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.


Europium, symbol Eu, soft, silvery metallic element that is among the least abundant of the rare earth elements. Europium is in the lanthanide series of the periodic table; its atomic number is 63.

Europium was discovered spectroscopically by the French chemist Eugène Demarçay in 1896. It ranks 50th in order of abundance of the elements in the earth's crust; it occurs in monazite, bastnaesite, and other rare earth minerals, as well as in fission products of uranium, thorium, and plutonium. Europium melts at 822° C (1512° F) boils at about 1527° C (about 2781° F), and has a specific gravity of 5.2. The atomic weight of europium is 151.96.

Europium is used as a phosphor activator. The screen of a color-television tube is treated with europium, which, when bombarded with electrons, produces the color red. Because it readily absorbs neutrons, europium is used in the control of nuclear fission in reactors (see Nuclear Energy).