CH226.25
•The Halogens (17 / VII)
The Oxygen group (group 16/ VI)
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Halogens (group 17/ VII)
F, fluorine (the name is derived from the Latin fleure, meaning to flow, perhaps with
reference to the common fluoride mineral fluorspar or fluorite, CaF2, which melts when
heated in a flame).
Cl, chlorine (the name is derived from the Greek chloros, meaning pale green).
Br, bromine (from the Greek bromos meaning stench).
I, iodine (the name is derived from the Greek iodes, meaning violet).
At, astatine (the name comes from astatos, a Greek word meaning unstable).
Members of group VII have the electronic configuration ns2np5.
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Fluorine
Fluorine is a corrosive pale yellow-green gas (see it contained in a
single-crystal quartz tube). It is the most reactive and
electronegative of all the elements, and readily forms compounds
with most other elements. Fluorine even combines with the noble
gases krypton, xenon, and radon. It is so reactive that glass,
metals, and even water, as well as other substances, burn with a
bright flame in a jet of fluorine gas.
Fluorine occurs mainly in the minerals fluorspar (fluorite, CaF2)
and cryolite (Na3[AlF6]).
Fluorine gas can now be handled in bulk and even transported safely as a liquid. The
main uses of fluorine are in making uranium hexafluoride (UF6) (to separate isotopes
of uranium), and sulfur hexafluoride (SF6) for the electricity supply industry. The
element and its compounds are used in producing many fluorochemicals, including
high-temperature plastics, and especially Teflon. Hydrofluoric acid (HF) is used to etch
glass, including most of the glass used in light bulbs.
The element fluorine and the fluoride ion (F–) are toxic.
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Special characteristics of fluorine compounds
1. Fluorine is the most strongly oxidizing element of the group:
E°(F2/F–) = 2.87 > E°(Cl2/Cl–) = 1.36 > E°(Br2/Br–) = 1.07 > E°(I2/I–) = 0.54 V
X2 (g) + 2e– → 2 X– (aq)
ΔG° < 0
ΔG° is large negative (especially for fluorine) on the account of the weak X–X bond
and highly exothermic hydration of the X– ions.
2. Related to the oxidizing power, is the ability of fluorine to stabilize high oxidation
states of the atoms to which it is bonded (e.g. in SF6, PF5) and disfavor low oxidation
states.
3. Fluorine atoms are σ-acceptors and withdraw electrons from the atoms to which
they are attached. This leads to reduced Lewis basicity (e.g. in NF3) or enhanced
Brønsted acidity:
O
F3C
HO
S
O
O
Triflic acid, pKa = 3 (in CH3CN)
H3C
HO
S
O
Methylsulfonic acid, pKa = 6
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Chlorine
A yellowy-green dense gas with a choking smell. Since it
reacts directly with nearly every element, chlorine is never
found free in nature, but combined chiefly with sodium as
sodium chloride in common salt and the minerals carnallite
(magnesium potassium chloride) and sylvite (KCl).
The primary uses for chlorine are in chemicals (30%), manufacture of PVC (25%),
water purification (20%), solvents (15%) and bleaches (10%). Chlorine is widely used
in making many products such as paper, dyestuffs, textiles, petroleum products,
medicines, antiseptics, insecticides, solvents, paints and plastics. A further substantial
use for this element is in organic chemistry, both as an oxidizing agent and in
substitution reactions.
The chloride ion (Cl–) is essential to life and is relatively non-toxic. The element itself,
chlorine gas (Cl2), is very toxic; it can be fatal after a few deep breaths. It was used as
a war gas in 1915. Hydrochloric acid (HCl) is produced in the human stomach to help
break down food.
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Bromine
Bromine is the only nonmetallic liquid element. It is a heavy
reddish-brown liquid, volatilizing readily to a red vapor with a
strong disagreeable odor, resembling chlorine. It reacts
readily with many elements.
Bromine is used to make organobromo compounds which are widely used as sprays
to kill insects and other unwanted pests. The main example is methyl bromide, CH3Br,
which (until 2010) was widely used to protect tomatoes, celery, lettuce, strawberries,
grapes, tobacco and flowers, such as carnations. Despite all of the advantages, CH3Br
poses a threat to the ozone layer, and the use of this compound is now strictly
regulated. Bromine is used in areas such as dyestuffs, chemical intermediates and
flame-retardants.
Apparently, the element has no biological role. Bromine itself (Br2) is very toxic and
has a strong irritating effect on the eyes and throat; when spilled on the skin it
produces painful sores. It presents a serious health hazard, and maximum safety
precautions should be taken when handling it. Bromide (Br–) depresses mental activity
and also the sex drive. Bromide salts were used as sedatives in the 19th century.
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Iodine
Iodine is a non-metallic, dark purple-black, lustrous, solid
element. Iodine sublimes easily on heating to give a purple
vapor. Iodine is the most electropositive and the least reactive
of the halogens: χ = 2.66 similar to χ = 2.58 (S) and 2.55 (C)
Iodine compounds are important in organic chemistry and are very useful in medicine.
Iodine is used in medical treatment; it is employed in the preparation of certain drugs
and in the manufacture of some printing inks and dyes. Silver iodide was used in
photography. Iodine is added to almost all the table salt and is used as a supplement
to animal feed.
Iodine is an essential element, lack of which causes problems with the thyroid gland.
In the gland, iodine is found in two hormones: thyroxine and tri-iodothyronine, which
regulate several metabolic functions, most noticeably to control the body’s
temperature. The artificial radioisotope, 131I, with a half-life of 8 days, is used in
treating cancerous thyroid glands. A solution of KI and iodine, or of iodine in ethanol,
has germicidal effects, and was used for the external treatment of wounds.
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Astatine
A dangerously radioactive element made in nuclear reactors.
The half-life of the longer-lived isotope is only eight hours.
Astatine can be obtained in various ways, but not in weighable
amounts.
Uranium ore Carnotite,
K2(UO2)2(VO4)2·3H2O
“Astatine, francium, actinium, and protactinium are irritating to element collectors.
It's customary to say that all the elements up to uranium (92) are the "naturally
occurring" elements, while those beyond 92 are man-made. So people like to define
a "complete" element collection as one containing all the elements up to uranium.
The problem is that astatine, francium, actinium, and protactinium are absolutely
impossible to collect in any meaningful sense of the word. They are so fantastically
radioactive and short-lived that if you had a visible quantity of any of them, you
would be dead and then it would vanish before your body was cold.
The customary solution is to take a chunk of uranium ore and explain that there's
probably a few atoms of the elements somewhere in it at any given time.”
(From http://www.theodoregray.com/PeriodicTable)
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Some properties of the elements
F is the most electronegative of all elements. However, the electron affinity of Cl is
greater, because of the strong electron-electron repulsion in the small F– ion.
For a similar reason, F-F bond is relatively weak in the molecule of F2, due to
repulsion between the nonbonding electron pairs.
F2
1.43 Å
159 kJ/mol
Cl2
1.98
243
Br2
2.27
193
I2
2.72
151
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