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.

Protactinium

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Protactinium, formerly protoactinium, symbol Pa, radioactive metallic element with an atomic number of 91. Protactinium is a member of the actinide series of the periodic table. It was discovered in 1918 by the Austrian-Swedish physicist Lise Meitner and the German physical chemist Otto Hahn. Protactinium is a member of the uranium-actinium radioactive-decay series and is found in uranium ores. Isotopes of protactinium ranging in mass number from 215 to 238 are known. Protactinium-233 has a half-life of 27 days. Protactinium-231, the most stable isotope, has a half-life of more than 32,000 years; by emission of an alpha particle it decays to actinium. Protactinium melts at about 1552° C (about 2826° F), boils at about 4227° C (about 7641° F), and has a specific gravity of about 15.37.

See also Radioactivity