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.


Rhodium (Greek rhodon, “rose”), symbol Rh, brilliant silvery white metallic element used principally in alloys. Rhodium is one of the transition elements of the periodic table (see Periodic Law). The atomic number of rhodium is 45.

Rhodium was discovered in 1803 by the British chemist William Hyde Wollaston. Rhodium metal is very durable. It is insoluble in ordinary acids and is very difficult to fuse. It has a hardness of 4. The compounds of rhodium span oxidation states of 1 to 6. Aqueous solutions of many of its salts are rose colored, from which its name is derived. The metal occurs as an alloy in platinum ores, in osmiridium, and in gold-rhodium ores called rhodite. Of the elements in the crust of the earth, it ranks 81st in order of abundance. Rhodium melts at about 1966° C (about 3571° F), boils at about 3727° C (about 6741° F), and has a specific gravity of 12.4. The atomic weight of rhodium is 102.905.

Rhodium is used mostly as an alloy with platinum; the resulting alloy has the desirable properties of platinum and is also hard and durable. Rhodium-platinum alloys are used in thermocouples, measuring high temperatures. Pure rhodium is used as a mirror surface in searchlights and as a plating finish for jewelry and silverware. Rhodium black is a finely divided metal that contains some oxide and hydride. It is used both as a catalyst and as a black pigment for porcelain ware.