The only free and comprehensive online etymological dictionary of the Spanish language
radio (3) m. (Noun) "radium"
A modern Latinism from radium "radiation," "ray." See rayo for continued etymology.
Real Academia Española. Diccionario de la lengua española. Online. 23rd ed. Madrid: Espasa Calpa. 2014. http://www.rae.es/.
Sihler, A. New Comparative Grammar of Greek and Latin. Oxford University Press. 2008.