Noble gas: definition in chemistry
Noble gas: Any element of group 18: helium, neon, argon, krypton, xenon and radon, along with the synthetic element oganesson. The naturally occurring ones are colorless, odorless gases that rarely react.
Noble gases have full outer electron shells, so they have very little tendency to gain, lose or share electrons, and they exist as single atoms rather than molecules. Their low reactivity makes them useful wherever a reaction must be avoided, such as argon shielding the metal during welding. They are not completely inert: xenon forms compounds with fluorine and oxygen, the first of which was made by Neil Bartlett in 1962.
Examples
- Helium — filling balloons and cooling MRI magnets
- Neon — glowing red-orange in advertising signs
- Argon — about 0.93% of air, used in welding
- Xenon — in some car headlights and spacecraft ion engines
On the periodic table
The noble gases form group 18, the last column on the right of the table, and each one ends a period.