn into wires,
• high melting points and high densities,
• usually strong and sonorous (they ring when struck).

These properties make metals suitable for wiring, tools, cooking pots, and building.

A simple picture of metal structure

In a metal, the atoms are packed in a regular pattern of layers. These layers can slide over one another, which is why pure metals are malleable and can be shaped. The atoms are held together by strong metallic bonding, which gives metals their high melting points.

What an alloy is

An alloy is a mixture of a metal with one or more other elements, often other metals. For example:

• steel is iron mixed with a little carbon,
• brass is copper mixed with zinc,
• bronze is copper mixed with tin.

Why alloys are harder and more useful

In a pure metal, the atoms are all the same size and the layers slide easily, so it is soft. In an alloy, the added atoms are a different size, so they disrupt the regular layers. The layers can no longer slide over each other easily, which makes the alloy harder and stronger than the pure metal. Alloys can also be made more resistant to corrosion or given other useful properties.

Examples in context

Example 1. Steel in construction. Pure iron is too soft and rusts easily, so it is rarely used alone. Adding a little carbon makes steel, which is much harder and stronger and is used for buildings, bridges, and cars. This is the single most important alloy in modern construction.

Example 2. Coins made from alloys. Coins are made from alloys rather than pure metals because they must resist wear from constant handling. The harder alloy keeps the design sharp for years, showing how the hardness gained by alloying is put to practical use.

Try this

Q1. State what is meant by an alloy. [1 mark]

• Cue. An alloy is a mixture of a metal with one or more other elements (often other metals).

Q2. Explain why a pure metal is usually softer than its alloy. [2 marks]

• Cue. In a pure metal the identical atoms form regular layers that slide over each other easily; an alloy has different-sized atoms that disrupt the layers, so it is harder.

Q3. Give one use of a metal and the physical property that makes it suitable. [2 marks]

• Cue. For example, copper is used in wiring because it conducts electricity well, or aluminium is used for cans because it is malleable and low in density.