Daryl Andresen, Giao Tran,
Jonathan Parrish, Branden Bacon
Halogens
Group 17, the halogens, are the most electronegative group and are able to withdraw
electrons from the central atom. Electronegativity decreases down the group. fluorine is the most
electronegative and astatine is the least electronegative. The boiling point and melting of
halogens are relatively low and increase down the group. This follows the general trend in the
periodic table because as atomic mass increases the melting and boiling point also increases.
Each halogen has its own distinctive color, making them notable among the elements.
These colors include near colorless fluorine, yellow-green chlorine, red-brown bromine, and
purple iodine. Covalent and ionic radius is slightly lower than group 16 and tends to decrease
down the group due to nuclear charge effect. For the same reason, halogens have the highest first
ionization energies beside the noble gases. Since halogens are very electronegative, they have
high electron affinities. Surprisingly, chlorine has strongest electron affinity instead of fluorine.
This is due to the fact that electron repulsion in fluorine overcomes the effective nuclear charge.
Also, halogens are the most reactive non-metals on the periodic table.
The second group halogen that demonstrates the uniqueness principle is fluorine. In
general, fluorine is special because it is the most electronegative element. Despite being the
smallest atom in the group, F2 has a weaker bond than Cl2 because of the electron-electron
repulsion from the small fluorine orbitals (Coulombic repulsion). Fluorine is never found in a
positive oxidation state (except in the transient gas phase F2+) because there is no element that is
more electronegative to withdraw electrons from it.
Interhalogens are molecules in which one or two larger halogens bond to one or more
smaller halogens. Fluorine is often the smaller molecule because of its ability to promote high
oxidation states as in IF7. Fluorine gas is unstable and hard to work with because it reacts with
Daryl Andresen, Giao Tran,
Jonathan Parrish, Branden Bacon
most inorganic and organic molecules as well as krypton, xenon, and radon. It is stored in steel
or monel metal, which is a nickel/copper alloy, because it reacts with these materials to form a
passivating metal fluoride surface film. Fluorine gas is difficult to isolate and the only
economical way to produce it industrially is from the electrolysis of 1:2 molten KF and HF.
One distinctive characteristic of fluorine among the halogens is that it promotes volatility
in its molecular form. Compounds containing fluorine are significantly more volatile than their
chlorine counterparts. This is because fluorine compounds have low polarizabilities which causes
variations in the strength of the dispersion interaction (the interaction between instantaneous
transient electric dipole moments).These variations cause the molecules to be very volatile.
Fluorine also causes the strengths of Lewis and Bronsted acids to be increased. Fluorine
can withdraw electrons from the atoms to which is bonded and this in turn makes the atom more
acidic (or conversely, less basic). This withdrawing effect is much stronger than that of the other
halogens. Fluorine is very good at stabilizing high oxidation states; only oxygen is better.
Fluorine actually disfavors low oxidation states in some compounds. For example, copper bonds
with one chlorine, bromine, or iodine atom but will automatically favor bonding to two fluorine
atoms rather than just one. Fluorine is known for getting cations to their highest oxidation states
such as the rare Ag (III) in KAgF4.
Due to their high reactivity, halogens are used in many areas of chemical research. One
such area is in the creation of Perfluorocarbons (PFCs). PFCs are carbon chains that have been
reacted with fluorine gas to replace hydrogen with fluorine.2 Due to the large pockets of air that
the bulky fluorinated carbons create in a liquid solution, PFCs are excellent for gas transfer. Dr.
Leland Clark’s research involves testing the oxygen-carrying ability of these compounds as a
blood substitute called Oxycyte.1
Daryl Andresen, Giao Tran,
Jonathan Parrish, Branden Bacon
Another application of halogens is in organic synthesis. Combined with magnesium,
these compounds form Grignard reagents. These metal/halogen complexes are extremely useful
for building onto carbon compounds and adding important functional groups. In addition to be a
cheap and easy way to synthesize organic molecules, organomagnesium compounds do not
interfere with sensitive groups such as nitro, hydroxyl and boronic esters. There use in
heteroaromatic systems is also well documented.3
Volatile organohalogens break down ozone. When they escape into the atmosphere, they
interfere and modulate chemical processes. Some volatile organohalogens include methyl
chloride, methyl bromide, methyl iodide, chloroform, and bromoform, form highly reactive
halogen radicals. These volatile organohalogens are produced by marine macroalgae. It has been
discovered that UV light actually catalyzes the production of volatile organohalogens by
macroalgae. This creates a vicious cycle where the volatile organohalogens break down ozone,
more UV light gets through, and more volatile organohalogens are produce which breaks down
more ozone. This is especially true for chloroform, methyl iodide, bromoform, and
dibromomethane.4
Halogens act in fluidized bed combustion (FBC) of coal by creating side reactions with
oxygen free radicals. This causes two major changes in FBC. Adding halogens or salt halides to
combustible materials greatly increases the CO and hydrocarbon concentrations in the emission
gases of FBC. This prevents CO from oxidizing to CO2. I2 actually increases CO concentrations
more than Cl2 which is related to the trend of activity for halogens. Halogens also increase NO
concentrations while decreasing N2O concentrations in nitrous fuels because they inhibit the
reduction of NO to N2O. The reactions of NO to N2O and CO to CO2 both involve the radicals
which the halogens react with.5
Daryl Andresen, Giao Tran,
Jonathan Parrish, Branden Bacon
1. Clark, L. Artificial blood and method for supporting oxygen transport in animals. US
Patent 3,911,138, October 7, 1975.
2. Clark, L.; et al. Synthesis of Unusual Perfluorocarbon Ethers and Amines Containing
Bulky Fluorocarbon Groups: New Biomedical Materials. J. Org. Chem. 1988, 53, 78-85.
3. Ila, H.; et al. Preparation and Reactions of Heteroaryl Organomagnesium Compounds.
Chem. Lett. 2006, 35, 2-7.
4. Laturnus, F.; Svensson, T.; Wiencke, C.; Oberg, G. Ultraviolet Radiation Affects
Emission of Ozone-Depleting Substances by Marine Macroalgae: Results from a
Laboratory Incubation Study. Environ. Sci. Technol.; (Article); 2004; 38(24); 6605-6609.
5. Lu, D. Y.; Anthony, E.; Talbot, R.; Winter, F.; Loffler, G.; Wartha, C. Understanding of
Halogen Impacts in Fluidized Bed Combustion. Energy & Fuels; (Article); 2001; 15(3);
533-540.