Astatine is the chemical element with the symbol At, and the atomic number 85. Any primordial astatine has decayed away, and is a mere trace element in nature, coming from the decay of radioactive elements. 2 isotopes are known to exist in nature via decay, Astatine-218 and Astatine-219. Its most stable isotope, Astatine-210, is synthetically produced and is known to have a half-life of about 8 hours. This synthetic isotope has been proved useful in radiotherapy.
Astatine, despite its radioactivity, is to date the only element tested on a non-primate. Astatine was once injected into a guinea pig, and it was shown to accumulate in the thyroid gland, so it probably has similar properties to the rest of the halogens, especially Iodine. It is expected at room temperature to be a black solid.