<Table> <Tr> <Td> </Td> <Td> This article relies too much on references to primary sources . Please improve this by adding secondary or tertiary sources . (January 2015) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article relies too much on references to primary sources . Please improve this by adding secondary or tertiary sources . (January 2015) (Learn how and when to remove this template message) </Td> </Tr> <P> Noble gas compounds are chemical compounds that include an element from the noble gases, group 18 of the periodic table . Although the noble gases are generally unreactive elements, many such compounds have been observed, particularly involving the element xenon . From the standpoint of chemistry, the noble gases may be divided into two groups: the relatively reactive krypton (ionisation energy 14.0 eV), xenon (12.1 eV), and radon (10.7 eV) on one side, and the very unreactive argon (15.8 eV), neon (21.6 eV), and helium (24.6 eV) on the other . </P> <P> The heavier noble gases have more electron shells than the lighter ones . Hence, the outermost electrons are subject to a shielding effect from the inner electrons that makes them more easily ionized, since they are less strongly attracted to the positively charged nucleus . This results in an ionization energy low enough to form stable compounds with the most electronegative elements, fluorine and oxygen, and even with less electronegative elements such as nitrogen and carbon under certain circumstances . </P>

Name an element of group 18 which can form compounds