Atoms and molecules
3 marks · NormalHelium exists as separate atoms, while oxygen exists as O₂ molecules. Use your knowledge of atoms and molecules to explain why.
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Must show: atoms form molecules by bonding, not all atoms bond, helium's stability, oxygen molecules
Common mistakes: Stating all elements form molecules; Confusing atoms with molecules
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Marking points
B1 · 1 mark · States that molecules form when atoms bond together.
B1 · 1 mark · Mentions helium atoms do not bond with other helium atoms and exist as individual atoms because they are stable.
B1 · 1 mark · Recognises that oxygen atoms bond in pairs, forming O₂ molecules.
Worked solution
Molecules form when atoms bond together. Helium atoms do not bond because helium has a full outer electron shell, making it stable on its own. Oxygen atoms bond together in pairs to form O₂ molecules because that makes them more stable.