Group 7A — The Halogens

1A 2A 3A 4A 5A 6A 7A 8A
(1) (2) (13) (14) (15) (16) (17) (18)
3B 4B 5B 6B 7B 8B 1B 2B
(3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
1 H He
2 Li Be B C N O F Ne
3 Na Mg Al Si P S Cl Ar
4 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
5 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
6 Cs Ba La   Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
7 Fr Ra Ac   Rf Db Sg Bh Hs Mt Ds Rg Uub Uuq
6   Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
7   Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

 

Group 7A (or VIIA) of the periodic table are the halogens:  fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).  The name "halogen" means "salt former", derived from the Greek words halo- ("salt") and -gen ("formation").

The Group 7A elements have seven valence electrons in their highest-energy orbitals (ns2np5).  This is one electron away from having a full octet of eight electrons, so these elements tend to form anions having -1 charges, known as halides:  fluoride, F-; chloride, Cl-, bromide, Br-, and iodide, I-.  In combination with other nonmetals, the halogens form compounds through covalent bonding.

In their elemental form, the halogens form diatomic molecules, X2, connected by single bonds.  Since all of the halogens have one unpaired electron in their atomic forms, it is easy for them to "pair up" to form diatomic molecules.  The X2 molecules are nonpolar, so the only interactions between them are fairly weak London forces, but as the size of the atoms increase, the London forces become stronger, increasing their melting and boiling points:  fluorine is a gas which liquefies at -188C, chlorine is a gas which liquefies at a much higher temperature of -34C; bromine is a liquid which boils at 59C; and iodine is a solid which melts at 113C and boils at 184C.

The halogens are extremely reactive (especially fluorine), and are not found naturally in their elemental forms.  They are usually found in combination with various metals in minerals, or in combination with other nonmetals in molecular compounds.  The halogens also form compounds with carbon easily; organic molecules containing carbon are often known as alkyl halides, or organohalides, and have many different household and industrial uses.  In combination with hydrogen (which also has one unpaired electron), the halogens form the hydrohalic acids:  hydrofluoric acid (HF), hydrochloric acid (HCl), hydrobromic acid (HBr), and hydroiodic acid (HI).

 

Fluorine (F, Z=9).

In its elemental form, fluorine (F2) is a pale yellow gas; it is extremely reactive and toxic.  (In fact, a large number of chemists who tried to isolate elemental fluorine — which turned out to be an extremely difficult task — died at relatively early ages.  See Isaac Asimov's book, Asimov on Chemistry (1974), "Death in the Laboratory" for more.)  The name of the element is derived from Latin word fluere, which means "to flow."  It is found in the Earth's crust at a concentration of 950 ppm, making it the 13th most abundant element; it is also found in seawater at a concentration of 1.3 ppm.  It is found in the ores fluorite [also known as fluorspar, calcium fluoride, CaF2], cryolite [Na3AlF6], and fluorapatite {[Ca3(PO4)2]3CaF2}.

In its ionic form, fluoride (F-), it essential in the diet, but only in small doses.  It strengthens bones and teeth by becoming incorporated into the hydroxyapatite crystals, [Ca3(PO4)2]3Ca(OH)2, of bone and enamel, converting some of it to the even harder (and more acid-resistant) form of fluorapatite, [Ca3(PO4)2]3CaF2.  Fluoride is used in toothpaste, and is often added to municipal drinking water, at concentrations at or below 1 ppm, to protect against tooth decay.  (At least in small doses, it has no effect on anyone's "precious bodily fluids.")

Fluorine atoms form very strong bonds to carbon atoms, so fluorine is incorporated into many organic molecules, including the chlorofluorocarbons, which contain carbon, chlorine, and fluorine, which were widely used as propellants and refrigerants until their ozone-destroying properties were discovered (see entry on Freon-12 in the Alkanes section of the Molecule Gallery), and also in Teflon (see entry on Teflon in the Polymers section of the Molecule Gallery).

Fluorine is also found in hydrogen fluoride, or hydrofluoric acid, HF, a weak acid.  (When dealing with acids and bases, "weak" means that only a small percentage of the acid form dissociates into "H+" and "F-" ions.)  It is used in etching glass, cleaning stainless steel, and in processing uranium ore.  (In the processing of uranium, uranium in the ore is transformed into uranium hexafluoride, UF6, which can be sublimed into the gas phase; in this form, fissionable uranium-235 isotopes can be separated from non-fissionable uranium-238 isotopes by gas diffusion.)  Hydrofluoric acid is toxic and corrosive, and eats through glass (it must be stored in plastic bottles); it penetrates the skin quickly, and causes intense pain.  Concentrated solution can also start reactions with calcium ions in the body, causing hypocalcemia (an electrolyte disturbance resulting from loss of calcium), cardiac arrest, or death.

 

Chlorine (Cl, Z=17).

Chlorine in its elemental form (Cl2) is a yellow-green gas; it is poisonous (it was the first toxic gas to be used in gas warfare during World War I), and too reactive to be found in nature in the elemental form.  The name of the element is derived from the Latin word for greenish-yellow, chloros.  It is found in the Earth's crust at a concentration of 130 ppm, making it the 20th most abundant element; in seawater, its concentration is about 1.8%.  It is found in the form of chloride anions, Cl-, in the minerals halite [sodium chloride, NaCl] and sylvite [potassium chloride, KCl], chlorargyrite [silver chloride, AgCl], and in seawater.

Industrially, chlorine is produced from the electrolysis of sodium chloride.  Chlorine is used to disinfect drinking water and wastewater, in bleaches, and in the manufacture of chlorinated organic compounds (such as the vinyl chloride used in making the plastic PVC, polyvinyl chloride).

Chlorine is also found in hydrogen chloride, a colorless gas with a sharp, irritating smell.  Aqueous solutions of hydrogen chloride are known as hydrochloric acid; concentrated hydrochloric acid is about 37% HCl (about 12 moles/L).  Hydrochloric acid is also known as "muriatic acid," and under this name is often sold with swimming-pool supplies.  It is used in the synthesis of organochlorine compounds, the "pickling" of steel and other metals to dissolve scale from their surfaces, and many other uses.  Hydrochloric acid is also produced in the stomach, where it serves to break down complex foods.

Chlorine is found in bleaches and cleaners, usually in the form of sodium hypochlorite, NaOCl, which is also used to kill bacteria in drinking water.

Carbon tetrachloride, CCl4, used to be used in dry cleaning and as a spot remover; this substance is now restricted by the Montreal Protocols (link) because of its effect on the ozone layer.  Chloroform, or trihalomethane, is a very commonly used organic solvent; chloroform vapor is a anesthetic:  James Young Simpson was the first to use chloroform as an anesthetic during childbirth in 1846 (presumably, not on himself!), and it was widely used in surgery in the 19th and early 20th centuries.  However, since chloroform is carcinogenic, and toxic to the liver, it is not widely used for this purpose anymore.  (It's also useful for knocking out giant apes.)

 

Bromine (Br, Z=35).

Bromine is a dark, reddish-brown liquid at room temperature (the only nonmetallic element that is a liquid at room temperature) with a terrible smell.  The name "bromine" is derived from the Greek word for "stench," bromos.  It is found in the Earth's crust at a concentration of 0.4 ppm, making it the 62nd most abundant element; it is also found in seawater at a concentration of 65 ppm.  It is found as bromide ions, Br-, in the ore bromargyrite [silver bromide, AgBr], in seawater, and some natural sea-salt deposits and brines.

Bromine is often incorporated into organic compounds; organobromo compounds are very useful in many organic synthesis reactions.  Bromine is also found in compounds called halons, which contain carbon atoms to which fluorine, fluorine, and sometimes chlorine, are also attached.  These compounds are used in fire extinguishers, since they do not damage electronic equipment.  Methyl bromide, CH3Br, used to be used as a soil fumigant to kill insects and bacteria, but its use is being phased out under the Montreal Protocols.

 

Iodine (I, Z=53).

Iodine forms dark, shiny, purple crystals at room temperature.  The name come from the Greek word iodes, meaning "violet."  It is found in the Earth's crust at a concentration of 0.14 ppm, making it the 64th most abundant element; it is also found in seawater at a concentration of 0.06 ppm.  It is found in the ores iodargyrite [silver iodide, AgI] and lautarite [calcium iodate, Ca(IO3)2], in seawater, and some natural sea-salt deposits and brines.

Iodine is toxic, but it is so much less reactive than the other halogens that it is not as dangerous, and in low concentrations it can be used as an antibacterial agent.  "Tincture of iodine" is a solution of 3% elemental iodine in a mixture of ethanol and water, commonly used as a disinfectant for cleaning wounds and sanitizing water.  Iodine (in the form of the iodide anion, I-) is essential in the diet; it accumulates in the thyroid gland, where it is incorporated into hormones that help to regulate metabolic functions.  Iodine deficiency results in a condition called goiter, in which the thyroid gland becomes enlarged.  Iodine is commonly added to salt (iodized salt) in the form of potassium iodide (KI), sodium iodide (NaI), and potassium iodate (KIO3).  Radioactive iodine-131, a beta emitter which decays to xenon-131 with a half-life of 8 days, is used to diagnose thyroid problems.  Silver iodide, AgI, is light-sensitive, and is used in photography; it is also used in seeding clouds to promote the formation of rain.

 

Astatine (At, Z=85).

Astatine is a radioactive element.  The name of the element is derived from from the Greek word astatos, which means "unstable."  It is found in the Earth's crust in only trace amounts, and is one of the ten least abundant compounds.

Astatine is found in trace amounts in some uranium ores, where it is produced as a part of the decay series of uranium and thorium, but since all of its isotopes have fairly short half-lives (the longest, astatine-210, has a half-life of 8 hours), there is not much of this element around.  (It is estimated that there is less than 30 grams of astatine in the Earth's crust.)

 

 

References

John Emsley, The Elements, 3rd edition.  Oxford:  Clarendon Press, 1998.

John Emsley, Nature's Building Blocks:  An A-Z Guide to the Elements.  Oxford:  Oxford University Press, 2001.

David L. Heiserman, Exploring Chemical Elements and their Compounds.  New York:  TAB Books, 1992.