Francium

Chemists of the late 1800s expected another alkali metal, with atomic number 87, to exist below cesium on the periodic table. But its discovery proved extremely elusive until 1939. Marguerite Perey, of the Curie Institute in Paris (where she had been mentored by Marie Curie), identified francium after purifying a sample of the radioactive isotope actinium-227. The actinium decayed into something with an unexpected energy, and she wanted to identify what it could be.

After eliminating various possibilities (thorium, radium, lead, bismuth, and thallium), she concluded it might be the long-sought element 87. She correctly surmised that some of the actinium-227 underwent alpha decay to produce an isotope of the new element—francium-223. She later named the element francium, after her home country.

It is very difficult to measure the properties of francium because it decays so fast. No weighable quantity of the element has been prepared or isolated, so even the atomic mass is difficult to quantify. Chemists report an atomic mass of 223 to match that of its longest-lived and naturally occurring isotope. The other chemical properties of francium are expected to resemble those of cesium, which is an extremely reactive metal that melts to a liquid around room temperature.

Because of the remarkable radioactive instability of francium, it does not stick around long enough to have any practical uses beyond scientific research.

The only francium naturally occurring on Earth is in tiny amounts in uranium minerals. The isotope uranium-235, which comprises less than 1% of uranium in nature, undergoes a series of radioactive decays that includes francium-223. However, francium-223, though the longest-lived francium isotope, has only a 22-minute half-life! Francium-223 is the only francium found in nature. At any given time, it has been estimated that there is probably only about 30 grams (1 ounce) of francium in the entire crust of the earth.