Lithium takes the record as the lightest of all metals at just 0.5g per cubic centimeter. That's half the weight of water which means, of course, that it could float on water just as wood. Quite unlike wood, however, lithium does not at all like floating on water. The metal is reactive enough in air that it tarnishes to black before your eyes. In water though it fizzes, boils then catches on fire and explodes!

Since lithium is so reactive it is very difficult to keep it in its oxide-free elemental state. It is so reactive, in fact, that when molten it can even attack glass which is impervious to every other element on the periodic table except fluorine. Keeping it oxide-free therefore requires encasing the sample in glass under a noble gas which works, but is more costly. Stored in mineral oil, however, lithium will darken but remains quite affordable!

Note other lithium samples on the Ampules section

CAS 7439-93-2