Argon, discovery properties

After helium and argon had been discovered the existence of neon, krypton, xenon, and radon was clearly indicated by the periodic law, and the search for these elements in air led to the discovery of the first three of them radon was then discovered during the investigation of the properties of radium and other radioactive substances. While studying the relation between atomic structure and the periodic law Niels Bohr pointed out that element 72 would be expected to be similar in its properties to zirconium. G. von Hevesy and D. Coster were led by this observation to examine ores of zirconium and to discover the missing element which they named hafnium. 

Iodine has a lower relative atomic mass than tellurium and hence should be placed in Mendeleev s group 6. However, Mendeleev placed it in his group 7 since it clearly has similar properties to the other halogens. The discovery of the noble gases introduced a similar reversal of relative atomic mass between argon and potassium. 

---