Bromine Br 2 is a reddish-orange liquid with an unpleasant, choking odor. The name of the element, in fact, comes from the Greek stem bromos , "stench. Iodine is an intensely colored solid with an almost metallic luster. This solid is relatively volatile, and it sublimes when heated to form a violet-colored gas.
Iodine has been used for many years as a disinfectant in "tincture of iodine. Silver iodide AgI plays an important role in the photographic process and in attempts to make rain by seeding clouds. Iodide is also added to salt to protect against goiter, an iodine deficiency disease characterized by a swelling of the thyroid gland.
Some of the chemical and physical properties of the halogens are summarized in the table below. There is a regular increase in many of the properties of the halogens as we proceed down the column from fluorine to iodine, including the melting point, boiling point, intensity of the color of the halogen, the radius of the corresponding halide ion, and the density of the element.
On the other hand, there is a regular decrease in the first ionization energy as we go down this column. As a result, there is a regular decrease in the oxidizing strength of the halogens from fluorine to iodine. Methods of Preparing the Halogens from their Halides.
The halogens can be made by reacting a solution of the halide ion with any substance that is a stronger oxidizing agent. Iodine, for example, can be made by reacting the iodide ion with either bromine or chlorine.
Bromine was first prepared by A. Balard in by reacting bromide ions with a solution of Cl 2 dissolved in water. To prepare Cl 2 , we need a particularly strong oxidizing agent, such as manganese dioxide MnO 2. The synthesis of fluorine escaped the efforts of chemists for almost years. Part of the problem was finding an oxidizing agent strong enough to oxidize the F - ion to F 2.
The task of preparing fluorine was made even more difficult by the extraordinary toxicity of both F 2 and the hydrogen fluoride HF used to make it. The best way of producing a strong reducing agent is to pass an electric current through a salt of the metal.
Sodium, for example, can be prepared by the electrolysis of molten sodium chloride. In theory, the same process can be used to generate strong oxidizing agents, such as F 2. Attempts to prepare fluorine by electrolysis, however, were initially unsuccessful. Humphry Davy, who prepared potassium, sodium, barium, strontium, calcium, and magnesium by electrolysis repeatedly tried to prepare F 2 by the electrolysis of fluorite CaF 2 , and succeeded only in ruining his health.
Joseph Louis Gay-Lussac and Louis Jacques Thenard, who prepared elemental boron for the first time, also tried to prepare fluorine and suffered from very painful exposures to hydrogen fluoride. George and Thomas Knox were badly poisoned during their attempts to make fluorine, and both Paulin Louyet and Jerome Nickles died from fluorine poisoning. Finally, in Henri Moissan successfully isolated F 2 gas from the electrolysis of a mixed salt of KF and HF and noted that crystals of silicon burst into flame when mixed with this gas.
All of these except HF are strong chemical acids when dissolved in water. However, hydrofluoric acid does have quite destructive properties towards animal tissue, including that of humans. When in aqueous solution, the hydrogen halides are known as hydrohalic acids. The names of these acids are as follows:. All of these acids are dangerous and must be handled with great care. Some of these acids are also widely used in chemical manufacturing plants. Hydrogen astatide should also be a strong acid hydroastatic acid , but it is seldom included in presentations about hydrohalic acids because of the extreme radioactivity of astatine via alpha decay and the fact that it readily decomposes into its constituent elements hydrogen and astatine.
The halogens form many compounds with metals. These include highly ionic compounds such as sodium chloride, monomeric covalent compounds such as uranium hexafluoride, and polymeric covalent compounds such as palladium chloride.
Metal halides are generally obtained through direct combination or, more commonly, through neutralization of a basic metal salt with a hydrohalic acid. Silver Chloride : Silver chloride is the precipitate formed when silver nitrate solution is added to chloride solution. The halogens react with each other to form interhalogen compounds.
The properties and behavior of a diatomic interhalogen compound tend to be intermediates of those of its parent halogens. Some properties, however, are found in neither parent halogen. Many synthetic organic compounds, such as plastic polymers, as well as a few natural organic compounds, contain halogen atoms; these are known as halogenated compounds, or organic halides.
Chlorine is by far the most abundant of the halogens and is the only one needed as chloride ions in relatively large amounts by humans. For example, chloride ions play a key role in brain function by mediating the action of the inhibitory transmitter GABA. They are also used by the body to produce stomach acid. Iodine is needed in trace amounts for the production of thyroid hormones, such as thyroxine.
On the other hand, neither fluorine nor bromine is believed to be essential for humans. Organohalogens are also synthesized through the nucleophilic abstraction reaction.
Polyhalogenated compounds are industrially created compounds substituted with multiple halogens. Many of them are very toxic and bioaccumulate in humans, but they have many possible applications.
Despite its toxicity, fluoride can be found in many everyday products, including toothpaste, vitamin supplements, baby formulas, and even public water. Many dental products contain fluoride in order to prevent tooth decay, but overconsumption of fluoride can be fatal. Chlorine accounts for about 0.
Chlorine is primarily used in the production of hydrochloric acid, which is secreted from the parietal cells in the stomach and is used in maintaining the acidic environment for pepsin.
It plays a vital role in maintaining the proper acid-base balance of body fluids. It is neutralized in the intestine by sodium bicarbonate. Both chlorine and bromine are used as disinfectants for drinking water, swimming pools, fresh wounds, spas, dishes, and surfaces. They kill bacteria and other potentially harmful microorganisms through a process known as sterilization. Chlorine and bromine are also used in bleaching. Sodium hypochlorite, which is produced from chlorine, is the active ingredient of most fabric bleaches.
Chlorine-derived bleaches are also used in the production of some paper products. Iodine is an essential mineral for the body. It is used in the thyroid gland but can also be found in breast tissue, salivary glands, and adrenal glands. Without iodine, thyroid hormones cannot be produced, which leads to a condition called hypothyroidism. Without treatment, the thyroid gland will swell and produce a visible goiter. Children with hypothyroidism may develop mental retardation. In women, hypothyroidism can lead to infertility, miscarriages, and breast and ovarian cancer.
Thyroid problems have been a common issue for many years, particularly in middle aged women; studies correlate this with the fact that iodine levels in the general population have significantly decreased in recent years.
Because of certain health problems, many people have been consuming less salt, which usually contains iodine. In drug discovery, the incorporation of halogen atoms into a lead drug candidate results in analogues that are usually more lipophilic and less water-soluble.
Iodine has many important roles in life, including thyroid hormone production. This will be discussed in Part VI of the text. Astatine - Astatine is a radioactive element with an atomic number of 85 and symbol At. It is the only halogen that is not a diatomic molecule and it appears as a black, metallic solid at room temperature.
Astatine is a very rare element, so there is not that much known about this element. In addition, astatine has a very short radioactive half-life , no longer than a couple of hours.
It was discovered in by synthesis. Also, it is thought that astatine is similar to iodine. However, these two elements are assumed to differ by their metallic character. The periodic trends observed in the halogen group:. The melting and boiling points increase down the group because of the van der Waals forces. The size of the molecules increases down the group. This increase in size means an increase in the strength of the van der Waals forces.
In addition, more energy levels are added with each period. This results in a larger orbital, and therefore a longer atomic radius. If the outer valence electrons are not near the nucleus, it does not take as much energy to remove them. Therefore, the energy required to pull off the outermost electron is not as high for the elements at the bottom of the group since there are more energy levels. Also, the high ionization energy makes the element appear non-metallic.
The number of valence electrons in an atom increases down the group due to the increase in energy levels at progressively lower levels. The electrons are progressively further from the nucleus; therefore, the nucleus and the electrons are not as attracted to each other. An increase in shielding is observed. An electron will not be as attracted to the nucleus, resulting in a low electron affinity.
However, fluorine has a lower electron affinity than chlorine. This can be explained by the small size of fluorine, compared to chlorine. This is due to the fact that atomic radius increases in size with an increase of electronic energy levels.
This lessens the attraction for valence electrons of other atoms, decreasing reactivity. This decrease also occurs because electronegativity decreases down a group; therefore, there is less electron "pulling. A halide is formed when a halogen reacts with another, less electronegative element to form a binary compound. Hydrogen, for example, reacts with halogens to form halides of the form HX:. Hydrogen halides readily dissolve in water to form hydrohalic hydrofluoric , hydrochloric , hydrobromic , hydroiodic acids.
The properties of these acids are given below:. It may seem counterintuitive to say that HF is the weakest hydrohalic acid because fluorine has the highest electronegativity. A strong bond is determined by a short bond length and a large bond dissociation energy.
Of all the hydrogen halides, HF has the shortest bond length and largest bond dissociation energy. A halogen oxoacid is an acid with hydrogen, oxygen, and halogen atoms. The acidity of an oxoacid can be determined through analysis of the compound's structure.
The halogen oxoacids are given below:. In each of these acids, the proton is bonded to an oxygen atom; therefore, comparing proton bond lengths is not useful in this case. Instead, electronegativity is the dominant factor in the oxoacid's acidity. Acidic strength increases with more oxygen atoms bound to the central atom. The halogens' colors are results of the absorption of visible light by the molecules, which causes electronic excitation.
Fluorine absorbs violet light, and therefore appears light yellow. Iodine, on the other hand, absorbs yellow light and appears violet yellow and violet are complementary colors, which can be determined using a color wheel.
The colors of the halogens grow darker down the group:. In closed containers, liquid bromine and solid iodine are in equilibrium with their vapors, which can often be seen as colored gases. Although the color for astatine is unknown, it is assumed that astatine must be darker than iodine's violet i. As a general rule, halogens usually have an oxidation state of However, if the halogen is bonded to oxygen or to another halogen, it can adopt different states: the -2 rule for oxygen takes precedence over this rule; in the case of two different halogens bonded together, the more electronegative atom takes precedence and adopts the -1 oxidation state.
Chlorine is more electronegative than iodine, therefore giving it the -1 oxidation state. Adding both of these values together, the total oxidation state of the compound so far is One third exception to the rule is this: if a halogen exists in its elemental form X 2 , its oxidation state is zero.
Electronegativity increases across a period, and decreases down a group. Therefore, fluorine has the highest electronegativity of all of the elements, indicated by its position on the periodic table. If fluorine gains one more electron, the outermost p orbitals are completely filled resulting in a full octet.
Because fluorine has a high electronegativity, it can easily remove the desired electron from a nearby atom. Fluorine is then isoelectronic with a noble gas with eight valence electrons ; all its outermost orbitals are filled. Fluorine is much more stable in this state. Fluorine : Although fluorine is very reactive, it serves many industrial purposes.
For example, it is a key component of the plastic polytetrafluoroethylene called Teflon-TFE by the DuPont company and certain other polymers, often referred to as fluoropolymers. Chlorofluorocarbons CFCs are organic chemicals that were used as refrigerants and propellants in aerosols before growing concerns about their possible environmental impact led to their discontinued use.
Hydrochlorofluorocarbons HFCs are now used instead. Fluoride is also added to toothpaste and drinking water to help reduce tooth decay. Fluorine also exists in the clay used in some ceramics.
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