| Cesium was discovered in 1860 by using the spectrophotometer to look
at the lines identifying elements in Durkheim mineral water. Spectral
lines for sodium, potassium, lithium, calcium, and strontium were observed.
When these metals were chemically removed, the absence of their lines from
the spectrum was observed. As the lines began to disappear, two blue
lines were unexpectedly observed. The discovery of new spectral lines
suggested a new element was to be discovered. Its name was taken
from the Latin word coesius meaning "sky blue". Cesium is an alkali
metal. In its solid form, cesium is the softest of all metals. It
is silvery white, ductile and easily melts at 28.4oC. It often exists
as a liquid that looks much like mercury. In fact, mercury is the
only metal that has a melting point lower than cesium. Dissolved
cesium salts, such as cesium clahounate and cesium chloride are widely
distributed in low concentrations in brines and mineral waters.
The main source of cesium is found in the mineral pollucite, CsAlSi2O6.
Cesium ores usually contain rubidium, which makes refining cesium metal
difficult as rubidium has many critical properties that are nearly identical
to cesium. As a result, rubidium and cesium can be difficult to separate
from one another. Today, the separation is accomplished by reducing
the metals with elemental sodium. In this process, the ores are finely
ground and heated to about 650oC with sodium metal. The result is
an alloy of sodium, cesium, and rubidium. The three metals are then
separated by fractional distillation, much the same way
the atmospheric gasses are separated from liquid air.
Because cesium reacts readily with most kinds of gases, it is used as
a getter (i.e. an absorber of unwanted gases) in electronic vacuum tubes
and cathode ray tubes. It is also used in the production of photoelectric
devices and atomic clocks. The cesium atomic clocks are so accurate
that they vary no more than five seconds in ten generations. |
