# Describe the properties of noble gases that make them chemically unreactive, and explain why helium is used in weather balloons rather than hydrogen.

> Cambridge International IGCSE Chemistry (0620) — 8.5 Noble gases · Describe and Explain · 4 marks

## Mark scheme (4 marks)

1. Noble gases have a full outer shell of electrons (stable electronic configuration)
2. Because they have a full outer shell, noble gases do not need to gain, lose, or share electrons (so they do not form chemical bonds / do not react)
3. Helium is less dense than air / provides lift (like hydrogen)
4. Helium is non-flammable / does not burn, whereas hydrogen is highly flammable / can explode, making helium safer

## Key terms

- [noble gases](https://www.gradenine.co.uk/glossary/noble-gases)
- [chemically unreactive](https://www.gradenine.co.uk/glossary/chemically-unreactive)

## Related

- [Revision notes for Cambridge International IGCSE Chemistry (0620)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe and Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/describe-the-properties-of-noble-gases-5c1d9f4d) · Published by Druglandscape Ltd.