There are many words that break the "gender" rule in Spanish, which can drive a person crazy....
Sometimes it's because it's a shortening of the word -- for example, la foto, which is a shortened version of la fotografia. That may be why radio goes either way, but I don't know what the original word might have been....El radio can also be "the radius" in a geometric way or a spoke, so of course context is important....Peter
Right on Peter.
There are tons of exceptions and those are learned by rote. In the case of "radio" as we use it, the receiving box is usually called "el radio"
. Radio as in broadcast station is "la radio" which is a contraction of "estación radiodifusora" .
I think that they were created to confuse non native speakers....
Then again, English has its own batch of oddities too; you'd think that since the plural of foot is feet, boot would be beet... etc, etc.