Noble gases

Having learned that $\text{O}_2$, $\text{N}_2$, $\text{H}_2$ and $\text{Cl}_2$ are diatomic, candidates over-apply the pattern to Group VIII and write "$\text{He}_2$" or "$\text{Ar}_2$". A noble-gas atom already has a full outer shell, so it has no tendency to bond — not even to another atom of its own kind. Each particle is a single atom.

The spec statement asks for reactivity to be explained in terms of electronic configuration. Answering "noble gases are unreactive" or "they are inert" only DESCRIBES the property the question already named, so it earns no explanation mark. The reason must be given: the atom already has a full outer shell, so there is no tendency to gain, lose or share electrons.

Examiners record candidates swapping the two — arguing that argon fills balloons or that helium fills lamps. Argon fills filament lamps: its unreactive atmosphere stops the hot filament reacting with oxygen. Helium fills balloons and airships: it is much less dense than air and, being unreactive, will not catch fire. Learn each gas with its own use AND its own reason.

Answers describing helium as filling a "hot-air balloon" are marked wrong. A hot-air balloon rises because the ORDINARY AIR inside it is heated and so becomes less dense than the air outside; no gas is added. A helium balloon rises because helium is itself less dense than air. Two different routes to buoyancy — do not merge them into one answer.

Helium has only 2 electrons in total, so candidates conclude its outer shell cannot be full and that helium must be reactive. Helium's only shell is the FIRST shell, whose maximum capacity is 2, not 8 — so helium's outer shell IS full, and helium is as unreactive as the rest of Group VIII. "Full" means full for that shell's own maximum.