The Halogens
A common characteristic between table salt, whiteners and fluoride toothpastes is that all of them contain one or more types of halogens. Halogens are the elements that make the seventeenth group in the periodic table. They are: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At). These elements are chemically related and have somewhat same characteristics. All the halogens end with the same number of electrons, thus they all have 7 valence electrons in the last shell. One of the common properties of the halogens is that all of them react strongly with metals resulting in different kinds of salts. That is why the group is called halogens, namely ‘salt producing’.
Physical and Chemical Properties of Halogens:
Halogens have many common characteristics, but they differ in some physical and chemical properties showing trends in those properties down the group.
Physical properties:
1-Physical state: At room temperature, fluorine and chlorine exist as gases, while bromine is liquid, and it is one of the two elements that are present in liquid state at room temperature in addition to mercury. Iodine and astatine exist normally as solids.
2- Color: There is a great degree of variation in halogens’ colors and they get darker when going down the group. Fluorine is a pale-yellow gas, chlorine is pale-green, bromine is red-brown. Lastly, iodine crystals are dark metallic, however Iodine is easily sublimized into purple vapor.
3-Other physical properties: Melting, boiling points and density are examples of other physical properties that increase when moving towards the bottom of the group as a result of the increase in atomic number, atomic size and London dispersion forces between the molecules. These properties increase from Fluorine to Iodine. The following graph shows the trend in melting and boiling points of Halogens.
Chemical properties:
1-Diatomicity: Halogens have seven valence electrons, so they need to gain one electron to satisfy the octet rule. Consequently, each two atoms of the same halogen share a pair of electrons and form covalent bond. That is why halogens are present as diatomic molecules in nature (F2, Cl2, Br2, I2).
2-Electronegativity (EN) and electron affinity (EA): The halogens have high electronegativity and electron affinity values because they need only one electron to complete the octet as mentioned previously. EN and EA decrease when going down the group as the atomic radii and shielding effect increase. Fluorine has the highest electronegativity value in the periodic table (=4.0). The following graph illustrates the trend in the EN of halogens.
3-Reactivity: Due to their high EN and EA, halogens are strongly reactive elements. Halogens are mostly found combined with other elements in nature. Reactivity of halogens decreases from Fluorine to Iodine. The reactivity of a halogen determines its ability to displace other halogens from their salt’s solutions. The more reactive halogen displaces the less reactive one. For example, Fluorine can displace all other halogens. Fluorine displaces Chlorine from NaCl solution according to the following reaction:
2 NACl(aq) + F2(g) = 2 NAF(s) + Cl2(g)
In addition to that, Astatine is highly radioactive that is why it has been difficult to determine many of its properties.
Reactions of Halogens:
Reaction with Metals:
Group 1 Metals: Group 1 Alkali metals react vigorously with halogens to form crystalline alkali metal halide salt. The white ionic solid formed is soluble in water apart from LiF.
?2Na(s)+Cl?_2 (g)?2NaCl(s)
Group 2 Metals: The reaction between Alkali Earth metals and halides forms hydrated halides. In these reactions, the alkali earth metal will lose 2 electrons and the halide would gain 1 electron, and result in ?MH?_2, where M is the metal and H represents Halogen.
?Mg(s)+Cl?_2 (g)??MgCl?_2
Reaction with Hydrogen
Halogens react with Hydrogen to form Hydrogen Halide, which are colorless gases at room temperature. The higher the halogen is in the periodic table; the more vigorous the reaction is. These hydrogen halides have covalent bonds between them and the bond strength decreases down the group with HF having the strongest bond.
H_2+F_2?HF
Reaction with Water
Reactions of halogens with water depends on the oxidizing power and reduction potential of the halogens. Both Fluorine and Chlorine can oxidize water due to their large reduction potential, so they react with water to form acidic hydrogen halides.
2F2(g) + 2H2O(g) ? 4HF(g) + O2(g)
Due to the fact that Chlorine is not a powerful oxidizing agent, it does not form oxygen when it reacts with water, instead, it gives hydrochloric acid or hydrochlorus acid.
Cl2(g) + H2O(l) ? HCl(aq) + H^+(aq) + ?ClO?^-(aq)
Bromine and Iodine barely react with water as they have small reduction potential when compared to Oxygen. The following equation illustrates the reaction:
Br2(l) + H2O(l) ? HBr(aq) + HOBr(aq)
Reaction with Oxygen
Halogens react with oxygen to form halogen oxides, but most compounds formed are unstable. For example, reaction of Fluorine and Oxygen forms Dioxygen Difluoride.
O (g)+F2(g)?F2O2(g)
Compounds Formed by Halogens’ Reactions:
Halogen + Oxygen ? Halogen Oxide
Halogen + Water ? Hydrogen Halide
Halogen + Hydrogen ? Hydrogen Halide
Halogen + Metal ? Metal Halide Salt
Uses of Halogens:
Halogens are used in the manufacturing of a variety of products. The most widespread use of Fluorine is toothpastes. It is used in toothpastes due to its ability to prevent tooth decay. It is also used to etch glass into different shapes. Chlorine is also used in different ways. It is one of the two main components of table salt (NaCl). In addition to that, it is used in disinfectants and whiteners due to its high ability to sterilize. Swimming pools are treated with this element as well. While Bromine which has fire-resistant properties is used in flame retardants. It is also found in insecticides. Iodine played an ideal role in the development of photography. Silver iodide is made and later used in producing the films required for photography. However, Astatine is radioactive, and this property make it less used in industries.
Biological Role of Halogens:
Chlorine and Iodine are the two main elements which have very important biological roles. Chlorine is used as the main digestive substance in the human body. It is produced in the form of hydrochloric acid in the stomach. It activates several proteins which are necessary in the digestion of different food components. The high acidity of HCl also helps in killing the harmful bacteria that might enter to the stomach by food. On the other hand, Iodine is necessary for the production of Thyroxine (T4) and Triiodothyronine (T3) which takes place in the thyroid gland. Iodine is also added to wound cleaning antiseptic solution due to its ability to disinfect injuries.
Toxicity of Halogens:
In general halogens are toxic but they decrease in toxicity towards iodine, therefore safety precautions should be taken when dealing with them. Chlorine and Fluorine are used only in fume hood hence breathing Fluorine or Chlorine gases at high concentration is potentially lethal. Iodine causes damage to skin, gloves and goggles must be worn when using it. Moreover, a variety of halogens compounds are also toxic, especially fluorine and chlorine compounds. For instance, one of the most toxic Florine compounds is HF (Hydrofluoric acid), it has the ability to penetrate the skin and cause severe burns. Some Chlorine compounds are poisonous too, as a result they were used as chemical weapon, for example, sulfur mustard (mustard gas) is an organic compound which contains Chlorine and was used as a chemical weapon in world wars. This figure shows mustard gas formula:
Taking into consideration the environmental hazard associated with halogens usage, using CFC compound (contain Chlorine and Fluorine) for cooling purposes is extremely dangerous, as it is able to react with the Ozone gas in the Ozone layer and lead to its corrosion. Furthermore, pesticides like D.D.T (an organic chlorine compound) can readily be adsorbed to soils and water due to its biodegradable characteristics thus D.D.T causes long-term damage to human and fish. However, fortunately, it was forbidden internationally since 1972.
All in all, the halogens group is an inspirational group. Elements in that same group differ from each other in terms of color, physical state, atomic size …etc. Chemical properties also vary between the elements, as reactivity, electronegativity and electron affinity all decrease while descending through the group. Halogens have the ability to react with different elements such as group 1A and 2A elements resulting salts, and with different compounds such as water. The uses of halogens vary depending on their respective properties as mentioned. Some of them also have very important biological roles. Besides the beneficial uses, halogens are highly toxic. Precautions must be taken while handling them to avoid complications.
