iodine

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A lustrous, grayish-black, corrosive, poisonous halogen element having radioactive isotopes, especially I 131, used as a medical tracer and in thyroid disease diagnosis and therapy. Iodine compounds are used as germicides, antiseptics, and dyes. Atomic number 53; atomic weight 126.9045; melting point 113.5°C; boiling point 184.35°C; specific gravity (solid, at 20°C) 4.93; valence 1, 3, 5, 7. See Table at element.
2. n. An antiseptic preparation containing iodine in solution, used to treat wounds.

Century Dictionary and Cyclopedia

1. n. Chemical symbol, I; atomic weight, 126.9. In chem., a peculiar non-metallic elementary solid substance, forming one of the group of halogens. It exists in the water of the ocean and mineral springs, in marine mollusks, in seaweeds, and in the nitrate deposits of western South America. At ordinary temperatures it is a solid crystalline body. Its color is bluish-black or grayish-black, with a metallic luster. It is often in scales, resembling those of micaceous iron ore; sometimes in brilliant rhomboidal plates or in elongated octahedrons. The specific gravity of solid iodine is 4.947. It fuses at 225°F., and boils at 347°. Its vapor, which is very dense, is of an exceedingly rich violet color, a character to which it owes the name of iodine. It is a non-conductor of electricity, and, like oxygen and chlorin, is electronegative. It is very sparingly soluble in water, but dissolves copiously in alcohol and in ether, forming dark-brown liquids. It possesses strong powers of combination, and forms with the pure metals and most of the simple non-metallic substances compounds which are named iodides. With hydrogen and oxygen it forms iodic acid; combined with hydrogen it forms hydriodic acid. Like chlorin, it destroys vegetable colors, but with less energy. Iodine has a very acrid taste, and its odor somewhat resembles that of chlorin. It is an irritant poison, and is of great service in medicine. It is used externally as a counter-irritant, the skin or mucous membrane being painted with the tincture; and also internally, both as iodine and in combination, especially as iodide of potash. Starch is a characteristic test of iodine, forming with it a deep-blue compound. This test is so delicate that a solution of starch dropped into water containing less than a millionth part of iodine is tinged blue.

Wiktionary

1. n. A chemical element (symbol: I) with an atomic number of 53; one of the halogens.
2. n. An antiseptic incorporating the element.
3. n. countable, uncountable, obsolete An iodide.

GNU Webster's 1913

1. n. (Chem.) A nonmetallic element, of the halogen group of atomic number 53, occurring always in combination, as in the iodides. When isolated it is in the form of dark gray metallic scales, resembling plumbago, soft but brittle, and emitting a chlorinelike odor. Symbol I. Atomic weight 126.90. If heated, iodine volatilizes in beautiful violet vapors.

WordNet 3.0

1. n. a nonmetallic element belonging to the halogens; used especially in medicine and photography and in dyes; occurs naturally only in combination in small quantities (as in sea water or rocks)
2. n. a tincture consisting of a solution of iodine in ethyl alcohol; applied topically to wounds as an antiseptic