It depends. If you're talking about the tool that broadcasts voice and music, it's femminin and invariable "la/le radio". If you're talking about the bone in your forearm ("radius" in English) or the chemical element discovered by Marie Curie ("radium"), it's masculin and variable, so "il radio/i radi".

"Il radio" is an arm bone, "la radio" is the radio, the electronic device. The first noun is male, the second female. It's determined by the article.