Overview of the Element Astatine

Modern physicists have achieved the dream of the alchemists: transmutation. However, they do not bother with changing lead into gold. The energy costs would be too high and such an enterprise would not add to humanity’s store of knowledge. They do transmute elements though, and they go the ancient alchemists one better: they create new elements. One such element is Astatine.

Astatine is a halogen, meaning it’s from a group of highly reactive semi-metal elements, like iodine, chlorine, bromine, and fluorine. Its atomic number is 85(it has 85 protons in its nucleus), making it the heaviest of the known halogens. It’s radioactive, meaning it’s unstable and changes into less massive forms by giving off radiation. Its name, in fact, comes from the Greek word for unstable. Because of the rarity caused by its short half-life (explained below), the chemical properties of astatine are hard to determine by experiment or observation.

Mendeleyev, who was the first to organize the elements into a periodic table grouped by their properties, predicted the properties of astatine. Thus astatine is believed to be a black solid, because the halogens get darker as their mass increases, but no one has yet seen enough of it to judge its hue. However, it was synthesized at the University of California at Berkeley in 1940, by barraging bismuth with alpha particles in a cyclotron. Dale Corson, K.R. MacKenzie, and Emilio Segre share credit for the work.

Some Astatine does exist in nature it turns out, although in quantities that are impossibly small. When large radioactive atoms decay, they give off radiation, become smaller and lighter, and turn into different elements. These elements, if they are unstable, then decay, at a different rate than the parent element, giving off more radiation. This process repeats, as it has for millions of years. So whatever element the series begins with is transformed in a series of branching steps, until it finally reaches stability. These series are called decay chains. Various forms of astatine are produced in the decay chains of heavier radioactive elements, but they are so unstable that the longest lived of them has a half life of just 59 seconds. That means that after 59 seconds half of the minute quantity of the rare element that just blinked into existence is gone. In another 59 seconds, half of what was left is has become something else, and in another 59 seconds half of that infinitesimal quantity is gone. And so on. So the astatine that is studied is generally created astatine, one form of which has a half-life of just over 8 hours, a longer time for investigation.

These various forms are called isotopes. Astatine has 33 different isotopes. An element has a certain number of protons. Astatine has 85. Its different isotopes all have 85 protons, but they have different numbers of neutrons, which give them different masses and properties. Isotopes are identified by their mass number, which is the sum of the number of their protons and neutrons. (Protons can be visualized as the part of the atom that is positively charged, while electrons are negatively charged, and neutrons are neutral.) Some of the isotopes of astatine are 210At, with a half life of 8.3 hours; 213At, with a half-life of 125 nanoseconds; and 211At, which has a half life of 7.2 hours and is used in radiation therapy. All of astatine’s isotopes are radioactive.

Normally, people are cautioned about the dangers of radioactive elements, and of the fiercely reactive halogens, but there is unlikely to be a sizable problem with astatine, the rarest of all the elements on earth.