Year 9 Science · Unit 2 · Lesson 07

Valency and Ion Formation

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Learning Goals

Predict the ion an element forms from its electron configuration and group.
Identify ions from their numbers of protons and electrons.
Write and check ionic formulas, including ones with 3+ and 3− ions.

Warm Up

1. Define a cation and an anion, and give one example of each.

Recall 2 marks

Order the steps

The steps below explain how a sodium atom becomes a sodium ion, but they are jumbled. Number them 1 to 5 in the correct order.

Order	Step
	Sodium loses its outer-shell electron.
	It now has 11 protons and 10 electrons, so it is the ion Na⁺, with the configuration 2,8, like neon.
	Notice that the outer shell holds only 1 electron.
	Write sodium’s electron configuration: 2,8,1.
	Decide that losing 1 electron is easier than gaining 7.

Your Turn

Ion formation table

Complete the three empty columns for each element.

Element	Configuration	Group	Lost or gained	Ion symbol
Sodium	2,8,1
Magnesium	2,8,2
Aluminium	2,8,3
Oxygen	2,6
Sulfur	2,8,6
Chlorine	2,8,7

2. Describe the pattern between an element’s group and the charge on its ion.

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3. Magnesium and sulfur react. Explain how electrons move between the atoms, and write the formula of the compound formed.

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4. Why does sulfur gain 2 electrons rather than lose 6 to reach a full outer shell?

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Go Deeper

Data: five mystery particles

In a card game, each card shows just two numbers for a particle. Players must name the particle.

Table 1. The five cards

Particle	V	W	X	Y	Z
Protons	13	8	19	16	18
Electrons	10	10	18	18	18

Source: each card matches a real particle. Use a periodic table.

5. Identify each particle. Write its symbol, with a charge if it has one.

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6. Three of the particles have the same electron configuration, 2,8,8. Which three? Explain why only one of them is an atom.

Analyse 2 marks

7. Write the formula of the compound formed by particles V and W, and of the compound formed by X and Y. Show a charge check for each.

Analyse 3 marks

Show What You Know

8. Calcium burns in nitrogen to form calcium nitride. A student writes its formula as CaN. Use the configurations of calcium (2,8,8,2) and nitrogen (2,5) to explain the error and work out the correct formula.

Analyse 5 marks