Noble Gases (Group 18)

Helium can lift a balloon; argon can protect a hot light-bulb filament. Both are very unreactive, but unreactivity alone does not explain either choice. Start with their electron arrangements, then match each use to the property it needs.

Group 18: very unreactive gases

The noble gases are the Group 18 elements. At room temperature they are colourless gases made of separate atoms. Monatomic means that each particle is one atom: helium is He, not He₂; neon is Ne, not Ne₂.

A stable outer electron arrangement

In the school model, these are full outer shells. Noble gases have little tendency to lose, gain or share electrons in ordinary chemical reactions, so they are very unreactive, also called inert. Helium follows the same stability pattern with two outer electrons because its outer shell is the first shell. It does not need eight.

Why single atoms?

Neon atoms do not normally form covalent bonds with each other. Their stable arrangements do not favour the electron sharing that would make Ne₂. The gas therefore consists of separate atoms, rather than diatomic molecules.

Use very unreactive, rather than “never reacts”. Some heavier noble gases can form compounds under particular conditions. You do not need those reactions to explain helium, neon and argon’s uses here.

Explain a use through a relevant property

A colourless gas can emit coloured light. Neon glows reddish-orange in a discharge tube when electrically excited. This is different from an ordinary filament bulb, where the hot filament emits the light and argon provides the protective atmosphere. Unreactivity does not prevent light emission.

Check that the property answers the question

• Why does helium help a balloon rise? It is less dense than air.
• Why is helium preferred to hydrogen where fire is a concern? It is non-flammable. Both gases are less dense than air.
• Why does argon protect a filament? It is unreactive and replaces the air that would allow oxidation.
• Why is helium stable with only two electrons? Its first shell is full.

Write the whole property–use link

Name the gas, state the relevant property and explain its effect. “Argon is used in a bulb because it is a noble gas” is incomplete; “argon is unreactive, so it protects the hot filament from oxidation” gives the chemical reason.

Worked examples

Modelled example 1

Explain helium’s stability

Core

Problem

Helium has only two electrons. Explain why it is very unreactive.

Study the worked solution

1. Identify the occupied shell

   Method

   Place both electrons in helium’s first shell.

   Reason

   The first electron shell is also helium’s outer shell.

   Working

   Helium electron configuration: 2.

2. Test whether the shell is full

   Method

   Compare two electrons with the first-shell capacity.

   Reason

   The first shell holds a maximum of two electrons, not eight.

   Working

   2 of 2 positions occupied → full duplet.

3. Connect configuration to reactivity

   Method

   State that helium does not tend to lose, gain or share electrons.

   Reason

   Its outer shell is already full and stable.

   Working

   Full first shell → helium is very unreactive.

Guided practice 2

Predict monatomic behaviour

About 5 min

Problem

Why does neon exist as single atoms rather than Ne₂ molecules?

Connect electron arrangement to particle form

Neon's outer shell

FullOne electron shortEmpty

Particles under normal conditions

Single Ne atomsNe2 moleculesNe+ ions

Check my attempt

Hints

Hint 1: write the configuration

Neon has ten electrons arranged 2,8.

Hint 2: test the reason to bond

Consider whether electron sharing between two neon atoms is favoured when each already has a stable outer electron arrangement.

View solution step by step

1. Identify neon's stability

   Method

   Use its 2,8 electron configuration.

   Reason

   Eight electrons fill neon’s outer shell.

   Working

   Outer shell: 8 of 8 electrons.

2. Infer particle form

   Method

   State that neon does not need to form a covalent bond.

   Reason

   Its stable configuration does not require electron sharing with another neon atom.

   Working

   Neon exists as monatomic Ne, not Ne₂.

Common misconception 3

Identify the correct reason for balloons

Find and correct the mistake

Learner response

A student says: “Helium is used in balloons instead of hydrogen because helium is unreactive.” Explain why this is incomplete and give the two relevant properties.

Match each property to one design requirement

Property for lift

Low densityColoured gasHigh melting point

Safety advantage over hydrogen

Non-flammableConducts electricityDiatomic

Check my attempt

View solution step by step

1. Identify the missing lift property

   Method

   State that helium has lower density than air.

   Reason

   The lower gas density helps a suitable balloon weigh less than the air it displaces.

   Working

   Low density → lift.

2. Compare fire risk

   Method

   State that helium is non-flammable while hydrogen is flammable.

   Reason

   This makes helium safer when ignition sources may be present.

   Working

   Helium: non-flammable; hydrogen: flammable.

3. Complete the answer

   Working

   Helium is used because it is less dense than air and non-flammable.

Examiner practice 4

Explain neon’s use

2 marks

Examination question

Explain why neon is used in advertising lights. [2 marks]

Link the operating condition to the visible outcome

Write the property–use link

View solution step by step

1. State the condition

   1 mark

   Method

   Pass electricity through neon gas in a discharge tube.

   Reason

   The electrical discharge excites the gas.

   Working

   Electricity passes through neon.

2. State the useful result

   1 mark

   Method

   Observe coloured light emission.

   Reason

   The visible emission makes the gas suitable for illuminated signs.

   Working

   Coloured light → advertising display.

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Self-mark the electrical condition and emitted-light use link.

Mark points

1. 1 mark: States that electricity passes through neon gas.
2. 1 mark: Links coloured light emission to advertising signs.

Calculate my mark

Challenge 5

Choose the gas for steel manufacture

Minimal support

Industrial transfer

A plant needs an inert gas around hot steel to limit unwanted reactions. Helium, neon and argon are all chemically suitable, but the plant has been quoted a much lower supply cost for argon. Choose a gas and justify the choice using both chemical suitability and the supplied cost information.

Connect gas identity, property and process

Suitable choice for this plant

HeliumNeonArgon

Process benefit

Limits unwanted reactionsAdds carbon to the steelRaises steel density

Check my attempt

Hints

Hint 1: recall the syllabus use

All three gases are very unreactive. Compare the supplied cost information.

Hint 2: explain inertness

A full outer shell means the gas does not tend to lose, gain or share electrons.

View solution step by step

1. Choose the gas

   Method

   Select argon.

   Reason

   All three options are chemically unreactive; argon has the lowest quoted cost for this plant.

   Working

   Choice: Ar.

2. Explain the chemical property

   Method

   Use argon’s full outer electron shell.

   Reason

   It does not tend to lose, gain or share electrons and is very unreactive.

   Working

   Full outer shell → inert atmosphere.

3. Link to manufacture

   Method

   Separate hot steel from reactive surrounding gases.

   Reason

   This limits unwanted chemical reactions during processing.

   Working

   Argon atmosphere → reduced unwanted reaction.

Next step

Try Mind Stretcher 1 independently.

Try these independently

Mind stretcher 1: Fix the overclaimExtension

Question: A student writes: “Noble gases never form compounds.” Correct this to an exam-safe statement.

Show Answer

Noble gases are very unreactive because they have a full outer shell, so they do not usually form compounds under normal conditions.

Mind stretcher 2: Match property to useExtension

Question: Helium and argon are both unreactive. At the same temperature and pressure, helium is less dense than air but argon is denser than air. Why is helium the better choice for a balloon intended to rise?

Show Answer

Helium’s lower density helps provide lift. Argon is denser than air, so an argon-filled balloon cannot rise on that basis. Being unreactive makes both gases chemically suitable, but it does not make both suitable for lift.

Practise and check