Second off, if you didn't get sodium and chlorine and are wondering how anybody knows it, think of salt.
Apart from that, good quiz!
P.S. Look at your comment in http://www.jetpunk.com/quizzes/geography-by-letter-a
"Tons per Square Mile" doesn't make sense as a unit. Do you mean tons per cubic mile? By my calculations, Oxygen should be 403,868,946,000, which is a lot more than what you have.
Really, the numbers are only relevant for their relative magnitude, but the units should still make sense.