Element 87 in our International Year of the Periodic Table series is francium. Vanishingly rare, francium is unstable and radioactive, and also doesn’t conform to expectations about its reactivity.
Francium is vanishingly rare; it’s estimated that less than 30 grams of the element are present in the Earth’s crust at any given time. Its most stable isotope has a half-life of just 22 minutes.
You might expect that francium, sat at the foot of the group 1 elements, would be the most reactive of the bunch. The reactivity of the group 1 elements increases as you descend the group, with the elements’ outermost electron becoming easier to remove during reactions. However, francium bucks this trend, due to weird effects that occur as elements get heavier.
In the heaviest elements in the periodic table, relativistic effects lead to some weird behaviour. Put simply, the size and mass of francium means that its electrons are travelling at very high speeds. At these speeds the electrons become closer to the nucleus than expected, and they are consequently harder to remove.
No-one’s ever gotten enough francium in one place to produce a sample to throw into a bucket of water – and they’re not likely to be able to in the future, as francium’s so radioactive it would vaporise from the heat its radioactivity produces. However, if it were possible, it seems it would be likely that francium wouldn’t create as big a bang as the element above it in the group, caesium.
Remember, you can keep track of all of the previous entries in this series on the site here, or on the Royal Society of Chemistry’s dedicated page.