Group 18: the noble gas column

Full outer shells: ns² np⁶

From neon down, each configuration ends in ns² np⁶, eight outer electrons, as in argon ([Ne] 3s² 3p⁶). Helium is 1s², two electrons that fill its only shell. With nothing to gain and their electrons held tightly, these atoms do not form ions, so their common charge is listed as 0.

Helium’s 1s² matches the ns² pattern of group 2, and a few chemists argue it belongs above beryllium, but it behaves exactly like a noble gas.

Trends down the group

The atoms grow larger and hold their outer electrons less tightly: first ionization energy falls from 24.587 eV for helium, the highest of any element, to 15.76 eV for argon and 10.745 eV for radon. The weak attractions between atoms grow stronger, so boiling points rise from 4.22 K for helium to 87.3 K for argon and 211.3 K for radon.

This site lists electronegativity values only for krypton (3.0), xenon (2.6) and radon (2.2). Electronegativity describes how strongly an atom pulls on electrons in a bond, and helium, neon and argon form no stable compounds under ordinary conditions.

Where they come from

Neon, argon, krypton and xenon are separated from liquefied air, which is about 0.93% argon. Helium comes from natural gas wells, where it builds up from the radioactive decay of uranium and thorium in rocks; radon is itself a product of uranium decay and can seep into basements. Their names tell a story: neon means new, argon lazy, krypton hidden and xenon stranger.

Oganesson and the old group 0

When argon was discovered in 1894, the periodic table had no place for an unreactive element, so a new column was added, first called group 0. Oganesson (118), named after the physicist Yuri Oganessian, now completes it. Only a handful of its atoms have been made, so its properties are predicted; some calculations suggest it would be a solid at room temperature.