Noble gases — identity and properties
The noble gases are Group 0 (also Group VIII or 18): helium, neon, argon, krypton, xenon and radon (atomic numbers 2, 10, 18, 36, 54, 86); IGCSE focuses on He, Ne and Ar. They are colourless, monatomic gases — single atoms, so helium is , not . All are non-flammable and unreactive (inert), forming no compounds normally. Down the group boiling point and density increase, though all boiling points stay far below 0 °C.
Why noble gases are unreactive
A noble gas atom has a full outer electron shell — a duplet for helium (structure 2), an octet for neon (2.8), argon (2.8.8) and heavier ones. That is already the most stable arrangement, so the atom has no tendency to gain, lose or share electrons, and forms no ions or bonds. The two-element mark-scheme answer is: (1) full outer electron shell; (2) no tendency to gain, lose or share electrons — both parts needed for the marks.
Uses of the noble gases
Each use matches a property. Helium fills balloons and airships because it is less dense than air and non-flammable — safer than hydrogen. Neon glows red-orange when electricity passes through it, used in advertising signs. Argon stays inert even when hot, so it fills filament light bulbs (stopping the tungsten oxidising) and shields metal in welding; argon or krypton fills double glazing because they conduct heat poorly. Always quote the property behind each use.
Drawn from real examiner reports.
Unreactivity: give the consequence too
"Explain why noble gases are unreactive" needs two elements. The first — full outer electron shell — is usually given; the second — no tendency to gain, lose or share electrons, so no bonds form — is often missing, scoring one of two. "They are stable" earns nothing unless both the full outer shell AND the resulting inability to bond are stated.
Helium vs hydrogen: say non-flammable
Both helium and hydrogen are less dense than air, so "lower density" alone does not explain why helium is chosen. The deciding property is safety: hydrogen is flammable, helium non-flammable. Give the shared property (low density) AND the difference (non-flammable). "Helium is a noble gas" does not score the discrimination mark.
Noble gases are monatomic, not diatomic
Knowing hydrogen, nitrogen and the halogens are diatomic tempts students to write or — but noble gases are monatomic, single atoms. With a full outer shell there is no drive to form a covalent bond, so neon gas is and its equals its . In a dot-and-cross diagram a noble gas is drawn as one atom with no bond.
Shells vs group number confusion
Asked which element has the most occupied electron shells, students choose by group number instead of counting shells. The number of shells is the period (row), not the group (column). Argon (2.8.8) is in Group 0 yet has three occupied shells — like sodium or chlorine in that period. Count the numbers in the electronic structure to get the shells.
W22 P13 Q21; W22 P23 Q21; S23 P31 Q1(c) — group vs period confusion: the wrong element chosen for the most occupied electron shells.
Helium is a duplet, not an octet
Helium's full outer shell holds two electrons, not eight, because the first shell is complete at two. Helium therefore has a duplet, whereas neon, argon and the rest have an octet of eight. Saying helium "has eight outer electrons" or "obeys the octet rule" is wrong — it is the special case, structure 2.
Unreactive is not the same as absent
"Unreactive" does not mean the noble gases are absent or do not exist. They are present and useful: argon makes up about 1% of the air, and all are extracted and used industrially. They are chemically inert but physically present — which is exactly why they are used wherever a gas must not react.
Unreactivity: two-part template
For "explain why noble gases are unreactive", give two distinct points: "[gas] has a full outer electron shell [1]; it therefore has no tendency to gain, lose or share electrons and does not react [1]." Do not merge them into one vague sentence.
State a use AND its property
For "state a use and explain why", attach the property that makes the gas suitable — a bare use scores half. For helium give BOTH: low density so it rises, non-flammable so it is safe. For argon say "inert, so it does not react with the hot filament".
Learn the uses checklist
Must-know uses: helium — balloons/airships (low density, non-flammable); neon — advertising signs (glows red-orange); argon — filament bulbs and welding (inert when hot); argon/krypton — double glazing (poor heat conductor). Quote the property, not the use alone.
Noble gases are the elements in Group 0 (also called Group VIII or Group 18) of the Periodic Table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). Cambridge IGCSE focuses on He, Ne and Ar.
Key definition — why noble gases are unreactive (two-element mark-scheme form):
Both elements are required for full marks on a 2-mark "Explain" question.
Electronic structures:
| Noble gas | Electronic structure | Outer shell electrons |
|---|---|---|
| Helium (He) | 2 | 2 (duplet — first shell only needs 2) |
| Neon (Ne) | 2.8 | 8 (octet) |
| Argon (Ar) | 2.8.8 | 8 (octet) |
Physical properties:
Element X is in Period 3, is a colourless gas, and forms no compounds under normal conditions. Deduce its identity and explain, from its electronic structure, why it is unreactive.
