Noble gas

The noble gases are the chemical elements in group 18 (old-style Group 0) of the periodic table. This chemical series contains helium, neon, argon, krypton, xenon and radon. The noble gases were previously referred to as inert gases, but this term is not strictly accurate because several of them do take part in chemical reactions. Another older term was rare gases, although in fact they form a considerable part (0.93% by volume, 1.29% by mass) of the Earth's atmosphere.

Because of their unreactivity, the noble gases were not discovered until 1868, when helium was detected spectrographically in the Sun. The isolation of helium on Earth had to wait until 1895. The noble gases have very weak inter-atomic forces of attraction, and consequently very low melting points and boiling points. This is why they are all gases under normal conditions, even those with larger atomic masses than many normally solid elements.

Noble gas compounds

All noble gases have full s and p outer electron shells (i.e. 8 outer shell electrons), and so do not form chemical compounds easily. As the atoms get larger down the series, they become slightly more reactive. Xenon was shown in 1962 to react with fluorine to produce XeF₂, XeF₄, and XeF₆ compounds. Radon has reacted with fluorine to form radon fluoride (RnF) which glows with a yellow light in the solid state. Krypton is able to react with fluorine to form KrF₂, and short-lived excimers of Xe₂ and noble gas halides such as XeCl are used in excimer lasers. The discovery of argon fluoride (ArF₂) was announced in 2003.

In 2002, compounds were discovered where uranium forms molecules with argon, krypton, or xenon. This suggests that the noble gases may be able to form compounds with other metals too.

The periodic table contains an empty space on under radon, with atomic number 118. This indicates the existence, albeit short-lived, of an as yet undiscovered noble gas, which has been temporarily named ununoctium.

External link

• Ohio State University press release for uranium compounds with noble gases.