Cesium is the perfect example of controlled chaos; it is known as the radioactive waste product of nuclear explosions and is one of the five elements that can be found in a liquid state at room temperature.
It is extremely volatile, and this is what happens when it comes into contact with water:
However, its electronic transitions are so precise that it was used as the main standard for the definition of the second and gave rise to this definition: “A second is the duration of 9,192,631,770 periods of radiation emitted during the transition between the two hyperfine levels of the ground state of the cesium-133 atom at sea level (with a zero magnetic field).”
Atomic Clock:
Atomic clocks are so precise that they wouldn't lose a single second in 20 million years. It's insane. How could such an unstable element be used to define time.