The Chemistry of halogens


Halogens are very reactive elements. They belong to VII A group of the periodic table. Among the halogens, Fluorine, Chlorine, Bromine and Idoine are very well known and find many applications. The chemistry of these elements is discussed in this article.

Introduction

The elements in the last group of the periodic table are known as inert gases. They will not have chemical reactivity. Just one group before this is the group of halogens. This group is called VII-A. They will be just one electron less than the nearest inert gases. So, these halogens are very reactive and they tend to become negatively charged ions. There are five elements in this group. They are Fluorine(F2), Chlorine (Cl2), Bromine (Br2), Iodine (I2) and Astatine (At2). Among the five elements, the last one is radioactive and very unstable and the chemistry is not much known. Hence the other four elements are only well known as Halogens.

Why they are called halogens? As for the Greek language, hal means salt and gen means to generate. These four elements are the salt formers and hence they are called halogens. None of these halogens is found in nature in their elemental form. They are found as salts only. The Fluoride ions are found in Fluorite (CaF2) and Cryolite (Na3AlF6). Chloride ions are found in rock salt (NaCl), the oceans and in water in lakes that have a high salt content. Bromide and iodide ions are found in the oceans and brine wells.

Fluorine (F2) is a known highly reactive element. It is a colourless gas and very toxic. The very inert materials like asbestos, water, and silicon will give flame in presence of this element. This will form compounds even with the inert gases. It is so reactive that it can even form compounds with Kr, Xe, and Rn, elements that were once thought to be inert. Fluorine reacts with even glass and quartz. So, we can't store this gas in containers made of these materials. Separate containers made of copper and nickel alloy will be used to store this. Fluorine reacts with even these containers also but forms a protective coating on the surface and stops the further reaction. The compounds formed with fluorine are very stable. Teflon, a very well-known insulating material, is a compound containing fluorine. Freon is a gas containing carbon dichloride and fluorine that is used these days in all types of refrigeration systems. Maximum fluorine is used in making this compound only.

Chlorine (Cl2) is also a highly toxic gas. It is pale yellow-green in colour. It is a very strong oxidizing agent. It is a commercial bleaching agent and a disinfectant. It acts as a germicide. Large quantities of chlorine are used to make many solvents like carbon tetrachloride (CCl4), chloroform (CHCl3), dichloroethylene (C2H2Cl2) are some such solvents.

Bromine (Br2) is a liquid. It is reddish-orange in colour and has an unpleasant odour. Flame retardants, fire-extinguishing agents, sedatives, antiknock agents and insecticides are made using this bromine.

Iodine (I2) is solid. This is relatively volatile and sublimes when heated. "Tincture of iodine" a very well-known disinfectant, is made by using iodine only. Some of the Iodine compounds are used as catalysts in various chemical reactions, dyes and drugs.

Properties

  1. Fluorine is a gas at room temperature. The boiling point of this material is -188 ^ 0C. The melting point is -218.6 ^ 0C. The ionisation potential of fluorine is1680.6 kJ/mol. The electron affinity is 322.6kJ/mol. The ionic radius is 0.133nm.
  2. The chlorine gas boils at -34 ^ 0C. The melting point is -101 ^ 0C. It is a gas at room temperature. The ionisation potential is 1255.7 kJ/mol. The electron affinity is 348.5 ^ 0C. The ionic radius is 0.184 nm.
  3. Bromine is a liquid at room temperature. It will melt at -7.3^ 0C. The boiling point is 59.5. The ionisation potential is 1142.7 kJ/mol/ and the electronic affinity is 324.7 kJ/mol. The ionic radius is 0.196 nm.
  4. Iodine is solid at room temperature. The melting point is 113.6 ^ 0C. The boiling point is 185.22 ^ 0C. The ionisation potential is 1008.7kJ.mol. Electron affinity is 295.5 kJ/mol/ The ionic radius is 0.220 nm.


Halogen Compounds

Halogens are very reactive and they form different stable compounds. Some important compounds of halogens are discussed below.
  1. Hydrogen Halides: Halogens on reaction with the hydrogen form these compounds. Hydrogen and chlorine react to give hydrochloric acid. Similarly, hydrogen fluoride, hydrogen bromide will also be formed. They are acids and very dangerous.
  2. Metal halides: These halides will be formed when a salt of the metal is reacted with hydrohalic acid. Sodium hydroxide on reaction with hydrochloric acid forms sodium chloride.
  3. Interhalogen Compounds: These are made by reacting halogens with each other. The properties of these compounds will be intermediates of the two-parent halogens. Generally, they are made by reacting pure halogens in the specified conditions. When halogens react with each other they form interhalogen compounds. Their properties and behaviours are intermediates of those of the two-parent halogens. Some properties may differ though. All interhalogens apart from IF7 can be formed by directly combining pure halogens in set conditions.
  4. Organic halides: These are organic compounds and will contain halogen atoms. They are normally manufactured through nucleophilic abstraction reactions.


Uses of Halogens

  1. Halogen lamps will be using small amounts of a halogen.
  2. They are used as disinfectants. Used for purifying water, wounds etc. Bromine and chlorine are mainly used for this.
  3. Fluoride is used in toothpaste etc.
  4. Polyhalogenated compounds are used in pest control.


Conclusion

Halogens are very well-known reactive materials and have a variety of applications. They are very toxic and some fluoride compounds are very corrosive and hazardous. They should be handled very carefully. They, especially, fluorine, is having a very vital application in many industries.


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