1. Key Facts & Summary
Halogen have very high electronegativities
They have seven valence electrons (one short of a stable octet)
They are highly reactive, therefore toxics
The halogens are Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and
Astatine (At)
Down the group, atom size increases.
Halogens are a group of elements on the periodic table found in group 17. They are
non-metals, and the term "halogen" means "salt-former" because halogens react
with metals to produce many important salts. Let's think about the chlorine ion,
which is usually obtained from table salt (NaCl).
Fig1: Periodic table of the elements showing the group 17: the halogens
family.https://byjus.com/jee/faq-halogens-jee/
All halogens have 7 electrons in their outer shells, giving them a valency of -1. This is
the main reason why the halogens have the tendency to acquire an additional
electron making them strong oxidizers.
The halogens exist, at room temperature, in all three states of matter:
Solid- Iodine, Astatine
Liquid- Bromine
Gas- Fluorine, Chlorine
but halogens are so reactive that they do not occur as free elements in nature
Properties of the Halogens
2. As mentioned above, these reactive nonmetals have seven valence electrons.
Halogens range from solid (I2) to liquid (Br2) to gaseous (F2 and Cl2) at room
temperature. As pure elements, they form diatomic molecules with atoms
joined by non-polar covalent bonds.
They all have very high electronegativity. Fluorine has the highest
electronegativity of all elements, and they all form negatively charged ions
(H-
, F-
, Cl-
, Br-
, I-
, and At-
).
There is, however, a progressive change in properties from fluorine through
chlorine, bromine, and iodine to astatine.
Some of the properties of this group of elements are shown in figure 2 below
Fig 2. Scheme representing the trends of the halogens' properties in group 7
There is a regular increase in many of the properties of the halogens as we proceed
down the column from fluorine to iodine, including the melting point, boiling
point, the radius of the corresponding halide ion, and the density of the
element.
The halogen elements all form compounds with hydrogen, the hydrogen halides
which are:
hydrogen fluoride (HF),
hydrogen chloride (HCl),
hydrogen bromide (HBr),
hydrogen iodide (HI),
hydrogen astatide (HAt).
All of these acids are dangerous and must be handled with great care.
The halogens
3. Fluorine
F2 is a highly toxic, colourless gas, and it represents the most reactive element
known. As an example of its reactivity, asbestos, water, and silicon burst into
flame in its presence and it is difficult to find a container in which it can be
stored safely.
Fluorine is used in the manufacture of Teflon, or to make the freons used in
refrigerators.
Chlorine
Cl2 is a highly toxic gas as well, and a very strong oxidizing agent, which is
used commercially as a bleaching agent and as a disinfectant. It is strong
enough to oxidize dyes and bleaching out colours, and strong enough to
destroy bacteria. From here its use as a germicide. Large quantities of chlorine
are used each year as water purifying agent or to make solvents such as
carbon tetrachloride (CCl4, quite toxic), chloroform (CHCl3), dichloroethylene
(C2H2Cl2), and trichloroethylene (C2HCl3), mainly used in synthetic chemistry
laboratories.
Bromine
Br2 is a reddish-orange liquid, and its name comes from the Greek stem
bromos, "stench." Bromine is used to prepare fire-extinguishing agents,
sedatives and insecticides. It can give off suffocating vapours, corrode to the
skin, and may cause severe gastroenteritis if ingested.
Iodine
I2 is an intensely coloured solid with an almost metallic luster. As its molecular
weight is quite light, this solid is relatively volatile, and it sublimes when
heated to form a violet-coloured gas. Iodine has been used as a disinfectant in
"tincture of iodine." In organic chemistry, Iodine compounds are used as
catalysts, drugs, and dyes.
Astatine
As2 s the last of the known halogens and it is a synthetic element. In fact, it
was synthesized in 1940 by a group of scientist in California. Astatine is a
radioactive element and its most stable isotope is astatine 210 which has a
half-life of 8 hours.
Not much is known about the chemical properties of astatine but it is
expected to react like the other halogen.
