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.


Francium, symbol Fr, radioactive metallic element that closely resembles cesium in chemical properties. In group 1 (or Ia) of the periodic table, francium is one of the alkali metals. The atomic number of francium is 87. Marguerite Perey of the Curie Laboratory of the Radium Institute of Paris discovered the element in 1939.

Francium is produced when the radioactive element actinium disintegrates. Francium is naturally radioactive; its longest-lived isotope, francium-223, or actinium-K, has a half-life of 22 minutes. It emits a beta particle of 1,100,000 electron volts (eV) energy. Isotopes ranging in atomic weights from 204 to 224 are known.

Francium is the heaviest of the alkali metals; it is the most electropositive element. All its isotopes are radioactive and short-lived.

See also Radioactivity.