The noble gases
Group 18 contains the noble gases: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). They are colourless, odourless monatomic gases that exist as single atoms.
Why they are so unreactive
Their outermost shell is completely filled — helium has a duplet (2 electrons) and the rest have a stable octet (8 electrons). Because they already have a stable electron arrangement, they have little tendency to gain, lose or share electrons, making them chemically inert.
| Element | Electron arrangement |
|---|---|
| Helium | 2 |
| Neon | 2.8 |
| Argon | 2.8.8 |
Key idea
Noble gases are inert because they have a stable, fully filled valence shell (duplet or octet), so they do not readily form compounds.
Physical trends and uses
Going down the group, the atomic size increases, and both melting point and boiling point increase because larger atoms have stronger forces of attraction between them. Uses include: helium in balloons and airships (low density, non-flammable), neon in advertising lights, argon to fill light bulbs and shield welding, and helium/oxygen mixtures for divers.
Remember
- Monatomic and chemically inert.
- Full outer shell (duplet or octet).
- Boiling point increases down the group.