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.
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
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 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.
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.
Astatine occurs naturally from uranium-235 and
uranium-238 decay, but because of its short half-life is
only found in minute amounts.
Astatine is the rarest naturally-occurring element,
with the total amount in Earth's crust estimated to be
less than 1 oz (28 g) at any given time. This amounts to
less than one teaspoon of the element. Guinness World
Records has dubbed the element the rarest on Earth,
stating: "Only around 0.9 oz (25 g) of the element
astatine (At) occurring naturally". Isaac Asimov,
in a 1957 essay on large numbers, scientific notation,
and the size of the atom, wrote that in "all of
North and South America to a depth of ten miles",
the number of astatine atoms at any time is "only a