1. Structure and bonding
page 3
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Why are metals good conductors of heat and electricity?
Metallic bonds are made from a lattice of ions in a 'cloud' of free electrons. These free electrons are responsible for the ability of metals to
  1. conduct electricity
  2. conduct heat especially well.
Interactive graphic of electric circuit
Interactive graphic of electric current in wire
Picture 1.3 Animation showing electrons moving randomly and then the movement of electrons through a wire
1. Electrical conductivity
Electric current is the flow of electrons in a wire. In metals, the outer electrons of the atoms belong to a ‘cloud’ of delocalised electrons. They are no longer firmly held by a specific atom, but instead they can move freely through the lattice of positive metal ions. Normally they move randomly. However, when the wire is connected to a cell, they are pushed away from the negative terminal and drawn to the positive one.

The cloud of electrons drifts through the wire. The drift velocity of the cloud is about 3 mm s-1. The electrons within the cloud are still moving randomly (at much higher speeds) - rather like a swarm of bees leaving a hive.

2. Thermal conducivity
Metals are good conductors of heat. There are two reasons for this:
Interactive graphic of thermal conductivity
Picture 1.4 A cool lattice. If we heat the left hand end, then the energy will be carried along by conduction.
Ionic vibrations
The positive metal ions in a metal structure are packed closely together in a symmetrical geometric arrangement. They don’t move from their position in the lattice but they are constantly vibrating. If a metal is heated, the positive metal ions vibrate more vigorously. These ions collide with neighbouring ions and make them vibrate more vigorously too. In this way, the energy is passed, or conducted, through the metal.

However, metals are particularly good conductors of heat. In general, they are better than ionic compounds which also have strong bonds. So we need another mechanism to explain their especially good conductivity. It is their free electrons.

Interactive graphic of thermal conductivity
Interactive graphic of thermal conductivity
Picture 1.5 How a metal conducts by the movement of free electrons.
You can find out more in the On Your Mettle virtual lab which is part of the Learning Zone on the coruseducation web site.
Free electrons
The ions in the lattice are vibrating . The ions at the hot end of a piece of metal vibrate more. [Note the electrons have been left out of picture 1.5 to keep it clear.]

Let's look at just a few electrons.

  • The electrons at the hot end will speed up – they gain kinetic energy from the vigorously vibrating ions.
  • Some of them will move down to the cooler end and collide with ions that are vibrating less vigorously than those at the hot end.
  • In these collisions, the electrons will lose kinetic energy and make the ions vibrate more vigorously.

In effect, the electrons have carried the vibrational energy from the hot end to the cold end. And, because they are free to move through the lattice, they are able to do this more quickly than the bonds between the ions in the lattice.

Question 1-2.
a) How are the vibrating positive ions within a metal structure able to conduct heat from one end to another?

b) What feature of the structure of metals enable them to conduct electricity?


Summary                   Close
  • metals are good electrical and thermal conductors
  • the free, delocalised electrons contribute to both conductivities
  • heat is also conducted by lattice vibrations