| Group | 17 | ||
|---|---|---|---|
| Period | |||
| 2 | title="F, Fluorine" style="text-align:center; color:#000000; background-color:#ffff99; border:2px solid #6e6e8e; ;"| 9 F | ||
| 3 | title="Cl, Chlorine" style="text-align:center; color:#000000; background-color:#ffff99; border:2px solid #6e6e8e; ;"| 17 Cl | ||
| 4 | title="Br, Bromine" style="text-align:center; color:#000000; background-color:#ffff99; border:2px solid #6e6e8e; ;"| 35 Br |
! 5 | title="I, Iodine" style="text-align:center; color:#000000; background-color:#ffff99; border:2px solid #6e6e8e; ;"| 53 I |
| 6 | title="At, Astatine" style="text-align:center; color:#000000; background-color:#ffff99; border:2px dashed #773300; ;"| 85 At | ||
| 7 | title="Uus, ununseptium" style="text-align:center; color:#000000; background-color:#ffff99; border:2px solid transparent; ;"| 117 Uus |
In Inorganic Chemistry, Halogens or Halogen Elements are a series of elements from the Group 17 (old-style: VII or VIIA; Group 7 IUPAC Style) only present naturally in compounds as halide ions, and as diatomic molecules in synthesised natural forms. The term originates from 18th century scientific French nomenclature based on erring adaptations of Greek roots; the greek word "halos" meaning "salt", and "genes" meaning "production" -- referring to elements which produce a salt in union with a metal. At room temperature, the group consists of two solids, two gases, and one liquid, which makes the Halogens the only group with all three forms of matter .
Properties
Reactivity
In their natural states, halogens are radioactive and do not do not occur in their diatomic states (F2 , Br2) due to radioactivity and the process of decay into several isotopes within several hours of synthesis[1]. These elements, do occur in other compounds such as Sodium Chloride and fluorite, which are the most abundant halides, which are found in deposits of crustal rocks and yield small amounts of natural-state halogens that can be extracted and synthesised.[1] In this natural-state, they require an additional electron from another source such as a metal to obtain an octet, and form a single negatively-charged ion ( ) known as a halide ion.
Because of their high electronegativities, the halogens, particularly Fluorine are highly reactive and can cause serious harm to biological organisms if ingested, due to their reaction with organic compounds as corrosive compounds or toxic gases; fluorine is the most reactive element in existence, even attacking glass can form compounds with the heavier noble gases. However, because of their high reactivity they can be used as potent sterilant for the elimination of bacteria due to their rapid rate of reaction. Similarly, their reactive properties are also put to use in bleaching; Chlorine is the active ingredient of most fabric bleaches and is used in the production of most paper products.
Oxidation states
Halogens are by default, one electron less than that of a noble gas, and to obtain a stable structure require an electron to be recieved through bonding or ionisation of another element to produce a negative ion ( ) through an exothermic reaction.[2] There are numerous ionically-bonded halogen compounds which form noble gas configurations, as well as covalent compounds formed through covalent bonds with other halogens of non-polar electronegative elements. Under normal conditions, the formation of halogen ions can only usually result in a single negative charge and oxidation state, due to the completion of it's outer shell and the amount of energy required to promote an element's structure to it's next shell by electron gain.[3][4] For instance, the gain of electrons in the structure of fluorine to yield it's electron shells requires significantly higher amounts of energy than found under "normal conditions".[5]
However, because of unfilled d orbitals in halogens, they are still available for covalency and can contribute towards the formation of compounds formed from other halogens and oxygen, which can achieve oxidation states as high as +7, such as those observed in chlorate I, V, and VII anions (ClO-, ClO3- and ClO4-).[3][2][5]
| halogen | molecule | structure | model | d(X−X) / pm (gas phase) |
d(X−X) / pm (solid phase) |
|---|---|---|---|---|---|
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Halide ions combined with single hydrogen atoms form the hydrohalic acids (i.e., HF, HCl, HBr, HI), a series of particularly strong acids. (HAt, or "hydrastatic acid", should also qualify, but it is not typically included in discussions of hydrohalic acid due to astatine's extreme instability toward alpha decay.)
They react with each other to form interhalogen compounds. Diatomic interhalogen compounds (BrF, ICl, ClF, etc.) bear strong superficial resemblance to the pure halogens.
Properties
The halogens show a number of trends when moving down the group - for instance, decreasing electronegativity and reactivity, increasing melting and boiling point.
| 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 |
| Ununseptium | (291)* | * | * | * |
* Ununseptium has not yet been discovered; values are either unknown if no value appears, or are estimates based on other similar chemicals.
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.
See also
Notes
References
- Holliday, C., Chambers, A. K. (1975) Modern Inorganic Chemistry: an intermediate text Butterworth & Co (Publishers)
- Greenwood, N. N.,Earnshaw, A. (1997) Chemistry of the Elements 2nd ed. Oxford:Butterworth-Heinemann
- G. Thomas, Medicinal Chemistry: an Introduction, John Wiley & Sons, West Sussex, UK, 2000.
| bgcolor="Template:Element color/Halogens" | Halogens | Atomic numbers in Template:Element color/Gas are gases | Atomic numbers in Template:Element color/Liquid are liquids | Atomic numbers in Template:Element color/Solid are solids |
|---|---|---|---|
| style="border:Template:Element frame/Primordial;" | Solid borders indicate primordial elements (older than the Earth) | style="border:Template:Element frame/Natural radio;" | Dashed borders indicate radioactive natural elements | style="border:Template:Element frame/Synthetic;" | Dotted borders indicate radioactive synthetic elements | style="border:Template:Element frame/Undiscovered;" | No borders indicates undiscovered elements |