Science Makes Sense,Week21 : The Halogens, refrigerants, toothpaste and disinfectants.

What does an older refrigerator, bleach,toothpaste and pesticides have in common?  All contain one or more halogens, the Group VII elements of the Periodic Table: fluorine, chlorine, bromine or iodine. (Astatine  is radioactive and does not behave like the other halogens.)  Today we shall study these very reactive elements, sitting so close to the most inert elements, the Noble Gases. (Ref. 1)

The word ‘ halogen’ comes from Greek, meaning ‘ salt-producing’.   The electronic configuration of all the halogens, indicate two electrons in the outermost ‘s’ shell and seven electrons in the ‘p’ shell.  This makes the halogens electrophilic and non-metals . They are ready to react with metals , which are nucleophilic. (Ref. 1)

The Halogens are diatomic molecules, i.e., they exist as two atoms sharing electrons – covalent bonding.  Astatine alone is not diatomic.  Intermolecular forces, Van der Waals forces, are very weak as one moves down the group.  This explains why the first two, F and Cl are gases.  As the forces weaken, the molecules get closer, bromine, Br is a liquid and iodine, I and astatine , At  are solids. (Ref. 2)  Since the halogens are very reactive, they are never found in the elemental form. (Ref. 1)

Flourine:

F is the most reactive of all the halogens and was a challenge to isolate it.  Even if scientists were able to get it in the elemental form, it reacted with the vessels in which it was synthesized. Finally, Henri Moisson ( 1852-1907) managed to isolate the element using electrolysis which is the process of passing an electric current to cause a chemical reaction.  Using a platinum-iridium alloy that is resistant to attack by fluorine as the electrodes, an electric current was passed through HF,which lead to the isolation of F at one of the electrodes. (Ref. 1)

Flourine is used as an oxidizing agent in rocket fuels. (Ref. 3)  As hydrogen flouride,HF, it is used to etch glass. (Ref. 1)

Chlorine:

Sir Humphrey Davy identified this gas as a greenish- yellow gas.  Chlorine, Cl, is produced by the electrolysis of brine (sodium chloride solution).  It is reactive, but not as reactive like F. (Ref.3)

The ionic compounds of Cl are used to make disinfectants for sewage, water and for pools.  The organo compounds of Cl are used to manufacture pesticides, herbicides and fungicides. (Ref. 3)

Bromine:

Bromine, Br, is a foul-smelling, reddish- brown liquid isolated in 1825 by Carl Lowing and Antoine Jerome.  It is found in deposits of brine in Israel’s Dead Sea and in the U.S. in Arkansas and Michigan. The vapors are highly irritating to the throat and eyes.  Bromine is used in dyes, pesticides,disinfectants and as a flame retardant.  One of the major uses of bromine is in the manufacture of halogen lamps.  These lamps are used as automobile headlights, floodlights, spot lights.  Tungsten is used in lamps and is considered better than carbon, since it is more durable than the latter.  However,the tungsten, W, begins to thin and break causing dimming of the light source.  But the addition of bromine or other halogens inside the bulb alleviates this problem. (Ref.1)

Iodine:

First isolated from seaweed in 1811, the word is derived from the Greek word ‘iodos’ meaning ‘ violet colored’. (Ref. 1,3)  Ultra-pure iodine, I, may be prepared adding potassium iodide to copper sulfate.  Though I as a halogen is a non- metal, it possesses characteristics of a metal sometimes.  Radioactive isotope I-131 is used to treat thyroid disorders.  Insufficient iodine leads to goiter. (Ref. 5)

Iodine was once used in photography; today it is used in chemical analyses and the synthesis of organic compounds. (Ref. 1)

Astatine:

No more than 44 mg of astatine, At, are found on the earth’s crust. It is one of the rare elements, and poorly researched.  No practical applications exist right now. (Ref. 3)

Activities for Middle School Teachers:

Students can study the chemical reactions involved in adding bleach to clothes.  Is the bleaching process an oxidation reaction or not? Please explain.

When you go down the halogen group, what makes flourine the most reactive halogen and iodine the least reactive?

Look at the different chloro-flouro carbons. Find out how many different stereo isomers there are.

Nuggets of Information:

The most characteristic feature of halogens is their distinct coloration: F is pale yellow, Cl is greenish- yellow, Br is orange to red- brown, I is dark violet and At is dark black. (Ref. 2)

Flourine reacts explosively with water and is the only element to react with Noble Gases. (Ref. 3)

Henri Moisson received the Nobel Prize in 1906 for isolating the element Flourine using electrolysis. (Ref. 1)

Teflon, which is a polymer continuing Flourine is used in non-stick cookware. (Ref. 1)

Flourine was initially added to water, to prevent tooth decay.  Now it is only added to toothpaste. (Ref.1)

‘Chloros’ in Greek indicates the color , greenish-yellow, and hence the name Chlorine, for the second halogen in the series.   Chlorine is among the top ten chemicals produced in the U.S. (Ref. 3)

Cl is a highly poisonous gas that was used in World War 1 as an agent of war (Ref. 1)

In early 19th century it was use as a disinfectant during a cholera epidemic in Europe. (Ref. 1)

The single greatest use of chlorine is in the preparation of a large variety of compounds including organo-chlorides that are the starting point of plastics like PVC, polyvinyl chloride, and neoprene, a synthetic form of rubber. (Ref. 3)

DDT which is an ogano-chloride containing two benzene rings and five chlorine atoms was used for many years as a strong pesticide. Rachel Carson, in her book written in the 60’s, called ‘ Silent Spring’ talked about the devastating environmental impact of such a pesticide and that resulted in a ban on the use of DDT in the U.S. (Ref. 4)

A series of compounds are formed by replacing the hydrogen atoms in methane by halogens. When three of the hydrogen atoms are replaced by chlorine atoms, you get the old anesthetic chloroform, or trichloromethane. When all four hydrogen atoms are replaced by chlorine atoms you get carbon tetrachloride/tetrachloro methane. This is a very common organic solvent and is also used as a refrigerant. (Ref. 3)

Chlorofluorocarbons, CFC’s, were used as refrigerants.  These are usually methane or ethane molecules where several of the hydrogen atoms are replaced by chlorine or fluorine atom/atoms.  They were given the general name Freon.  These CFC’s were found to deplete the ozone layer and were late banned. (Ref. 5)

At one time,ethylene dibromide was used by the petroleum industry as an additive to leaded gasoline. It reacted with lead to form lead bromide and cleaned out lead emissions.  In the late 70’s leaded gasoline was slowly phased out due to environmental concerns and ethylene bromide was not needed then. (Ref. 1)

Iodine, I, binds to starch and colors it deep blue; this is used as a test for iodine or starch. (Ref. 6)

Most isotopes of I are radio active, except I-127. (Ref. 6)

Solid I is blue-black and shiny, at ordinary temperatures and pressures it sublimates (converts from solid state directly to gaseous state) into a gas. (Ref. 6)

The thyroid gland uses I to make the hormones thyroxine and triodotyorine. (Ref. 6) Hormones are chemical messengers that direct cells what to do. (Ref. 7).  Insufficient iodine leads to goiter, swelling of the thyroid gland.

I deficiency is considered one of the leading causes of mental retardation. (Ref. 6)

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References:

1.encyclopedia.com/topic/Halogen_elements.aspx

2. buzzle.com/articles/halogen- familyhtml

3. science clarified.com/Ga-He/Halogens.html

4. Carson, Rachel, Silent Spring (Houghton Miflin, 1962)

5. britannica.com/science/chloridifluoromethane

6.chemistry.about.com/od/elementfacts/a/iodine.htm

7.biology.about.com/od/molecular biology/ss/hormones.htm

 

 

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