Describe the properties of noble gases that make them chemically unreactive, and explain why helium is used in weather balloons rather than hydrogen.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Noble gases have a full outer shell of electrons, giving them a stable electronic configuration.
Because the outer shell is full, they neither need to gain nor lose electrons and they do not share electrons, so they do not form chemical bonds and are chemically unreactive.
Helium is used in weather balloons because it is less dense than air and provides lift, just like hydrogen.
Unlike hydrogen, helium is non‑flammable and will not burn or explode, making it safer for use in balloons.
Because the outer shell is full, they neither need to gain nor lose electrons and they do not share electrons, so they do not form chemical bonds and are chemically unreactive.
Helium is used in weather balloons because it is less dense than air and provides lift, just like hydrogen.
Unlike hydrogen, helium is non‑flammable and will not burn or explode, making it safer for use in balloons.
Examiner tips
- Use the phrase "full outer shell" and "stable electronic configuration" to show understanding of inertness. Mention both lack of need to gain/lose/share electrons and the resulting lack of bonds. State the two reasons for helium’s use: low density for lift and non‑flammability for safety. Keep the answer concise and use correct UK spelling.
Common mistakes
- Confusing the full outer shell with a full inner shell. Forgetting to mention that helium is non‑flammable. Using vague terms like "doesn't react" without explaining the electronic reason.
Mark scheme (4 marks)
- Noble gases have a full outer shell of electrons (stable electronic configuration)
- 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)
- Helium is less dense than air / provides lift (like hydrogen)
- Helium is non-flammable / does not burn, whereas hydrogen is highly flammable / can explode, making helium safer
Key terms in this question
noble gases · chemically unreactive
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →