Noble gases: full shells and separate atoms
Explain unreactive, monatomic gases while preserving helium's two-electron full shell.
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Use the first-twenty-elements shell model.
- Distinguish an individual atom from a diatomic molecule.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which shell arrangement belongs to neon?
What you will learn
- Describe the required noble gases as unreactive and monatomic.
- Explain this through full outer shells, including helium's first-shell exception.
The outer shell is already full
Group VIII noble gases in the syllabus notation include helium, neon and argon; many modern tables call this Group 18 or show Group 0 in a school convention. Helium has arrangement 2, neon 2,8 and argon 2,8,8. Their outer shells are full in the stated model, making the gain, loss or sharing used by many other elements unnecessary for this simple stable arrangement.
Helium's full first shell has two electrons, not eight. Therefore the explanation is full outer shell, rather than every noble gas has exactly eight outer electrons. Noble gases are very unreactive under the ordinary conditions described by this syllabus; this does not assert that no noble-gas compound can ever exist under specialised conditions.
Monatomic means the gas particles are single atoms
At the stated ordinary conditions, the required noble gases consist of separate atoms rather than diatomic molecules like Cl₂ or O₂. Write He, Ne and Ar, not He₂, Ne₂ or Ar₂ as their usual elemental gas formulas. A sample contains many atoms moving through space; monatomic does not mean the entire container contains only one atom.
The full outer shells explain low chemical reactivity, while the monatomic description states the kind of particle present. Do not confuse gas state with molecularity: both chlorine and neon are gases at r.t.p., but chlorine particles are diatomic molecules and neon particles individual atoms. Element formulas and electron arrangements answer different parts of that comparison.
Pause and explain
What does monatomic neon gas mean?
Worked example
Compare neon and chlorine gas particles and explain why neon is unreactive in the stated model.
Show the worked solution
- Neon particles are individual Ne atoms with arrangement 2,8.
- Chlorine particles are Cl₂ molecules; each chlorine atom's neutral shell arrangement is 2,8,7.
- Neon's full outer shell supports its low reactivity; being a gas alone does not distinguish their chemical behaviour.
Answer Ne is monatomic with a full outer shell; Cl₂ is diatomic and has different reactivity.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Give shell arrangements for helium, neon and argon and connect them to low reactivity.
Give me a hint
Check the full first shell separately from later shells.
Compare my reasoning
- Helium is 2; its first shell is full.
- Neon is 2,8 and argon 2,8,8, each with a full outer shell in the model.
- The full-shell arrangements support their unreactive behaviour under the stated ordinary conditions.
He 2; Ne 2,8; Ar 2,8,8; all have full outer shells.
Look for these in your work
- Helium's two-electron shell is retained.
- The explanation uses full shells rather than a universal eight rule.
Compare particle type and usual elemental formula for argon and chlorine at r.t.p.
Give me a hint
Gas state does not require the same number of atoms per particle.
Compare my reasoning
- Argon has separate atoms and formula Ar.
- Chlorine has two-atom molecules and formula Cl₂.
- Both can be gaseous at r.t.p. while having different particle types.
Argon is monatomic Ar; chlorine diatomic Cl₂.
Look for these in your work
- Both formulas match their particle types.
- Physical state is not confused with molecularity.
A report says helium must be reactive because it lacks eight electrons, and writes He₂ because it is a gas. Correct both rules.
Give me a hint
Full depends on the occupied shell; gas can contain atoms or molecules.
Compare my reasoning
- Helium's first shell is full with two electrons.
- The full-shell explanation supports low reactivity in the stated model.
- Ordinary helium gas is monatomic and written He, not He₂; gas state does not force a diatomic formula.
Helium has a full two-electron first shell and is monatomic He under the stated conditions.
Look for these in your work
- Both flawed universal rules are corrected.
- Electron arrangement and particle formula are distinguished.
Common mistakes
- Writing Ne₂ because every elemental gas is assumed diatomic.
- Assigning eight outer electrons to helium.
- Defining monatomic as one atom in the whole sample.
- Treating unreactive in ordinary conditions as a claim about every possible specialised reaction.
What can you explain now?
Why is helium's arrangement 2 compatible with the full-shell explanation?
Compare with the explanation
Its first shell is full with two electrons.
A full first shell has capacity two in this model; helium does not need eight electrons to fit the noble-gas explanation.
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Make it stick
Explain helium's full shell, recall He/Ne/Ar arrangements and contrast a noble-gas particle with a halogen molecule.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.