Metallic bonding and the structure of iron | 14–16 years

In this activity, learners decide whether a series of statements about the bonding in iron are true or false. This leads to further thinking and discussion about the bonding in metals. This resource is from the Assessment for learning series, which contains lesson plans and associated resources to actively involve students in their learning.

This activity checks on the misconceptions that:

• The structure of iron is an example of a giant molecule.
• The atoms of iron are held together by ionic bonds.
• Iron conducts electricity because iron atoms move through the solid.
• Iron expands when heated because the atoms get bigger.
• Iron metal is silver because iron atoms are silver.

Learning objectives

• Describe the bonding in iron.
• Explain the properties of iron using ideas about bonding.

Teaching sequence

Introduction (slide 2)

1. Give each student a nail or a paper clip to look at and feel as a stimulus to focus their thinking.
2. Share with the learners that they will:
   • Describe the bonding in iron.
   • Explain the properties of the metal using their ideas about bonding.

Activity: stage 1 (slide 3)

Give each learner a ’Metallic bonding and the structure of iron question sheet’ (MS Word | PDF) which has 10 statements about the structure and properties of iron. Ask them to work individually to:

1. Look at and feel their object.
2. Read each statement carefully.
3. Write down in the table whether they think that the statements are true or false.

Activity: stage 2 (slide 4)

Arrange learners into groups of three to:

1. Look at each statement in turn.
2. Discuss the answers they gave individually and share ideas.
3. Decide on a group answer and record this on the table.
4. Be ready to share ideas and how they might have changed. 

Activity: stage 3 (slides 5–9)

Issue mini whiteboards and:

1. Ask each learner to draw a picture to represent the bonding in iron.
2. Ask some learners to share their ideas.

Show the models of the structure of iron and highlight these key features:

• The outer shell of electrons become delocalised and are free to move through the giant lattice of positive ions.
• The electrons are sometimes shown as a ‘cloud’ and referred to as a ‘sea of delocalised electrons’.
• These models can explain the physical properties of metals: conduction of electricity and heat, expansion on heating, malleability and ductility.
• Extend ideas about bonding (and physical properties) to metals in general.

Reflection (slides 10–19)

Go through the answers in the lesson slides (PPT | PDF). Demonstrate these ideas and help to challenge any misconceptions using the animations and links to YouTube videos in the presentation.

Commentary

• Use a tactile stimulus to help learners appreciate the objectives for the session.
• Working in groups, students compare their ideas with others, re-evaluate their initial ideas and develop them where appropriate.
• Probing questions are fundamental to the process of ensuring that misconceptions are dispelled and that a correct model takes their place.
• Throughout the tasks, learners are prompted to check their understanding.

Equipment

For each learner:

• Nail or paper clip
• Mini whiteboard

Scaffolding

• Think carefully about pairings and groupings for this task to ensure learners work effectively to develop their ideas.
• Move around the groups through the task to probe understanding and offer support where needed.
• When asking learners to draw a diagram to represent metallic bonding, provide learners with prompts where needed, such as ‘you must include: positive metal ions, delocalised electrons’.
• Pictures, videos and animations are helpful. Some are provided in the lesson slides (PPT | PDF).

Answers and notes on responses

1. Iron has a type of bonding called metallic bonding.
   • TRUE. Iron is a metal and all metals have a type of bonding called metallic bonding which is different from covalent or ionic bonding. In metallic bonding the outer shells of adjacent atoms overlap, and the outer shell electrons are free to move through the lattice. The metal consists of metal cations and a balancing number of these ‘free’ electrons.
2. The structure of iron is an example of a giant molecule.
   • FALSE. In iron the positive ions are packed together in a giant lattice but we use the word molecule to imply that the structure contains covalent bonds which iron does not.
3. In the structure of iron there are positive ions.
   • TRUE. Because the electrons in the outer shell are free to move through the lattice they leave behind positive iron ions.
4. The atoms in iron are held together by ionic bonds.
   • FALSE. The bonding in iron is metallic bonding. This is different from ionic bonding because there are no negative anions present.
5. In the structure of iron, some electrons can move round the solid.
   • TRUE. The electrons from the outer shell of atoms are free to move through the lattice of positive ions.
6. If iron is heated to a very high temperature it would become a gas.
   • TRUE. If iron is heated it will melt. If molten iron is heated to a sufficiently high temperature it will boil.
7. Iron can conduct electricity because iron atoms can slip over their neighbours and move through the solid.
   • FALSE. The iron cations are normally fixed in their lattice positions and cannot move about.
8. Iron conducts electricity because it contains a ‘sea’ of electrons.
   • TRUE. The electrons from the outer shells of atoms are able to move about and will pass through the metal when it is connected to a battery.
9. Iron expands when it is heated because iron atoms get bigger.
   • FALSE. When iron is heated the positive cations vibrate more and move a little further apart.
10. Iron is a silvery grey metal because iron atoms are silvery grey.
    • FALSE. The colour of iron is a property of the arrangement of cations and electrons. A single atom would not have a colour.