Editing Period 6 element (section)

===Astatine===
{{main|Astatine}}
'''Astatine''' is a [[radioactive]] [[chemical element]] with the symbol '''At''' and [[atomic number]] 85. It occurs on the Earth only as the result of decay of heavier elements, and decays away rapidly, so much less is known about this element than its upper neighbors in the [[periodic table]]. Earlier studies have shown this element follows periodic trends, being the heaviest known [[halogen]], with [[melting point|melting]] and [[boiling point]]s being higher than those of lighter halogens.

Until recently most of the chemical characteristics of astatine were inferred from comparison with other elements; however, important studies have already been done. The main difference between astatine and [[iodine]] is that the HAt molecule is chemically a [[hydride]] rather than a [[halide]]; however, in a fashion similar to the lighter halogens, it is known to form ionic astatides with metals. Bonds to [[Nonmetal (chemistry)|nonmetal]]s result in positive [[oxidation state]]s, with +1 best portrayed by monohalides and their derivatives, while the higher are characterized by bond to oxygen and carbon. Attempts to synthesize astatine fluoride have been met with failure. The second longest-living astatine-211 is the only one to find a commercial use, being useful as an [[alpha decay|alpha emitter]] in medicine; however, only extremely small quantities are used, and in larger ones it is very hazardous, as it is intensely radioactive.

Astatine was first produced by [[Dale R. Corson]], [[Kenneth Ross MacKenzie]], and [[Emilio Segrè]] in the [[University of California, Berkeley]] in 1940. Three years later, it was found in nature; however, with an estimated amount of less than 28&nbsp;grams (1&nbsp;oz) at given time, astatine is the least abundant element in Earth's crust among non-[[transuranium element]]s. Among astatine isotopes, four (with [[mass number]]s 215, 217, 218 and 219) are present in nature as the result of decay of heavier elements; however, the most stable astatine-210 and the industrially used astatine-211 are not.