Halogen

The halogens are the non-metallic elements found in Group 17 of the periodic table of elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). The name "halogen" (Greek halos, salt and -genes, forming) is derived from their tendency to form a salt when reacted with a metal.

Chemistry

The valence shells of halogen atoms need only one more electron to form a completely filled shell of either eight or eighteen electrons. As such, the halogens have a strong tendency to form ions with a charge of -1 in order to complete their valence shells. They also do not exist as isolated atoms in their elemental forms; instead, they each share one electron with one other atom to form diatomic molecules.

A third consequence of the halogens' nearly filled valence shell is high reactivity towards cationic or low-ionization-potential species. Halogen anions in the body can bind to potassium, calcium and magnesium cations, rendering them useless. This is particularly true for fluoride ions, which can easily diffuse through the skin and enter the bloodstream.

Hydrohalic acids, diatomic compounds of one hydrogen atom and one halogen atom, are particularly strong acids owing to high halogen electron affinity. Care should be taken when handling these acids, particularly at high concentrations.

The halogens are found in some hypervalent compounds (eg. SF₆) and were instrumental in the formation of the first compounds of the noble gases (eg. XeF₄). They can also react with each other to form the so-called interhalogens.

Properties

The halogens demonstrate several periodic trends very well. Their electronegativity and electron affinities decrease going down the group, and their sizes, melting and boiling points increase.

Halogen	Atomic Mass (u)	Melting Point (K)	Boiling Point (K)	Electronegativity (Pauling)
Fluorine	18.998	53.53	85.03	3.98
Chlorine	35.453	171.6	239.11	3.16
Bromine	79.904	265.8	332.0	2.96
Iodine	126.904	386.85	457.4	2.66
Astatine	(210)	575	610 ?	2.2

Fluorine and chlorine are diatomic gases at room temperature, bromine is a liquid, and iodine and astatine are solids. Group 17 is the only group containing elements in all three phases at room temperature.

Organic compounds

Many synthetic organic compounds such as plastic polymers, and a few natural ones, contain halogen atoms; these are known as halogenated compounds or organic halides. Chlorine is by far the most abundant of the halogens, and the only one needed in relatively large amounts (as chloride ions) by humans. For example, chloride ions play a key role in brain function by mediating the action of the inhibitory transmitter GABA and are also used by the body to produce stomach acid. Iodine is needed in trace amounts for the production of thyroid hormones such as thyroxine. On the other hand, neither fluorine nor bromine are believed to be really essential for humans, although small amounts of fluoride can make tooth enamel resistant to decay.

Drug discovery

In drug discovery, the incorporation of halogen atoms into a lead drug candidate results in analogues that are more lipophilic and less water soluble. Consequently, halogen atoms are used to improve penetration through lipid membranes. However, there is an undesirable tendency for halogenated drugs to accumulate in lipid tissue.

The chemical reactivity of halogen atoms depends on both their point of attachment to the lead and the nature of the halogen. Aromatic halogen groups are far less reactive than aliphatic halogen groups, which can exhibit considerable chemical reactivity. For aliphatic carbon-halogen bonds the C-F bond is the strongest and usually less chemically reactive than aliphatic C-H bonds. The other aliphatic-halogen bonds are weaker, their reactivity increasing down the periodic table. They are usually more chemically reactive than aliphatic C-H bonds. Consequently, the most popular halogen substitutions are the less reactive aromatic fluorine and chlorine groups.

References

1. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
2. G. Thomas, Medicinal Chemistry an Introduction, John Wiley & Sons, West Sussex, UK, 2000.