Structure and Properties » Properties of Metals and Alloys

Why metals have high melting and boiling points

Remember, a metal is a giant structure of positive ions surrounded by a sea of delocalised electrons. The strong attraction between them is metallic bonding.

The spec puts it simply: metals have giant structures of atoms with strong metallic bonding. This means that most metals have high melting and boiling points.

Here is the chain of reasoning you need:

• The structure is giant, so there are a huge number of metallic bonds.
• Each of these bonds is strong.
• To melt or boil the metal, many of these strong bonds must be overcome.
• That takes a lot of energy, so the melting and boiling points are high.

For example, iron melts at about 1,538 °C and tungsten at about 3,422 °C. That is why tungsten was used for the glowing wire in old-style light bulbs.

Notice the word most. Mercury is a metal, but it is a liquid at room temperature because it melts at about −39 °C. So in an exam, say "most metals", not "all metals".

Layers of atoms: why pure metals bend

In a pure metal, all the atoms are the same element, so they are all the same size. They pack together neatly in layers.

When you push hard on a pure metal, the layers can slide over each other. The metallic bonding is not broken, because the delocalised electrons can move with the layers and keep holding everything together. So the metal changes shape instead of shattering.

This is why metals can be bent and shaped. Remember, this is what we mean when we say metals are malleable. It lets us hammer metal into sheets, press it into car body panels or bend it into pipes.

Key terms:

• Pure metal: a metal made of atoms of only one element.
• Alloy: a mixture of a metal with one or more other elements, usually other metals.

But the same feature has a downside. Because the layers slide so easily, pure metals are too soft for many uses. Pure iron, copper and aluminium are all fairly soft. A bridge, a tool or a coin made from a soft pure metal would bend, dent or wear away.

Alloys: why they are harder

To fix this, pure metals are mixed with other metals to make alloys, which are harder. This is the explanation the spec says you must be able to give.

Think of a pile of paper sheets. A neat stack slides apart easily if you push the top. Now slip a few marbles between the sheets. The sheets are bumpy and jam against each other, so they are much harder to slide. The different-sized atoms in an alloy act like those marbles.

The added atoms can be bigger or smaller than the main metal atoms. Either way, they disrupt the regular layers.

You will meet named alloys and what they are used for in the lesson Alloys as Useful Materials.

Metals as conductors of electricity

Metals are good conductors of electricity. The reason is the delocalised electrons. They are free to move through the whole structure, so they can carry electrical charge through the metal.

When a metal wire is connected to a cell, the delocalised electrons flow through it. That flow of charge is an electric current. This is why the pins of a plug are made of metal.

Notice it is the electrons that move, not the ions. The positive metal ions stay in their fixed positions in the structure.

Metals as conductors of thermal energy

Metals are also good conductors of thermal energy (heat). Again, the delocalised electrons are the key: energy is transferred by the delocalised electrons as they move through the metal.

So when one end of a metal object is heated, energy quickly reaches the other end. This is why saucepans are made of metal: thermal energy passes quickly from the hob to the food. It is also why a metal spoon left in hot soup soon feels hot.

Exam-style question

Pure aluminium is too soft for making aircraft parts, so an alloy of aluminium is used instead.

(a) Explain why the alloy is harder than pure aluminium. [3 marks]

(b) Explain why aluminium conducts electricity. [2 marks]