The halogens, which make up Group 7 A of the periodic table, show a graduation of chemical properties from the highly reactive fluorine to the comparatively unreactive iodine.

Fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At)—five chemically reactive nonmetals called the halogens—make up Group 7A of the periodic table. (The name halogen comes from Greek words meaning “salt producer.’! Halogens all have strong, nasty odors and do not dissolve well in water. The first two are gases, the third is a liquid, and the fourth and fifth are solids. Halogens react with metals to form the salts in seawater. Table salt, sodium chloride (NaCI), is the most familiar example. The halogens form negative ions by accepting electrons from other elements. These ions combine with positive metal ions to form salts known as halides. Halogens also react with nonmetals, producing many useful compounds, ranging from antiseptics and medicines to the additives that produce antiknock gasoline to hydrochloric acid (HCI).

Fluorine, chlorine, bromine, and iodine are so reactive that they never occur in pure form in nature but are instead combined with other elements. Astatine is an unstable, radioactive element of no practical use. Most of its 30 isotopes do not exist in nature but are created artificially. Astatine is an artificial element, first prepared at the University of California by Emilio Segre and others in 1940. Its 30 isotopes are radioactive. It was named after the Greek astatos, meaning unstable. Its atomic number is 85, and the atomic mass of its most stable isotope is 210.


This greenish-yellow gas, the most reactive of all elements, is always found combined with other elements. Flourine was first isolated in 1886 by the French chemist Henri Moissan (1852-1907). It occurs mainly in the mineral flourite, after which it is named. Its atomic number is 9, and its atomic mass is 18.9984.

Its melting point is -219.62° C, and its boiling point,-188.14° C.

Fluorine compounds, known as fluorides, are widely used in industry. Some are used to etch glass, others to make an inert plastic that withstands large differences in temperature. Fluorides are also used in making coolants for refrigerators and propellants for aerosol sprays, and they are added to toothpaste and drinking water to reduce tooth decay.


This yellow-green gas is strong smelling and toxic. Chlorine was discovered by the Swedish chemist Karl Scheele (1742-1786) in 1774. It was named later after its greenish-yellow color. Chlorosis greekfor greenish-yellow. Its atomic number is 17, and its atomic mass is 35.453. Its melting point is -100.98° C, and its boiling point,-34.6° C.

Chlorine kills bacteria in water, and so it is widely used to purify the water in swimming pools.

In nature, chlorine is usually combined with sodium in sodium chloride—table salt. Large quantities of the gas are used for bleaching textiles and paper and in making household bleach. It is added to reservoirs to kill bacteria in drinking water, and is also used to make weedkillers and insecticides.

Bromine and Iodine

Bromine, a dense, dark-red liquid, easily vaporizes into a reddish-brown gas with an irritating odor. Bromine was discovered independently in 1826 by the French chemist Antoine Baiard (1802-1876) and the German chemist Carl Lowig. Its name is derived from bromos, the Greek word for stench. Its atomic number is 35, and its atomic mass is 79.904. Its melting point is 7.25° C, and its boiling point, 59.47; C In nature, bromine is usually found in seawater and dry salt beds, combined with sodium or magnesium. It is used to make dyes, sedatives, anesthetics, photographic materials, and fire-retardant chemicals.

Bromine and chlorine can both be extracted from seawater. The process for producing bromine deft end befcm- relies on the tact that chlorine displaces bromine from its salts.

Iodine is a grayish-black, flaky solid with a slightly metallic luster. Iodine was discovered by the French chemist Bernard Courtois (1777-1838) in 1811. It is named after the violet color of its vapor. The Greek word iodes means violet. Its atomic number is 53, and its atomic mass is 126.905. Its melting point is 113.5° C, and its boiling point, 184= C Iodine occurs in very salty water, like bromine, and in some types of seaweed. It is used to make photographic film, antiseptics, germicides, drugs, and dyes. Iodine is an important trace element in the human body, essential to proper functioning of the gland that regulates metabolism—the thyroid.

Metal Halide lamps contain chemical compounds of a metal and a halogen. These lamps have a long life and a high light output.