This is Caesium! It’s number 55 on the periodic table and one of only three elements that are liquid at room temperature in their pure form (the other two being Gallium and Mercury). It has an atomic weight of 132.9.
It was discovered in 1860 Germany by Gustav Kirchhoff and Robert Bunsen (that’s right, the guy who invented the BUNSEN BURNER). The element gave off bright blue lines on its emission spectrum, therefore Kirchhoff and Bunsen named it after the Latin word, caesius, which means sky blue.
Caesium’s most popular application today is in atomic clocks. The atomic clock uses a caesium-133 atom as a reference point to define the “perfect second”. This standard was set down by the International System of Units, which states that a second is comprised of exactly 9,192,631,770 cycles of radiation for caesium-133 (a cycle of radiation is referring to the transition of energy levels on a caesium atom).
This has proven to be an incredibly precise method to measure time, being reportedly off by only 2 seconds for every 65 million years.
And if that doesn’t get you interested in caesium, it also reacts explosively with water. What’s not to like?
Atomic Clock [Wikipedia] http://en.wikipedia.org/wiki/Atomic_clock#Mechanism
Caesium [WebElements] http://www.webelements.com/caesium/
Caesium [Wikipedia] http://en.wikipedia.org/wiki/Caesium
(Image courtesy of Google Images)