0620 · 2.7Metallic bonding

Metallic bonding structure

0:00A copper wire carries electric current, yet the same metal can be bent into a different shape. Both properties come from the arrangement of particles inside the metal. This whole topic is Supplement content for the Extended paper. We will connect metallic bonding to electrical conductivity, malleability and ductility. The starting point is electrical charge. Electrons have negative charge, and positive charges attract negative charges. This attraction between opposite charges is called electrostatic attraction.

0:26A neutral atom has equal numbers of protons and electrons, while removing electrons leaves a positive ion. Inside a metal, particles form a giant metallic lattice. A lattice is a regular arrangement that repeats throughout the structure. Giant means that this arrangement extends through the metal, rather than forming separate small molecules. Our diagram shows only a small section of a structure containing enormous numbers of particles. In this structure, outer electrons are delocalised.

0:54Delocalised means that these electrons are not attached to one particular atom and can move throughout the metal. The remaining particles are positive metal ions. These ions still contain electrons, but their positive charge comes from having more protons than electrons. The delocalised electrons are often described as a sea of electrons. This is a picture of electrons spread throughout the structure, not a literal liquid. They occupy spaces throughout the lattice rather than sitting in a separate layer outside it.

1:20The positive ions are surrounded by this shared population of negative electrons. Metallic bonding is the electrostatic attraction between the positive ions in a giant metallic lattice and a sea of delocalised electrons. The opposite charges explain why the particles stay together. Positive ions attract the negative electrons throughout the structure. That attraction holds the metal together as one continuous material. Notice that the whole piece of metal is still electrically neutral.

1:47The electrons have become delocalised, not disappeared from the material. Their total negative charge balances the total positive charge of the ions. Having positive ions inside a metal therefore does not mean that the metal itself has an overall positive charge.

Electrical conductivity in metals

2:01An electric current involves the movement of electrical charge. Metals contain charged particles that can move through their structure: the delocalised electrons. When a metal forms part of a complete circuit, these electrons have an overall movement through the wire. They carry charge, which explains the metal’s good electrical conductivity. The electrons do not need to be freed by melting the metal first. They are already able to move through the solid structure.

2:25The positive ions remain in their lattice positions, although they vibrate. So, when explaining conduction in a solid metal, identify the electrons as the moving charge carriers. Saying that a metal contains electrons is not enough to explain conductivity. Other materials also contain electrons, but those electrons may be held in particular places. The important feature here is that the electrons are delocalised and mobile. Their ability to move through the structure connects metallic bonding to electrical conduction.

Malleability and ductility

2:54Metals can also change shape without immediately breaking. Malleable means that a material can be hammered or pressed into shape. A ductile material, on the other hand, can be drawn out into a wire. These are different ways of reshaping a material, but metallic bonding explains both properties. When a force is applied, layers of positive ions can slide over each other. Their positions change, but the ions remain surrounded by delocalised electrons.

3:19The electrostatic attraction between the ions and the electron sea remains. The metal can therefore change shape while its particles are still held together. Think of the electron sea as providing attraction throughout the structure. An ion does not depend on one fixed neighbouring ion to hold it in place. After a layer shifts, attraction to the surrounding electrons still holds the structure together. This explains why sliding does not automatically cause the metal to fall apart.

3:44One common mistake is to say that malleability happens because all the metallic bonds break. That would not explain why the reshaped metal remains in one piece. Another mistake is to describe layers of electrons sliding. The layers are formed by positive ions, while the delocalised electrons maintain attraction throughout the changing structure. Consider a worked example involving a copper wire that is bent but remains unbroken. We need to explain both its change of shape and its continued conductivity.

4:11Start by identifying what changes inside the wire. Layers of positive ions move into different positions as the metal bends. Next, identify what continues to hold the structure together. Attraction between the positive ions and the delocalised electrons remains as the layers slide. Finally, identify the particles that carry charge. The electrons can still move through the continuous metal structure, so bending does not remove its ability to conduct electricity.

4:36In an examination, you might be asked to describe metallic bonding and explain why a metal is ductile and conducts electricity. Describe means give the relevant features, while explain means connect those features to the properties. Begin with the structure, then identify either electron movement or ion-layer movement. Finish by stating how that movement produces the property. Metallic bonding is the electrostatic attraction between positive ions in a giant metallic lattice and a sea of delocalised electrons.

5:04A metal conducts electricity because its delocalised electrons move through the structure and carry charge. A metal is ductile because layers of positive ions can slide over each other. Attraction between the ions and the electron sea remains as the metal is drawn into a wire. The central idea is one structure explaining several properties. Attraction between positive ions and delocalised electrons holds the metal together. Mobile electrons explain good electrical conductivity.

5:30Sliding ion layers, with attraction maintained, explain malleability and ductility. Keep those two kinds of movement separate, and the explanations follow naturally from the structure.