Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Astatine (Greek astatos, “unstable”), symbol At, radioactive element that is the heaviest of the halogens. The atomic number of astatine is 85.

Originally called alabamine because of early research with the element at Alabama Polytechnic Institute, it was prepared in 1940 by bombarding bismuth with high-energy alpha particles. The first isotope synthesized had an atomic weight of 211 and a half-life of 7.2 hours. Subsequently, astatine-210 was produced and found to have a half-life of about 8.3 hours. Isotopes of astatine with mass numbers from 200 to 219 have been cataloged, some with half-lives measured in fractions of a second (see Radioactivity).

Astatine is the halogen that behaves most like a metal and that has only radioactive isotopes. It is highly carcinogenic: Mammary and pituitary tumors have been induced in laboratory animals by a single injection. See also Periodic Law.