I did A-level chemistry, and have no idea what Rubidium is! You could be very rude and "assume" he ment mass number...

*grin*

Rubidium is one of the Group 1 metals, which most people at my school tended to remember vividly because they're the ones that react explosively with water. Rubidium is the next one up from potassium; its reaction with water is a bit too vicious for a school chemistry teacher to actually demonstrate, but there are apparently videos of a demonstration floating around somewhere and it tends to be spoken of in hushed tones in boy's-school society (which is after all composed almost exclusively of raving pyromaniacs).

Rubidium's pretty good.
My favorite's caesium, though. Caesium actually explodes on contact with water, shattering the half-inch-thick glass basin being used for the demonstration.

Aah, it's *that one*. Our chemistry teacher made us memorise the periodic table up to and including the line before (final memorised line starting with Potassium). The mnemonic supplied involved simply stringing the symbols together and speaking them as one long nonsense-word. I can remember it to this day :)

And, I have seen such a video. It involved a large glass dish of water which was in several pieces by the end of the demonstration.

Our chemistry teacher promised us that if the school were ever to want rid of the (old falling-down Victorian) swimming pool, she would buy some caesium and let us all watch her drop it in the water. Strangely they never did.

Sounds like the thing one of our chemmie teachers was notorious for, though he had a silly tune to go with it.