Metallic bonding and the structure of iron

In this activity, students 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 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

Students will be able to:

• Describe the bonding in iron and use this to explain the physical properties of the metal.

Sequence of activities

Introduction

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 students that they are going to:
   1. Describe the bonding in iron.
   2. Explain the properties of the metal using their ideas about bonding.

Activity: stage 1

Give each student a ’Metallic bonding and the structure of iron question sheet’ which has 10 statements about the structure and properties of iron. Ask them to work individually to:

1. Read the questions.
2. Record in the table whether they think that the statements are true or false.

Activity: stage 2

Arrange students into groups of three to:

1. Look at each question in turn.
2. Share their responses and ideas.
3. Agree a group answer.
4. Record each answer.
5. Write down, individually, how they have modified their ideas as a result of the discussion.

Activity: stage 3

Issue mini whiteboards and ask:

1. Each student to draw a picture to represent the bonding in iron.
2. Some of the students to share their ideas.

Show an electronic animation, using a data projector, of the structure of a metal and what happens to it when heated or when connected to a battery. Pose questions to:

• Make explicit a picture of metallic bonding in terms of the outer shell of electrons being free to move through a giant lattice of positive ions.
• Make explicit the use of this model of metallic bonding in explaining physical properties including conduction of electricity, expansion on heating and appearance.
• Extend ideas about bonding (and hence their physical properties) to metals in general.

Before finishing

Provide an opportunity for students to:

1. Add to what is already on their ‘Metallic bonding and the structure of iron question sheet’.
2. Write in their name.

Feedback

Take in the sheets and comment on how their ideas have developed and draw attention to ways for the student to develop further.

Commentary

A tactile stimulus helps students to 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, students are prompted to check their understanding.

The final check at the feedback stage is coupled with an indication of the next steps that the student should take.

Equipment

For the demonstration (if possible):

• Electronic animation showing the structure of a metal and what happens to it when the metal is heated, or when it is connected to a battery.
• Data projector.

For each student:

• Nail or paper clip.
• Mini whiteboard.

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.