The quiz is BS. If you kept going according to the CSM to be scientific you have to memorize the Periodic table. Did you know Radon was heavier than Xenon? LOL.
Actually, yes. And I don't think that is quite as ridiculous as the ones I cited. But really, what matters is that you can give the answer to that question with a Periodic Table in front of you. I'm sure that I wouldn't be able to answer with certainty for the majority of element pairs. (And I just looked at a table and I see that I was lucky in my answer about Xenon. I thought it was lower in the table.)
ML/NJ