Writing formulas for ionic compounds | 14-16 years

In this activity, ion formula cards help students check, consolidate and demonstrate their ability to write correct formulas for ionic compounds. Students work cooperatively to demonstrate their mastery of the ideas to each other and the teacher. Following teacher validation of one group, the students themselves then progressively validate other groups.

Learning objectives

Students will be able to:

• Recall the names and formulae of common cations and anions.
• Write formulae of ionic compounds.

Sequence of activities

Introduction

1. Give each student a set of ’traffic light’ cards.
2. Ask them to indicate whether they know the names of the ions Na⁺, Fe²⁺, NO^3-, SO₄^2-, S^2-:
   
   • Green if they think that they know.
   • Red if they do not know.
   • Yellow if they are unsure.
3. Explain that they are going to:
   • Work in groups.
   • Check and demonstrate that they can associate the names and formulas of common ions.
   • Show that they can use the formulas of ions to construct formulas of ionic compounds.

Activity: stage 1

Give each student a mini whiteboard and an ‘Ion formulas’ sheet. Ask them to:

1. Write, on the whiteboard, the formulae of sodium bromide, potassium nitrate, calcium sulfate and sodium hydrogencarbonate.
2. Turn the board over.

Activity: stage 2

1. Organise students into groups of four and ask them to complete their ’Ion formulas’ sheets by writing the names of ions in appropriate places.
2. When one group believes it has completed this task successfully, check their work. If successful, designate them an ’Expert Group’. Authorise these group members to check the work of other groups.
3. Allow the checking to cascade.

Activity: stage 3

1. Give successful groups one set of ’Ion formula cards’ and a ’Formulas for ionic compounds’ sheet for each student. Ask them to:
   • Arrange the cards to form the formulas of the ionic compounds on their sheet.
   • Devise and ask questions of each other so that they can demonstrate that they can write down the formulas of ionic compounds (this could be in pairs).
2. When one group believes it is competent, check their work using random questions. If five questions are correctly answered, designate them an ‘Expert Group’. Authorise these group members to check the work of other groups and to designate them as an ‘Expert Group’.
3. Allow the checking to cascade.

Activity: stage 4

With the whole class, ask students to:

1. Write on their whiteboards, the formulae of sodium bromide, potassium nitrate, calcium sulfate and sodium hydrogencarbonate.
2. Compare this with what they wrote at the beginning of the session.
3. Write in an evaluation of their progress on the ’Formulas for ionic compounds’ sheet.

Commentary

The ’traffic light’ cards activity gives the learning objectives a personal relevance for the students.

In the two main activities, the focus is on the whole group rather than individuals. For the whole group to succeed, students will support and assess each other.

The element of personal evaluation, using the mini whiteboards at the start and end, promotes student confidence that they can improve.

Equipment

For each student:

• Set of ‘traffic light’ cards
• Mini whiteboard

Answers

The formulas featured in the ‘Formulas for ionic compounds’ sheet are:

• Sodium carbonate – Na₂CO₃
• Silver nitrate – AgNO₃
• Calcium bromide – CaBr₂
• Copper hydroxide – Cu(OH)₂
• Iron(II) nitrate – Fe(NO₃)₂
• Iron(III) iodide – FeI3
• Lead sulfate – PbSO₄
• Zinc nitrate – Zn(NO₃)₂
• Potassium sulfate – K₂SO₄
• Magnesium sulfide – MgS
• Aluminium hydroxide – Al(OH)₃
• Ammonium chloride – AlCl₃
• Sodium hydrogen carbonate – NaHCO₃
• Iron(III) carbonate – Fe2(CO₃)₃