http://www.rsc.org/periodic-table/element/16/sulfur
http://www.chemicool.com/elements/sulfur.html
http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/Periodic_Trends_of_Elemental_Properties/Periodic_Trends_in_Ionic_Radii
https://www.webelements.com/sulfur/isotopes.html