We are almost at the end of the p-block now, having arrived at Group 17, which are known as the
halogens. This group includes fluorine, chlorine, bromine, iodine, astatine, and tennessine. All
but the last two of these elements should be extremely familiar from organic chemistry,
as these first four are ubiquitous in organic synthesis. All of the halogens are nonmetals,
and they have very similar properties. They all have electron configurations ending ns2np5, which
means they have a strong tendency to gain one electron to complete their outermost shell. Thus
the -1 oxidation state is very common for these elements, and is the only oxidation state fluorine
will attain, while the others can also be +1, +3, +5, or +7, depending on what they are interacting
with, because of their access to d orbitals. Where can these elements be found?
They are actually too reactive to be found naturally in elemental form, but they can be
found in a variety of compounds and separated by electrolysis or oxidation. Fluorine is present in
ores like fluorite, which is widespread, with the largest deposits located in South Africa,
as well as fluoroapatite and cryolite, the latter of which is found mainly in the United States,
Canada, and Russia. Chlorine makes up 1.9% of seawater by mass, due to its salinity,
and is also found in underground brine and deposits of halite, sylvite, and carnalite.
Bromine is found in brine wells and the water of the Dead Sea.
Iodine is present in certain salts found in Chile, in a layer of soil called caliche,
as well as underground brines, sea kelp, and oil fields of Japan and the United States.
Astatine is formed from the decay of uranium and thorium in certain ores. It has 32 isotopes,
the most stable of which has a half-life of about 8 hours. It can also be made synthetically by
alpha bombardment of bismuth-209. Tennessine is another strictly synthetic transactinide element,
first made in 2009, and its most stable isotope has a half-life of 80 milliseconds.
Now let’s talk about properties. Of course we know that atomic radius increases going down the group,
and this has an impact on their physical properties. Fluorine is a yellow diatomic gas.
Chlorine is a greenish-yellow diatomic gas. Bromine is also diatomic,
but it is a red-brown liquid at room temperature, with a high vapor pressure.
Quite interestingly, this characteristic applies only to bromine and mercury. All other elements,
in their most stable allotrope, are either solid or gas at room temperature.
And finally iodine is a purple-black solid, whose crystals easily sublime to generate a deep violet
vapor. This is the only halogen with multiple allotropes, as apart from the diatomic form, I3-,
I5-, I7- and I9- are all possible. Another interesting feature of the halogens is their
ability to form interhalogen compounds. Since nonmetals like to make covalent bonds with one
another, this applies to the halogens, and works well when the two halogens have very
different electronegativities. Here is a list of some common interhalogen compounds.
Additionally, chlorine, bromine, and iodine form many different oxyacids, such as the
ones listed here, as well as the halides we are familiar with from organic chemistry.
Moving on to applications, silver bromide and silver iodide are used in
photographic film. Sodium hypochlorite is the primary component of bleach.
Chlorine is also important for bleaching wood pulp and cotton cloth. Sodium chloride is table salt,
ubiquitous in cooking. Potassium iodide is added to table salt in trace amounts
to counter iodine deficiency, which impacts thyroid function. Fluorine
is found in interesting compounds like teflon, which is useful in creating non-stick surfaces
for a variety of products, and there are also many important fluorinated pharmaceuticals.
Chlorofluorocarbons, such as freon, were used as nontoxic refrigerants, though this was eventually
discontinued as it was discovered that these compounds were damaging the ozone layer.
Fluoride, the anion of fluorine, is added to many public drinking water supplies
to enhance dental health, as we discussed at great length in this video here. Chlorine is used as a
disinfectant in community water supplies, while bromine is used as a disinfectant in hot water,
like in hot tubs and some pools. Iodine is a co-catalyst in the Monsanto acetic acid synthesis,
which we will cover later in the series. It is also used as a topical disinfectant,
while iodine in alcohol solution is used as an antiseptic. Silver iodide is used in seeding
clouds to induce rain. Astatine has no known applications, though it is under investigation
as to whether it has utility in radiation therapy for prostate cancer. And of course Tennessine,
as one might imagine, has no applications due to its extemely short half-life.
And with that we’ve covered another group on the periodic table, the halogens.