The halogens
fluorine
Atomic fluorine is univalent and is the most chemically reactive and
electronegative of all the elements. In its elementally isolated (pure) form,
fluorine is a poisonous, pale, yellowish brown gas, with chemical formula
F2. Like other halogens, molecular fluorine is highly dangerous; it causes
severe chemical burns on contact with skin.
Pure fluorine (F2) is a corrosive pale yellow or brown gas that is a powerful
oxidizing agent. It is the most reactive and most electronegative of all the
elements (4.0), and readily forms compounds with most other elements. It
has an oxidation number -1, except when bonded to another fluorine in F2
which gives it an oxidation number of 0. Fluorine even combines with
argon, krypton, xenon, and radon. Even in dark, cool conditions, fluorine
reacts explosively with hydrogen. It is so reactive that metals, and even
water, as well as other substances, burn with a bright flame in a jet of
fluorine gas. It is far too reactive to be found in elemental form. In moist air
it reacts with water to form also-dangerous hydrofluoric acid.
Chlorine
In its common elemental form (Cl2 or
"dichlorine") under standard conditions, it is a
pale green gas about 2.5 times as dense as
air. It has a disagreeable, suffocating odor
that is detectable in concentrations as low as
3.5 ppm and is poisonous. Chlorine is a
powerful oxidant and is used in bleaching and
disinfectants. As a common disinfectant,
chlorine compounds are used in swimming
pools to keep them clean and sanitary. In the upper atmosphere, chlorine
based molecules have been implicated in the destruction of the ozone
layer.
Chlorine gas, also known as bertholite, was first used as a weapon in
World War I by Germany on April 22, 1915 in the Second Battle of Ypres.
As described by the soldiers it had a distinctive smell of a mixture between
pepper and pineapple. It also tasted metallic and stung the back of the
throat and chest.
Chlorine gas is diatomic, with the formula Cl2. It combines readily with all
elements except O2 and N2 and the noble gases
bromine
Bromine is the only liquid nonmetallic
element at room temperature and one
of only six elements on the periodic
table that are liquid at or close to
room temperature. The pure chemical
element has the physical form of a
diatomic molecule, Br2. It is a dense,
mobile, reddish-brown liquid, that
evaporates easily at standard
temperature and pressures to give a
red vapor (its color resembles nitrogen
dioxide) that has a strong
disagreeable odor resembling that of chlorine. Bromine is a halogen, and is
less reactive than chlorine and more reactive than iodine. Bromine is
slightly soluble in water, and highly soluble in carbon disulfide, aliphatic
alcohols (such as methanol), and acetic acid. It bonds easily with many
elements and has a strong bleaching action.
Elemental bromine is toxic and causes burns. As an oxidizing agent, it is
incompatible with most organic and inorganic compounds.
iodine
Chemically, iodine is the least reactive of the halogens, and the most electropositive
halogen after astatine. Iodine is primarily used in medicine, photography and dyes. It is
required in trace amounts by most living organisms.
As with all other halogens (members of Group VII in the Periodic Table),
iodine forms diatomic molecules, and hence has the molecular formula of
I2.