Francium

Reference: http://www.lenntech.com/periodic/elements/fr.htm

Francium is a really rare element, that's why its density is unknown. The origin of the word Francium  is from France, which is the country of its discoverer.
It is also know for being the heaviest alkali metal.
Alkali Metals reaction to Water
Minute 4:49 (For Fr)

Bibliography:
http://chemistry.about.com/od/elementfacts/a/francium.htm
http://www.lenntech.com/periodic/elements/fr.htm