Visual Elements Periodic Table
Group 18 - The Noble Gases
The elements of Group 18, the noble gases, are:
Radon is a hazardous radioactive gas and will not be considered further here.
As the name suggests, all the elements in this Group are gases.
These elements are generally considered unreactive, because they have closed-shell configurations.
Occurrence and Extraction
The noble gases are all found in minute quantities in the atmosphere, and are isolated by fractional distillation of liquid air. Helium can be obtained from natural gas wells where it has accumulated as a result of radioactive decay.
All these gases are monatomic. They boil at low temperatures as only dispersion forces act between the atoms. Helium has the lowest boiling point of any substance at 4.2K.
Atomic radii increase on descending the Group.
This Group was originally named the "inert gases", as it was thought they formed no compounds. However, compounds of these gases are now well documented. Helium, neon and argon form no known compounds.
Krypton forms KrF2, a colourless solid, on reaction with fluorine.
Xenon forms a wide range of compounds with oxygen and fluorine.
Oxidation States and Ionisation Energies
The first ionisation energy decreases on descending the Group, as the valence shell becomes further away from the nucleus and electrons easier to remove. The first ionisation energy of xenon is comparable with that of bromine, which explains why xenon forms compounds with oxygen and fluorine relatively easily. The oxidation numbers of xenon in its compounds are +2, +4, +6 and +8.
The noble gases do have certain important industrial functions.
Helium is used by divers to dilute the oxygen they breathe.
Argon is widely used to provide an inert atmosphere for high-temperature metallurgical processes.
Neon and argon are used for filling discharge tubes.
For further information look up the individual elements.