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.


Iridium, symbol Ir, white, brittle, extremely hard, metallic element. The atomic number of iridium is 77; it is one of the transition elements of the periodic table (see Periodic Law).

Iridium is extremely inert chemically, resisting even the action of aqua regia. In its chemical compounds it forms tetravalent and trivalent salts. It is an extremely rare metal, ranking 77th in order of abundance of the elements in the crust of the earth. The atomic weight of iridium is 192.22. It melts at about 2410° C (about 4370° F) and boils at about 4130° C (about 7466° F), and has a specific gravity of 22.6. Iridium and osmium are considered the most dense elements.

Iridium is found in alluvial deposits alloyed with platinum as platiniridium and with osmium as osmiridium. Iridium is used chiefly as an alloying material for platinum; the alloy, which contains about 10 percent iridium, is much harder than pure platinum. Platinum-iridium alloys containing larger percentages of iridium are used in making precision instruments, surgical tools, pen points, and standard weights and lengths.

Iridium was discovered by the British chemist Smithson Tennant in 1804 and was named for the iridescent nature of some of its compounds.