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= Halogen =
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Introduction
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↓ Period
2
3
4
5
6
7
colspan="2" ---- 'Legend' {
|Primordial}}; background:; padding:0 2px;" |primordial element
|from decay}}; background:;" |element from decay
|Synthetic}}; background:;" | Synthetic
|}
The halogens () are a group in the periodic table consisting of six
chemically related elements: fluorine (F), chlorine (Cl), bromine
(Br), iodine (I), and the radioactive elements astatine (At) and
tennessine (Ts), though some authors would exclude tennessine as its
chemistry is unknown and is theoretically expected to be more like
that of gallium. In the modern IUPAC nomenclature, this group is known
as group 17.
The word "halogen" means "salt former" or "salt maker". When halogens
react with metals, they produce a wide range of salts, including
calcium fluoride, sodium chloride (common table salt), silver bromide,
and potassium iodide.
The group of halogens is the only periodic table group that contains
elements in three of the main states of matter at standard temperature
and pressure, though not far above room temperature the same becomes
true of groups 1 and 15, assuming white phosphorus is taken as the
standard state. All of the halogens form acids when bonded to
hydrogen. Most halogens are typically produced from minerals or salts.
The middle halogens--chlorine, bromine, and iodine--are often used as
disinfectants. Organobromides are the most important class of flame
retardants, while elemental halogens are dangerous and can be toxic.
History
======================================================================
The fluorine mineral fluorspar was known as early as 1529. It is
believed to be found in the foot bones of early dinosaurs. Early
chemists realized that fluorine compounds contain an undiscovered
element, but were unable to isolate it. In 1869, George Gore, an
English chemist, ran a current of electricity through hydrofluoric
acid and probably produced fluorine, but he was unable to prove his
results at the time. In 1886, Henri Moissan, a chemist in Paris,
performed electrolysis on potassium bifluoride dissolved in anhydrous
hydrogen fluoride, and successfully isolated fluorine.
Hydrochloric acid was known to alchemists and early chemists. However,
elemental chlorine was not produced until 1774, when Carl Wilhelm
Scheele heated hydrochloric acid with manganese dioxide. Scheele
called the element "dephlogisticated muriatic acid", which is how
chlorine was known for 33 years. In 1807, Humphry Davy investigated
chlorine and discovered that it is an actual element. Chlorine gas was
used as a poisonous gas during World War I. It displaced oxygen in
contaminated areas and replaced common oxygenated air with the toxic
chlorine gas. The gas would burn human tissue externally and
internally, especially the lungs, making breathing difficult or
impossible depending on the level of contamination.
Bromine was discovered in the 1820s by Antoine Jérôme Balard. Balard
discovered bromine by passing chlorine gas through a sample of brine.
He originally proposed the name 'muride' for the new element, but the
French Academy changed the element's name to bromine.
Iodine was discovered by Bernard Courtois, who was using seaweed ash
as part of a process for saltpeter manufacture. Courtois typically
boiled the seaweed ash with water to generate potassium chloride.
However, in 1811, Courtois added sulfuric acid to his process and
found that his process produced purple fumes that condensed into black
crystals. Suspecting that these crystals were a new element, Courtois
sent samples to other chemists for investigation. Iodine was proven to
be a new element by Joseph Gay-Lussac.
In 1931, Fred Allison claimed to have discovered element 85 with a
magneto-optical machine, and named the element Alabamine, but was
mistaken. In 1937, Rajendralal De claimed to have discovered element
85 in minerals, and called the element dakine, but he was also
mistaken. An attempt at discovering element 85 in 1939 by Horia
Hulubei and Yvette Cauchois via spectroscopy was also unsuccessful, as
was an attempt in the same year by Walter Minder, who discovered an
iodine-like element resulting from beta decay of polonium. Element 85,
now named astatine, was produced successfully in 1940 by Dale R.
Corson, K.R. Mackenzie, and Emilio G. Segrè, who bombarded bismuth
with alpha particles.
In 2010, a team led by nuclear physicist Yuri Oganessian involving
scientists from the JINR, Oak Ridge National Laboratory, Lawrence
Livermore National Laboratory, and Vanderbilt University successfully
bombarded berkelium-249 atoms with calcium-48 atoms to make
tennessine.
Etymology
===========
In 1811, the German chemist Johann Schweigger proposed that the name
"halogen" - meaning "salt producer", from αλς [hals] "salt" and γενειν
[genein] "to beget" - replace the name "chlorine", which had been
proposed by the English chemist Humphry Davy. Davy's name for the
element prevailed. However, in 1826, the Swedish chemist Baron Jöns
Jacob Berzelius proposed the term "halogen" for the elements fluorine,
chlorine, and iodine, which produce a sea-salt-like substance when
they form a compound with an alkaline metal.
The English names of these elements all have the ending -ine.
Fluorine's name comes from the Latin word 'fluere', meaning "to flow",
because it was derived from the mineral fluorite, which was used as a
flux in metalworking. Chlorine's name comes from the Greek word
'chloros', meaning "greenish-yellow". Bromine's name comes from the
Greek word 'bromos', meaning "stench". Iodine's name comes from the
Greek word 'iodes', meaning "violet". Astatine's name comes from the
Greek word 'astatos', meaning "unstable". Tennessine is named after
the US state of Tennessee, where it was synthesized.
Chemical
==========
The halogens fluorine, chlorine, bromine, and iodine are nonmetals;
the chemical properties of astatine and tennessine, two heaviest group
17 members, have not been conclusively investigated. The halogens show
trends in chemical bond energy moving from top to bottom of the
periodic table column with fluorine deviating slightly. It follows a
trend in having the highest bond energy in compounds with other atoms,
but it has very weak bonds within the diatomic F2 molecule. This means
that further down group 17 in the periodic table, the reactivity of
elements decreases because of the increasing size of the atoms.
Halogen bond energies (kJ/mol)
X X2 HX BX3 AlX3 CX4
F '159' '574' '645' '582' '456'
Cl |243 |428 |444 |427 |327
Br |193 |363 |368 |360 |272
I |151 |294 |272 |285 |239
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 high electronegativity of the atoms due to their high
effective nuclear charge. Because the halogens have seven valence
electrons in their outermost energy level, they can gain an electron
by reacting with atoms of other elements to satisfy the octet rule.
Fluorine is the most reactive of all elements; it is the only element
more electronegative than oxygen, it attacks otherwise-inert materials
such as glass, and it forms compounds with the usually inert 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 (SiF4). Thus, fluorine must be handled with
substances such as Teflon (which is itself an organofluorine
compound), extremely dry glass, or metals such as copper or steel,
which form a protective layer of fluoride on their surface.
The high reactivity of fluorine allows some of the strongest bonds
possible, especially to carbon. For example, Teflon is fluorine bonded
with carbon and is extremely resistant to thermal and chemical attacks
and has a high melting point.
Diatomic halogen molecules
============================
The stable halogens form homonuclear diatomic molecules.
Due to relatively weak intermolecular forces, chlorine and fluorine
form part of the group known as "elemental gases".
halogen molecule structure model 'd'(X−X) / pm (gas phase)
'd'(X−X) / pm (solid phase)
fluorine F2 45px 45px 143 149
chlorine Cl2 70px 63px 199 198
bromine Br2 80px 72px 228 227
iodine I2 70px 84px 266 272
The elements become less reactive and have higher melting points as
the atomic number increases. The higher melting points are caused by
stronger London dispersion forces resulting from more electrons.
Hydrogen halides
==================
All of the halogens have been observed to react with hydrogen to form
hydrogen halides. For fluorine, chlorine, and bromine, this reaction
is in the form of:
: H2 + X2 → 2HX
However, hydrogen iodide and hydrogen astatide can split back into
their constituent elements.
The hydrogen-halogen reactions get gradually less reactive toward the
heavier halogens. A fluorine-hydrogen reaction is explosive even when
it is dark and cold. A chlorine-hydrogen reaction is also explosive,
but only in the presence of light and heat. A bromine-hydrogen
reaction is even less explosive; it is explosive only when exposed to
flames. Iodine and astatine only partially react with hydrogen,
forming equilibria.
All halogens form binary compounds with hydrogen known as the hydrogen
halides: hydrogen fluoride (HF), hydrogen chloride (HCl), hydrogen
bromide (HBr), hydrogen iodide (HI), and hydrogen astatide (HAt). All
of these compounds form acids when mixed with water. Hydrogen fluoride
is the only hydrogen halide that forms hydrogen bonds. Hydrochloric
acid, hydrobromic acid, hydroiodic acid, and acid are all strong
acids, but hydrofluoric acid is a weak acid.
All of the hydrogen halides are irritants. Hydrogen fluoride and
hydrogen chloride are highly acidic. Hydrogen fluoride is used as an
industrial chemical, and is highly toxic, causing pulmonary edema and
damaging cells. Hydrogen chloride is also a dangerous chemical.
Breathing in gas with more than fifty parts per million of hydrogen
chloride can cause death in humans. Hydrogen bromide is even more
toxic and irritating than hydrogen chloride. Breathing in gas with
more than thirty parts per million of hydrogen bromide can be lethal
to humans. Hydrogen iodide, like other hydrogen halides, is toxic.
Metal halides
===============
All the halogens are known to react with sodium to form sodium
fluoride, sodium chloride, sodium bromide, sodium iodide, and sodium
astatide. Heated sodium's reaction with halogens produces
bright-orange flames. Sodium's reaction with chlorine is in the form
of:
:
Iron reacts with fluorine, chlorine, and bromine to form iron(III)
halides. These reactions are in the form of:
:
However, when iron reacts with iodine, it forms only iron(II) iodide.
:
Iron wool can react rapidly with fluorine to form the white compound
iron(III) fluoride even in cold temperatures. When chlorine comes into
contact with a heated iron, they react to form the black iron(III)
chloride. However, if the reaction conditions are moist, this reaction
will instead result in a reddish-brown product. Iron can also react
with bromine to form iron(III) bromide. This compound is reddish-brown
in dry conditions. Iron's reaction with bromine is less reactive than
its reaction with fluorine or chlorine. A hot iron can also react with
iodine, but it forms iron(II) iodide. This compound may be gray, but
the reaction is always contaminated with excess iodine, so it is not
known for sure. Iron's reaction with iodine is less vigorous than its
reaction with the lighter halogens.
Interhalogen compounds
========================
Interhalogen compounds are in the form of XYn where X and Y are
halogens and n is one, three, five, or seven. Interhalogen compounds
contain at most two different halogens. Large interhalogens, such as
can be produced by a reaction of a pure halogen with a smaller
interhalogen such as . All interhalogens except can be produced by
directly combining pure halogens in various conditions.
Interhalogens are typically more reactive than all diatomic halogen
molecules except F2 because interhalogen bonds are weaker. However,
the chemical properties of interhalogens are still roughly the same as
those of diatomic halogens. Many interhalogens consist of one or more
atoms of fluorine bonding to a heavier halogen. Chlorine and bromine
can bond with up to five fluorine atoms, and iodine can bond with up
to seven fluorine atoms. Most interhalogen compounds are covalent
gases. However, some interhalogens are liquids, such as BrF3, and many
iodine-containing interhalogens are solids.
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 in seawater, 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. Organohalogens are also synthesized
through the nucleophilic abstraction reaction.
Polyhalogenated compounds
===========================
Polyhalogenated compounds are industrially created compounds
substituted with multiple halogens. Many of them are very toxic and
bioaccumulate in humans, and have a very wide application range. They
include PCBs, PBDEs, and perfluorinated compounds (PFCs), as well as
numerous other compounds.
Reactions with water
======================
Fluorine reacts vigorously with water to produce oxygen (O2) and
hydrogen fluoride (HF):
:
Chlorine has maximum solubility of ca. 7.1 g Cl2 per kg of water at
ambient temperature (21 °C). Dissolved chlorine reacts to form
hydrochloric acid (HCl) and hypochlorous acid, a solution that can be
used as a disinfectant or bleach:
:
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):
:
Iodine, however, is minimally soluble in water (0.03 g/100 g water at
20 °C) and does not react with it. However, iodine will form an
aqueous solution in the presence of iodide ion, such as by addition of
potassium iodide (KI), because the triiodide ion is formed.
Physical and atomic
=====================
The table below is a summary of the key physical and atomic properties
of the halogens. Data marked with question marks are either uncertain
or are estimations partially based on periodic trends rather than
observations.
!Halogen !Standard atomic weight(Da) !Melting point(K) !Melting
point(°C) !Boiling point(K) !Boiling point(°C) !Density(g/cm3 at 25
°C) !Electronegativity(Pauling) !First ionization energy(kJ·mol−1) !!
Covalent radius(pm)
Fluorine 18.9984032(5) 53.53 −219.62 85.03 −188.12 0.0017
3.98 1681.0 71
Chlorine [35.446; 35.457] 171.6 −101.5 239.11 −34.04
0.0032 3.16 1251.2 99
Bromine 79.904(1) 265.8 −7.3 332.0 58.8 3.1028 2.96
1139.9 114
Iodine 126.90447(3) 386.85 113.7 457.4 184.3 4.933 2.66
1008.4 133
Astatine [210] 575 302 ? 610 ? 337 ? 6.2-6.5 2.2
899.0 ? 145
Tennessine [294] ? 623-823 ? 350-550 ? 883 ? 610 ?
7.1-7.3 - ? 743 ? 157
!'Z' !! Element !! Electrons per shell
9 fluorine 2, 7
17 chlorine 2, 8, 7
35 bromine 2, 8, 18, 7
53 iodine 2, 8, 18, 18, 7
85 astatine 2, 8, 18, 32, 18, 7
117 tennessine 2, 8, 18, 32, 32, 18, 7 '(predicted)'
Sublimation or boiling point (oC) of halogens at various pressures
colspan="2" rowspan="1" |Tmelt (оС) −100.7 −7.3 |112.9
!log(P[Pa]) !mmHg !Cl2 !Br2 !I2
|2.12490302 |1 −118 −48.7 |38.7
|2.82387302 |5 −106.7 −32.8 |62.2
|3.12490302 |10 −101.6 −25 |73.2
|3.42593302 |20 −93.3 −16.8 |84.7
|3.72696301 |40 −84.5 −8 |97.5
|3.90305427 |60 −79 −0.6 |105.4
|4.12490302 |100 −71.7 |9.3 |116.5
|4.42593302 |200 −60.2 |24.3 |137.3
|4.72696301 |400 −47.3 |41 |159.8
|5.00571661 |760 −33.8 |58.2 |183
!log(P[Pa]) !atm !Cl2 !Br2 !I2
|5.00571661 |1 −33.8 |58.2 |183
|5.30674661 |2 −16.9 |78.8
|5.70468662 |5 |10.3 |110.3
|6.00571661 |10 |35.6 |139.8
|6.30674661 |20 |65 |174
|6.48283787 |30 |84.8 |197
|6.6077766 |40 |101.6 |215
|6.70468662 |50 |115.2 |230
|6.78386786 |60 |127.1 |243.5
Isotopes
==========
Fluorine has one stable and naturally occurring isotope, fluorine-19.
However, there are trace amounts in nature of the radioactive isotope
fluorine-23, which occurs via cluster decay of protactinium-231. A
total of eighteen isotopes of fluorine have been discovered, with
atomic masses ranging from 13 to 31.
Chlorine has two stable and naturally occurring isotopes, chlorine-35
and chlorine-37. However, there are trace amounts in nature of the
isotope chlorine-36, which occurs via spallation of argon-36. A total
of 24 isotopes of chlorine have been discovered, with atomic masses
ranging from 28 to 51.
There are two stable and naturally occurring isotopes of bromine,
bromine-79 and bromine-81. A total of 33 isotopes of bromine have been
discovered, with atomic masses ranging from 66 to 98.
There is one stable and naturally occurring isotope of iodine,
iodine-127. However, there are trace amounts in nature of the
radioactive isotope iodine-129, which occurs via spallation and from
the radioactive decay of uranium in ores. Several other radioactive
isotopes of iodine have also been created naturally via the decay of
uranium. A total of 38 isotopes of iodine have been discovered, with
atomic masses ranging from 108 to 145.
There are no stable isotopes of astatine. However, there are four
naturally occurring radioactive isotopes of astatine produced via
radioactive decay of uranium, neptunium, and plutonium. These isotopes
are astatine-215, astatine-217, astatine-218, and astatine-219. A
total of 31 isotopes of astatine have been discovered, with atomic
masses ranging from 191 to 227.
There are no stable isotopes of tennessine. Tennessine has only two
known synthetic radioisotopes, tennessine-293 and tennessine-294.
Production
======================================================================
Approximately six million metric tons of the fluorine mineral fluorite
are produced each year. Four hundred-thousand metric tons of
hydrofluoric acid are made each year. Fluorine gas is made from
hydrofluoric acid produced as a by-product in phosphoric acid
manufacture. Approximately 15,000 metric tons of fluorine gas are made
per year.
The mineral halite is the mineral that is most commonly mined for
chlorine, but the minerals carnallite and sylvite are also mined for
chlorine. Forty million metric tons of chlorine are produced each year
by the electrolysis of brine.
Approximately 450,000 metric tons of bromine are produced each year.
Fifty percent of all bromine produced is produced in the United
States, 35% in Israel, and most of the remainder in China.
Historically, bromine was produced by adding sulfuric acid and
bleaching powder to natural brine. However, in modern times, bromine
is produced by electrolysis, a method invented by Herbert Dow. It is
also possible to produce bromine by passing chlorine through seawater
and then passing air through the seawater.
In 2003, 22,000 metric tons of iodine were produced. Chile produces
40% of all iodine produced, Japan produces 30%, and smaller amounts
are produced in Russia and the United States. Until the 1950s, iodine
was extracted from kelp. However, in modern times, iodine is produced
in other ways. One way that iodine is produced is by mixing sulfur
dioxide with nitrate ores, which contain some iodates. Iodine is also
extracted from natural gas fields.
Even though astatine is naturally occurring, it is usually produced by
bombarding bismuth with alpha particles.
Tennessine is made by using a cyclotron, fusing berkelium-249 and
calcium-48 to make tennessine-293 and tennessine-294.
Disinfectants
===============
Both chlorine and bromine are used as disinfectants for drinking
water, swimming pools, fresh wounds, spas, 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.
Lighting
==========
Halogen lamps are a type of incandescent lamp using a tungsten
filament in bulbs that have small amounts of a halogen, such as iodine
or bromine added. This enables the production of lamps that are much
smaller than non-halogen incandescent lightbulbs at the same wattage.
The gas reduces the thinning of the filament and blackening of the
inside of the bulb resulting in a bulb that has a much greater life.
Halogen lamps glow at a higher temperature (2800 to 3400 kelvin) with
a whiter colour than other incandescent bulbs. However, this requires
bulbs to be manufactured from fused quartz rather than silica glass to
reduce breakage.
Drug components
=================
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. As a consequence, halogen atoms are used to
improve penetration through lipid membranes and tissues. It follows
that there is a 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. As a consequence, the most common halogen
substitutions are the less reactive aromatic fluorine and chlorine
groups.
Biological role
======================================================================
Fluoride anions are found in ivory, bones, teeth, blood, eggs, urine,
and hair of organisms. Fluoride anions in very small amounts may be
essential for humans. There are 0.5 milligrams of fluorine per liter
of human blood. Human bones contain 0.2 to 1.2% fluorine. Human tissue
contains approximately 50 parts per billion of fluorine. A typical
70-kilogram human contains 3 to 6 grams of fluorine.
Chloride anions are essential to a large number of species, humans
included. The concentration of chlorine in the dry weight of cereals
is 10 to 20 parts per million, while in potatoes the concentration of
chloride is 0.5%. Plant growth is adversely affected by chloride
levels in the soil falling below 2 parts per million. Human blood
contains an average of 0.3% chlorine. Human bone typically contains
900 parts per million of chlorine. Human tissue contains approximately
0.2 to 0.5% chlorine. There is a total of 95 grams of chlorine in a
typical 70-kilogram human.
Some bromine in the form of the bromide anion is present in all
organisms. A biological role for bromine in humans has not been
proven, but some organisms contain organobromine compounds. Humans
typically consume 1 to 20 milligrams of bromine per day. There are
typically 5 parts per million of bromine in human blood, 7 parts per
million of bromine in human bones, and 7 parts per million of bromine
in human tissue. A typical 70-kilogram human contains 260 milligrams
of bromine.
Humans typically consume less than 100 micrograms of iodine per day.
Iodine deficiency can cause intellectual disability. Organoiodine
compounds occur in humans in some of the glands, especially the
thyroid gland, as well as the stomach, epidermis, and immune system.
Foods containing iodine include cod, oysters, shrimp, herring,
lobsters, sunflower seeds, seaweed, and mushrooms. However, iodine is
not known to have a biological role in plants. There are typically
0.06 milligrams per liter of iodine in human blood, 300 parts per
billion of iodine in human bones, and 50 to 700 parts per billion of
iodine in human tissue. There are 10 to 20 milligrams of iodine in a
typical 70-kilogram human.
Astatine, although very scarce, has been found in micrograms in the
earth. It has no known biological role because of its high
radioactivity, extreme rarity, and has a half-life of just about 8
hours for the most stable isotope.
Tennessine is purely man-made and has no other roles in nature.
Toxicity
======================================================================
The halogens tend to decrease in toxicity towards the heavier
halogens.
Fluorine gas is extremely toxic; breathing in fluorine at a
concentration of 25 parts per million is potentially lethal.
Hydrofluoric acid is also toxic, being able to penetrate skin and
cause highly painful burns. In addition, fluoride anions are toxic,
but not as toxic as pure fluorine. Fluoride can be lethal in amounts
of 5 to 10 grams. Prolonged consumption of fluoride above
concentrations of 1.5 mg/L is associated with a risk of dental
fluorosis, an aesthetic condition of the teeth. At concentrations
above 4 mg/L, there is an increased risk of developing skeletal
fluorosis, a condition in which bone fractures become more common due
to the hardening of bones. Current recommended levels in water
fluoridation, a way to prevent dental caries, range from 0.7 to 1.2
mg/L to avoid the detrimental effects of fluoride while at the same
time reaping the benefits. People with levels between normal levels
and those required for skeletal fluorosis tend to have symptoms
similar to arthritis.
Chlorine gas is highly toxic. Breathing in chlorine at a concentration
of 3 parts per million can rapidly cause a toxic reaction. Breathing
in chlorine at a concentration of 50 parts per million is highly
dangerous. Breathing in chlorine at a concentration of 500 parts per
million for a few minutes is lethal. In addition, breathing in
chlorine gas is highly painful because of its corrosive properties.
Hydrochloric acid is the acid of chlorine, while relatively nontoxic,
it is highly corrosive and releases very irritating and toxic hydrogen
chloride gas in open air.
Pure bromine is somewhat toxic but less toxic than fluorine and
chlorine. One hundred milligrams of bromine is lethal. Bromide anions
are also toxic, but less so than bromine. Bromide has a lethal dose of
30 grams.
Iodine is somewhat toxic, being able to irritate the lungs and eyes,
with a safety limit of 1 milligram per cubic meter. When taken orally,
3 grams of iodine can be lethal. Iodide anions are mostly nontoxic,
but these can also be deadly if ingested in large amounts.
Astatine is radioactive and thus highly dangerous, but it has not been
produced in macroscopic quantities and hence it is most unlikely that
its toxicity will be of much relevance to the average individual.
Tennessine cannot be chemically investigated due to how short its
half-life is, although its radioactivity would make it very dangerous.
Superhalogen
======================================================================
Certain aluminium clusters have superatom properties. These aluminium
clusters are generated as anions ( with 'n' = 1, 2, 3, ... ) in helium
gas and reacted with a gas containing iodine. When analyzed by mass
spectrometry one main reaction product turns out to be . These
clusters of 13 aluminium atoms with an extra electron added do not
appear to react with oxygen when it is introduced in the same gas
stream. Assuming each atom liberates its 3 valence electrons, this
means 40 electrons are present, which is one of the magic numbers for
sodium and implies that these numbers are a reflection of the noble
gases.
Calculations show that the additional electron is located in the
aluminium cluster at the location directly opposite from the iodine
atom. The cluster must therefore have a higher electron affinity for
the electron than iodine and therefore the aluminium cluster is called
a superhalogen (i.e., the vertical electron detachment energies of the
moieties that make up the negative ions are larger than those of any
halogen atom). The cluster component in the ion is similar to an
iodide ion or a bromide ion. The related cluster is expected to
behave chemically like the triiodide ion.
See also
======================================================================
* Halogen bond
* Halogen addition reaction
* Halogen lamp
* Halogenation
* Interhalogen
* Pseudohalogen
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=========
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Original Article:
http://en.wikipedia.org/wiki/Halogen
