Bismuth

Category: post-transition metal Atomic number: 83 Colour: pinkish silver Melting point: 272°C (521°F) Boiling point: 1,564°C (2,847°F) First identified: fifteenth century

Bismuth was known to the Incas in the fifteenth century; a knife found at Machu Picchu is made of an alloy containing the metal. Western alchemists had also recognized bismuth, and it was being mined by 1460, although it was often mistaken for a type of lead. In the nineteenth century, it was used in cosmetics: when dissolved with nitric acid and then poured into water, it produces a white, flaky material known as ‘pearl white’, which could be made into a face powder. This is much less toxic than lead white, but tended to turn rather brown in cities, due to sulphur pollution from coal burning.

Bismuth is a heavy but brittle metal, often used in alloys such as pewter. With cadmium or tin, it forms alloys with a low melting point, which can be used in fuses or solders. It is still used (as bismuth chloride oxide) to create a pearly effect in cosmetics and (as bismuth oxide) for yellow pigments. Bismuth carbonate is sometimes used as an indigestion cure (known as ‘bismuth mixture’).

A Brief Guide to Radioactivity

In the nuclei of stable atoms, there is enough force to bind the protons and neutrons together. In unstable atoms, though, especially heavy ones such as uranium, the force is not strong enough, so the nucleus emits energy and particles, which we call ‘radioactive decay’. (Note that some usually stable elements do have radioactive isotopes.) The term radioactivity refers to the particles emitted: the atom will gradually decay until it becomes stable. For instance, uranium-238 decays gradually through eighteen stages, forming atoms of thorium, radium, radon and polonium, before becoming a stable atom of lead-206. It’s impossible to estimate how long a single atom will take to decay, so instead we use the concept of ‘half-life’, which is the average amount of time taken for half of the nuclei of a given isotope to decay.

It used to be thought that bismuth wasn’t radioactive. In fact, it is, but only very, very slightly. In 2003, a group of researchers in France detected alpha particles resulting from the decay of bismuth-209 (the only naturally occurring isotope of bismuth). However, it has a half-life of 2×1019: only a handful of substances have longer half-lives, so its radioactivity is not at all dangerous, unlike most of the substances beyond it in the periodic table.