Caesium

Category: alkali metal Atomic number: 55 Colour: silvery gold Melting point: 28°C (83°F) Boiling point: 671°C (1,240°F) First identified: 1860

Caesium would be a lovely element to play with if it weren’t so extraordinarily reactive – it becomes liquid just above room temperature, so can be melted with the warmth of your hands. It is one of only three gold-coloured metals (along with copper and, of course, gold), although the gold colour disappears in 100 per cent pure samples, as it is caused by tiny traces of oxygen. The problem is that it is extremely reactive in air, so it has to be stored under oil or in an inert gas like argon, and if you drop it in water, it is even more explosive than its fellow alkali metals: lithium, potassium, sodium and rubidium. (For an explanation of why caesium is also slightly more reactive than the heavier alkali metal, francium, see here.)

You may remember that Robert Bunsen and Gustav Kirchhoff used the spectroscope invented by Kirchhoff to discover rubidium in 1861. At that point they had already discovered caesium – they had been studying a sample of mineral water when they noticed unexpected blue lines in the spectrum, indicating a new element, which they named after the Greek word for ‘sky blue’. They were able to produce caesium chloride, but it would be another twenty-two years before a sample of caesium was isolated from molten caesium cyanide (by Carl Theodor Setterberg at the University of Bonn).

Just a Second

If you’re ever asked to give the internationally recognized definition of a second, just memorize this: in 1967, the official definition was set by the International Bureau of Weights and Measures at 9,192,631,770 cycles of the radiation that gets an atom of caesium-133 to vibrate between two energy states. Simple!

Caesium compounds are used as drilling fluid and in the production of optical glass. But the most significant use of this element is in the caesium clock – it has replaced rubidium as most commonly used for this. The principle behind atomic clocks is the frequency of a change of energy levels caused by the magnetic field of the nucleus, and the stable isotope caesium-133 is currently the best candidate (although, as well as rubidium, strontium could also be used and, in theory, ytterbium).