Noble gases (group 18)

Why the noble gases are so unreactive

Every noble gas has a completely filled outer shell. Helium has two electrons (1s²), which fill its only shell, and neon to radon have eight outer electrons in an ns² np⁶ arrangement, a stable octet. With a full shell they have no tendency to gain electrons and very high ionization energies, so they rarely lose them either; helium has the highest first ionization energy of any element.

Properties and trends down the group

The forces between noble gas atoms are very weak, so all six are gases at room temperature. Helium has the lowest boiling point of any element, 4.2 K (−269 °C), and cannot be frozen at normal pressure. Going down the group the atoms get larger and hold their outer electrons less tightly: ionization energy falls, while atomic radius, density and boiling point rise, and radon boils at about −62 °C.

Can noble gases react?

They were once called the inert gases, until Neil Bartlett made the first noble gas compound, xenon hexafluoroplatinate, in 1962. Xenon combines with fluorine when the mixture is heated or exposed to light, Xe + F₂ → XeF₂, and also forms XeF₄, XeF₆ and oxides. Krypton forms krypton difluoride (KrF₂), but no stable compounds of helium, neon or argon are known under ordinary conditions.

Uses of the noble gases

Helium is light and does not burn, so it fills balloons and airships, and liquid helium cools the magnets of MRI scanners. Neon glows red-orange in advertising signs. Argon, about 0.93% of the air, shields hot metal during welding and fills light bulbs and double-glazed windows. Krypton and xenon are used in bright lamps and camera flashes, and xenon powers the ion engines of some spacecraft.

Radon is radioactive. It seeps out of rocks and soil that contain uranium and can build up in basements, where it is the second leading cause of lung cancer after smoking.

Oganesson, the unknown member

Oganesson (118) completes period 7 below radon. Only a handful of its atoms have been made, each lasting about a millisecond, so none of its properties have been measured. Calculations suggest it may be a solid at room temperature and more reactive than the other noble gases.