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.


Iodine, symbol I, chemically reactive element, a blue-black solid at room temperature. In group 17 (or VIIa) of the periodic table, iodine is one of the halogens. Its atomic number is 53.

Iodine was first isolated from seaweed residues in 1811 by Bernard Courtois, a French manufacturer of saltpeter. The discovery was confirmed and announced by the French chemists Charles Desormes and Nicholas Clément. The nature of the element was further established in 1813 by the French chemist Joseph Louis Gay-Lussac, who also gave iodine its name.


The atomic weight of iodine is 126.905. Unlike the lighter halogens, iodine is a crystalline solid at room temperature. The lustrous, blue-black, soft substance sublimes when heated, giving off a violet vapor with a stinging odor like that of chlorine. The vapor rapidly condenses again on a cold surface. Iodine melts at 113.6° C (236° F) and boils at 185° C (365° F). The only isotope that occurs in nature is stable, but several radioactive ones have been produced artificially. The element, in pure form, is poisonous.

Iodine, like all halogens, is chemically active. It is only slightly soluble in water, but it dissolves readily in an aqueous solution of potassium iodide. It is also soluble in alcohol, chloroform, and other organic reagents. With seven electrons in the outer shell of its atom, iodine has several oxidation states, of which the main ones are -1, +1, +3, +5, and +7. It combines readily with most metals to form iodides, and it also combines with other halides (chemical compounds made up of a halogen atom and a metal atom). Reactions with oxygen, nitrogen, and carbon are accomplished with more difficulty.

Iodine is a relatively rare element, ranking about 62nd in abundance on earth, but its compounds are widespread in seawater, soil, and rocks. Iodine is obtained from brines and from Chilean nitrate ores in which it occurs as an impurity. To a lesser extent, iodine is also derived from sea organisms, such as brown seaweeds, that concentrate iodine in their tissues.


Iodine is medicinally very important because it is an essential trace element, present in a hormone of the thyroid gland that is involved in growth-controlling and other metabolic functions. Without iodine, stunted growth and conditions such as goiter can result. Thus in areas where iodine is not sufficiently abundant naturally, iodine-containing salt serves to make up the deficit. In medicine, iodine-alcohol solutions and iodine complexes have been used as antiseptics and disinfectants. Radioisotopes of iodine are used in medical and other fields of research. More broadly, various iodine compounds find use in photography, the making of dyes, and cloud-seeding operations. In chemistry, various iodine compounds serve as strong oxidizing agents, among other uses.