General Chemistry Principles, Patterns, and Applications, 2011

an ns 2 np 5 valence electron configuration, and all but astatine are diatomic molecules in which the two
halogen atoms share a pair of electrons. Diatomic F2 and Cl2 are pale yellow-green and pale green gases,
respectively, while Br2 is a red liquid, and I2 is a purple solid. The halogens were not isolated until the 18th
and 19th centuries.
Because of their relatively high electronegativities, the halogens are nonmetallic and generally react by
gaining one electron per atom to attain a noble gas electron configuration and an oxidation state of
−1. Halides are produced according to the following equation, in which X denotes a halogen:
Equation 7.16
2 E + nX 2 → 2 EXn
If the element E has a low electronegativity (as does Na), the product is typically an ionic halide (NaCl). If
the element E is highly electronegative (as P is), the product is typically a covalent halide (PCl5). Ionic
halides tend to be nonvolatile substances with high melting points, whereas covalent halides tend to be
volatile substances with low melting points. Fluorine is the most reactive of the halogens, and iodine the
least, which is consistent with their relative electronegativities (Figure 7.15 "Pauling Electronegativity
Values of the "). [1] In fact, fluorine reacts with nearly all elements at room temperature. Under more
extreme conditions, it combines with all elements except helium, neon, and argon.
The halogens react with hydrogen to form the hydrogen halides (HX):
Equation 7.17
H 2 (g) + X 2 (g,l,s) → 2 HX(g)
Fluorine is so reactive that any substance containing hydrogen, including coal, wood, and even water, will
burst into flames if it comes into contact with pure F2.
Because it is the most electronegative element known, fluorine never has a positive oxidation state in any
compound. In contrast, the other halogens (Cl, Br, I) form compounds in which their oxidation states are
+1, +3, +5, and +7, as in the oxoanions, XOn − , where n = 1–4. Because oxygen has the second highest
electronegativity of any element, it stabilizes the positive oxidation states of the halogens in these ions.
All of the halogens except astatine (which is radioactive) are commercially important. NaCl in salt water is
purified for use as table salt. Chlorine and hypochlorite (OCl − ) salts are used to sanitize public water
supplies, swimming pools, and wastewater, and hypochlorite salts are also used as bleaches because they
oxidize colored organic molecules. Organochlorine compounds are used as drugs and pesticides. Fluoride
(usually in the form of NaF) is added to many municipal water supplies to help prevent tooth decay, and
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