Rubidium

Category: alkali metal Atomic number: 37 Colour: silvery white Melting point: 39°C (102°F) Boiling point: 688°C (1,270°F) First identified: 1861

Just as the noble gases (in the column on the right-hand side of the periodic table) share many properties, such as not being very reactive, the elements of group 1 on the left-hand side of the table are soft metals with low melting points, which are highly reactive. School chemistry teachers will sometimes demonstrate the strong reaction of lithium or sodium in water, but rubidium (which becomes liquid at a relatively low temperature) makes for a more dramatic and dangerous reaction – it can catch fire spontaneously in air and has to be stored in a vacuum or in a gas such as argon. When added to water, the explosion is instant, and so violent that the hydrogen being released from the water will often ignite.

This alkali metal has a handful of quite interesting applications: one of its isotopes is the radioactive rubidium-87. It has a half-life of 50 billion years: bearing in mind that it has only been about 14 billion years since the Big Bang, this is an extraordinarily slow rate of radioactive decay. It forms strontium-87 as it decays, which means that this property can be used to age ancient rocks accurately, by comparing the levels of rubidium and strontium using a spectrometer.

The Bunsen Burner and the Spectroscope

In the unlikely event that you are asked what these two brilliant scientific instruments have in common, the answer is Robert Bunsen: he invented the Bunsen burner, and was also (in 1859) the co-inventor (with Gustav Kirchhoff) of the spectroscope, which has been used to identify many new elements since. By 1861, the pair had used it to identify caesium and rubidium – the latter was found in the mineral lepidolite in 1861. It was named after the Latin rubidus (ruby) because of the vivid ruby-red lines that showed up on the mineral’s spectrum.

Rubidium is also (along with caesium) used in atomic clocks, which align microwave radiation with the activity of electrons as they circle the atom and emit bursts of radiation. It is not naturally found in the human body, but it is harmless, as we can easily excrete it – it has been used to study the way that potassium moves around the body (since our bodies treat the two in the same way) and the radioactive isotope rubidium-82 has also been used to locate brain tumours.