Everything about Halogen totally explained
halogens or
halogen elements are a
series of
nonmetal elements from
Group 17 (old-style: VII or VIIA; Group 7
IUPAC Style) of the
periodic table, comprising
fluorine, F;
chlorine, Cl;
bromine, Br;
iodine, I; and
astatine, At. The undiscovered element 117, temporarily named
ununseptium, may also be a halogen.
The group of halogens is the only group which contains elements in all three familiar
states of matter at
standard temperature and pressure.
Abundance
Owing to their high
reactivity, the halogens are found in the environment only in
compounds or as
ions.
Halide ions and
oxoanions such as
iodate (IO
3−) can be found in many minerals and in seawater.
Halogenated organic compounds can also be found as natural products in living organisms. In their elemental forms, the halogens exist as
diatomic molecules, but these only have a fleeting existence in nature and are much more common in the laboratory and in industry. At room temperature and pressure, fluorine and chlorine are gases, bromine is a liquid and iodine and astatine are solids; Group 17 is therefore the only periodic table group exhibiting all three
states of matter at room temperature.
Etymology
The term
halogen originates from
18th century scientific
French nomenclature based on adaptations of
Greek roots:
hals (sea) or
halas (salt), and
gen- (to
generate) — referring to elements which produce a
salt in union with a
metal.
Properties
The halogens show a number of trends when moving down the group - for instance, decreasing
electronegativity and reactivity, and increasing
melting and
boiling point.
| Halogen |
Standard Atomic Weight (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 |
Diatomic halogen molecules
| halogen |
molecule |
structure |
model |
d(X−X) / pm
(gas phase) |
d(X−X) / pm
(solid phase) |
| fluorine |
F2 |
|
|
143 |
149 |
| chlorine |
Cl2 |
|
|
199 |
198 |
| bromine |
Br2 |
|
|
228 |
227 |
| iodine |
I2 |
|
|
266 |
272 |
Chemistry
Reactivity
Halogens are highly
reactive, and as such can be harmful or lethal to
biological organisms in sufficient quantities. This high reactivity is due to the atoms being one electron short of a full outer shell of eight electrons. They can gain this electron by reacting with atoms of other elements.
Fluorine is the most reactive element in existence, attacking otherwise inert materials such as glass, and forming compounds with the heavier
noble gases. It is a corrosive and highly toxic gas. The reactivity of fluorine is such that if used or stored in laboratory glassware, it can react with glass in the presence of small amounts of water to form
silicon tetrafluoride (SiF
4). Thus fluorine must be handled with substances such as
Teflon, extremely dry glass, or metals such as copper or steel which form a protective layer of fluoride on their surface.
Both chlorine and bromine are used as
disinfectants for drinking water, swimming pools, fresh wounds, dishes, and surfaces. They kill
bacteria and other potentially harmful
microorganisms through a process known as
sterilization. Their reactivity is also put to use in
bleaching.
Sodium hypochlorite, which is produced from chlorine, is the active ingredient of most
fabric bleaches and chlorine-derived bleaches are used in the production of some
paper products.
Hydrogen halides
The halogens all form binary compounds with hydrogen, the
hydrogen halides
HF,
HCl,
HBr,
HI, and
HAt), a series of particularly strong
acids. When in aqueous solution, the hydrogen halides are known as
hydrohalic acids. HAt, or "hydrastatic acid", should also qualify, but it isn't typically included in discussions of hydrohalic acid due to astatine's extreme instability toward
alpha decay.
Interhalogen compounds
The halogens react with each other to form interhalogen compounds. Diatomic interhalogen compounds such as
BrF,
ICl, and
ClF) bear resemblance to the pure halogens in some respects. The properties and behaviour of a diatomic interhalogen compound tend to be intermediate between those of its parent halogens. Some properties, however, are found in neither parent halogen. For example, Cl
2 and I
2 are soluble in
CCl4, but ICl isn't since it's a
polar molecule due to the relatively large
electronegativity difference between I and Cl.
Organohalogen 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 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 usually more
lipophilic and less water soluble. Consequently, halogen atoms are used to improve penetration through
lipid membranes and tissues. Consequently, there's an tendency for some halogenated drugs to accumulate in
adipose 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 common halogen substitutions are the less reactive aromatic fluorine and chlorine groups.
Solubility in water
Fluorine reacts vigorously with water to produce
oxygen (O
2) and
hydrogen fluoride (HF):
» 2 F
2(g) + 2 H
2O(l) → O
2(g) + 4 HF(aq)
Chlorine has minimal solubility in water, with maximum solubility at 49.3°F (9.6°C) when approximately 1% is dissolved. Dissolved chlorine reacts to form
hydrochloric acid (HCl) and
hypochlorous acid, a solution that can be used as a
disinfectant or
bleach:
» Cl
2(g) + H
2O(l) → HCl(aq) + HClO(aq)
Bromine has a solubility of 3.41 g per 100 g of water, but it slowly reacts to form
hydrogen bromide (HBr) and
hypobromous acid (HBrO):
» Br
2(g) + H
2O(l) → HBr(aq) + HBrO(aq)
Iodine, however, is minimally soluble in water (0.03 g/100 g water @ 20 °C) and doesn't react with it.
Further Information
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