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198 Halogen Chemistry

More negative lattice energies indicate that the ions are were discovered, the meaning was extended to include all
held more tightly in the crystal. members of this family.
Lewis acid Electron-pair acceptor (electrophile). Compounds containing halogens have been known for
Lewis base Electron-pair donor (nucleophile). thousands of years. Archeological evidence indicates that
Oxidation state or oxidation number Number of elec- rock salt (NaCl) was used by people as early as 3000 B.C.
trons lost or gained by the element as a result of com- and the Bible contains numerous references to the im-
pound formation. The number of electrons lost due to portance of this substance. Dibromoindigo (Tyrian pur-
oxidation denotes a positive oxidation state, while the ple) was employed as a dye by the Greeks and Romans,
number of electrons gained due to reduction denotes a and medieval alchemists used aqua regia, a mixture of
negative oxidation state. hydrochloric (HCl) and nitric (HNO 3 ) acids, to dissolve
Standard reduction potential Potential difference de- gold.
veloped between two electrodes, where the reaction at Chlorine was the first halogen to be isolated as a free
one electrode is the oxidation of hydrogen gas at 1 atm element. In 1774, C. W. Scheele liberated chlorine by re-
+
pressure to H at unit activity (about 1 M concentra- action of sodium chloride, sulfuric acid, and manganese
tion) and the reaction at the second electrode is the dioxide,
reduction of interest. Higher positive values indicate a
greater tendency for reduction. heat
4NaCl + 2H 2 SO 4 + MnO 2 −→ 2Na 2 SO 4
+ MnCl 2 + 2H 2 O + Cl 2 (g), (1)
THE HALOGENS, members of Group 17 of the periodic
table (Group VIIA, older nomenclature), include the five but erroneously identified the yellowish-green gas as a
elements fluorine (F), chlorine (Cl), bromine (Br), iodine compound. The true identity of the substance was not rec-
(I), and astatine (At). Astatine, a radioactive element, does ognized until 1810, when H. Davy presented proof of its
not occur naturally except in trace amounts. The remain- elementary nature to the Royal Society and suggested the
ing halogens do not exist in the free form, but are found name chlorine (Greek chloros, “yellowish-green”) for the
primarily as anionic constituents in a large number of min- new element.
erals and in marine water. All halogens are nonmetals Iodine was isolated in 1811 by the industrial chemist
and display many similarities in chemical behavior and B. Courtois. He treated seaweed ashes with hot, concen-
in the properties of their compounds with other elements. trated sulfuric acid and condensed the resultant violet va-
There is a regular modification in these properties with por to black crystals, which he called “Substance X.” In
increasing atomic weight of the halogen, and fluorine, the 1813, J. L. Gay Lussac recognized Substance X as an el-
lightest halogen, displays some unique characteristics that ement similar to chlorine and proposed the name iodine
tend to differentiate it from the remaining members of the (Greek ioeidea, “violet-colored”).
group. Halogens and halogen-containing compounds are In1826,A.J.Balardisolatedfreebrominefromconcen-
widely employed as refrigerants, propellants, lubricants, trated natural brines. After first precipitating the sodium
bleaches, antiseptics, water purification agents, pesticides, chloride and sulfate, he passed chlorine gas through the
and gasoline additives. Other important commercial appli- liquid residues and heated the resulting material with man-
cations include photography and the prevention of tooth ganese dioxide and sulfuric acid. A red vapor was pro-
decay. duced that condensed to form a dark liquid with an irri-
tating smell. The similarity of this reaction to that used
by Scheele to produce chlorine [Eq. (1)], led Balard to
conclude that he had isolated a new element, muride, sim-
ilar to chlorine. The element was later renamed bromine
I. THE ELEMENTS
(Greek bromos, “stench”) because of its odor.
Fluorine had been suspected as an element present in
A. History
the mineral fluorite (CaF 2 ) and had been officially named
The name halogen, derived from the Greek roots hal-(“sea in 1812 by A. M. Ampere and H. Davy (Latin fluere, “to
salt”) and -gen (“to produce”), denotes the ability of the flow,” from the use of fluorite as a flux). Its isolation,
five elements in Group 17 of the Periodic Table to combine however, remained one of the chief unsolved problems of
with sodium to give compounds that either exist in, or inorganic chemistry for more than 70 years because of
have properties similar to the components of, sea salt. The the great reactivity of this element. In 1886, the French
term halogen was first used by J. S. C. Schweigger in chemist H. Moissan successfully prepared elemental flu-
1811 to describe chlorine, the only member of Group 17 orine by the electrolysis of a cooled solution of dry potas-
known at that time. As additional elements of Group 17 sium hydrogen fluoride (KHF 2 ) in anhydrous hydrofluoric