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.


Osmium, symbol Os, bluish-white, brittle metallic element. Osmium and iridium are generally considered the most dense elements. The atomic number of osmium is 76. The element is one of the transition elements of the periodic table (see Periodic Law).

Osmium was discovered in 1803 by the British chemist Smithson Tennant. Osmium has a hardness of 7; it melts at 2700° C (4892° F) and has a specific gravity of 22.6. The atomic weight of osmium is 190.2.

Osmium is not attacked by ordinary acids, but dissolves in aqua regia or fuming nitric acid. Osmium forms salts in which it has valences of 1 through 8. The metal occurs naturally in platinum ores and as an alloy, osmiridium, with iridium. Osmium ranks about 74th in natural abundance among the elements in crustal rock. The chief use of the metal is in the alloy osmiridium. Alloyed with platinum, it is used for standard weights and measures.