Noble gases: stable atoms with important jobs
Group 18 elements usually react very little. A filled outer electron shell helps explain their chemical stability, but it does not mean that reaction is absolutely impossible.
Big idea and learning goals
A noble gas is an element in Group 18, the far-right column of the periodic table. At room conditions each exists mainly as separate atoms, so it is a monatomic gas. You will learn to locate Group 18; name He, Ne, Ar, Kr, Xe and Rn; connect filled outer shells with low reactivity; explain useful applications; and identify important physical and radiological hazards.
1. The Group 18 family
Elements in one group have related outer-electron patterns. Group 18 contains helium, neon, argon, krypton, xenon, radon and the extremely short-lived laboratory-made oganesson. For Grade 7, helium, neon and argon give the clearest shell-model examples.
| Element | Symbol | Introductory shell count | Connection |
|---|---|---|---|
| Helium | He | 2 | balloons; low-temperature science |
| Neon | Ne | 2, 8 | sealed discharge lighting |
| Argon | Ar | 2, 8, 8 | shielding gas; lamps; insulated windows |
| Krypton / xenon | Kr / Xe | filled outer shell | special lamps and technologies |
| Radon | Rn | filled outer shell | radioactive indoor-air hazard |
A shell count is a model of energy levels, not a photograph of tiny planets moving on visible tracks.
Group 18 map
2. Filled shells and chemical stability
An atom has a nucleus surrounded by electrons. In the introductory electron-shell model, electrons occupy energy levels called shells. The occupied shell farthest from the nucleus is the outer shell, and its electrons strongly influence reactions.