The number of electrons in the nucleus determines chemical properties, such as color, taste, smell and so on. In an electrically neutral atom, the number of electrons in an atomic orbital is equal to the total number of protons and neutrons in the nucleus.
Astatine is a chemical element with the symbol At and atomic number 85. It was first synthesized by Dale Corson, Kenneth Ross Mackenzie and Emilio Segre at the University of California, Berkeley in 1940.
It is a radioactive halogen in group 17 and the p-block of the periodic table. It is produced by bombarding bismuth-209 with alpha particles in a device called a cyclotron.
One molecule of the stable isotope of astatine, 211At, is produced by the bombardment of bismuth-209 with a single alpha particle. The resulting isotope has two free neutrons and a mass of 210.
This isotope decays by emitting one alpha particle and a small amount of neutrons to form a new isotope with an identical atomic weight, the other isotope having lost a large number of neutrons.
The name of this radioactive element is derived from the Greek word “astatos,” which means “unstable.” It is the rarest naturally occurring element in the universe and only found as an outgrowth on a break down pathway for uranium and thorium. It is estimated to occur in minuscule quantities (less than 25 grams per kilogram of Earth’s crust) at any given time. This makes it a very difficult chemical element to study in laboratory experiments.