Francium

Category: alkali metal Atomic number: 87 Colour: unknown Melting point: 21°C (70°F) Boiling point: 650°C (1,202°F) First identified: 1939

In 1929, five years before her death, caused by radiation exposure, Marie Curie hired a new lab assistant, Marguerite Catherine Perey. The brilliant Perey discovered the elusive element francium (named after France) in 1939, and would go on to be the first woman to be elected a member of the French Academy of Sciences.

When a particle loses an alpha particle, its atomic number falls by two; when it emits a beta particle, its atomic number rises by one. When it comes to artificially creating radioactive elements, the trick is to consider the possible decay pathways from other elements. In the case of francium, Perey purified a sample of actinium (atomic number 89) of its known radioactive impurities, but found a remaining trace of radioactivity, which turned out to be the new element.

Speedy Electrons

We have seen that the other alkali metals (lithium, sodium, potassium, rubidium and caesium) become more reactive as you go down the periodic table. Francium is an exception for an interesting reason – as the atoms of the elements have more and more protons, the electrons move around at an incredible pace, approaching the speed of light. The laws of relativity mean that they are thus slightly smaller than they would be at lower speeds, and the electrons are packed closer to the nucleus and fractionally harder to remove. As a result, caesium has been shown to be more reactive than francium (although you wouldn’t want to drop a lump of either of them into your bath).

Most actinium decays by emitting a beta particle, to form thorium (with an atomic number of 90, which in turn loses an alpha particle to become radium with an atomic number of 88). However, a tiny proportion of actinium atoms lose an alpha particle instead and become element 87, francium, which can thus only ever be found naturally in tiny quantities and has a very short half-life.