Chemistry
Halogens
Halogens are a group of non-metal elements that include fluorine, chlorine, bromine, iodine, and astatine. They are highly reactive and are known for their ability to form salts when they react with metals. Halogens are commonly used in various industrial and commercial applications, such as disinfectants, water treatment, and the production of plastics and pharmaceuticals.
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9 Key excerpts on "Halogens"
eBook - ePubOccupational Exposures
Chemical Carcinogens and Mutagens
- Frances Alston, Onwuka Okorie(Authors)
- 2023(Publication Date)
- CRC Press(Publisher)
Chapter 6 Halogenated compoundsDOI: 10.1201/9781003220114-66.1 Introduction
The chemicals that are known as Halogens are listed in group 17 of the periodic table. These elements are fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Together, these elements form what is commonly known as the halogen group. Astatine has no natural or synthetic isotopes that can be considered stable2 ,3 and will not be of focus in this chapter. Fluorine (F2 ) has the highest crustal abundance of the Halogens (544 mg/kg) while iodine has the lowest (0.25 mg/kg). Chlorine is the most abundant halogen in the universe. The chemical composition of the four naturally occurring Halogens has some similarities with fluorine, chlorine, and bromine being classified as lithophile elements while iodine has a more chalcophile nature. Iodine and chlorine are essential elements for mammals, and fluorine has been shown to have beneficial effects on bone and tooth formation.4 ,5Fluorine is the most electronegative and most reactive of all the periodic elements. It can form binary compounds with all other elements apart from helium, neon, and argon. It is a pale-yellow, highly reactive gas and a strong oxidant. Fluorine is a toxic irritant that attacks the skin and the mucous membranes of the exposed individual’s nose and eyes.7 Fluorine chemistries have important industrial uses. For example, preparation of Freons in the 1930s was a major advance in the refrigeration field. F2 has also increasingly become an important laboratory reagent for the synthesis of some fluorinated molecules, many having important biological and medical applications.3Chlorine gas is produced commercially by electrolysis of sodium chloride (NaCl) brine in asbestos diaphragm cells or mercury cathode cells. Of all the Halogens, Cl2 has the most expansive industrial use and is ranked high among the large volume of chemicals manufactured in the United States in 1979. Some primary uses include the production of organic compounds; use in bleaches for paper, pulp, and textile, sanitation, and disinfection of water supplies; and production of inorganic chemicals. Chlorine gas (Cl2 ) reacts with water to produce strong oxidizing solution. This reaction can damage the moist tissue lining of the respiratory tract when exposed to chlorine.7
eBook - PDF- Bernard Moody(Author)
- 2013(Publication Date)
- Arnold(Publisher)
23 Group VII: the Halogens Fluorine, chlorine, bromine and iodine 9 The name halogen (Greek, F hals genon = sea-salt pro-2,7 ducing) was used by 17 Berzelius because he wished CI to indicate that chlorine, 2,8,7 bromine and iodine occurred 35 in the sea as salts. Sodium Br chloride is sea-salt. The !,8,18,7 Halogens are a distinctive 53 family of diatomic non-I metallic elements showing a 2,8, 18,18,7 progressive gradation in 85 reactivity. Fluorine shows At many anomalies as the first 2,8, 18,32,18,7 member of the family, and indeed, it could be argued that fluorine is so distinctive that it is not a halogen, in the family sense, at all. The extreme non-metallic properties of fluorine are toned down with suc-cessive members of the family until some slight metallic characteristics appear with iodine. The radioactive element, astatine, is not described here. As the atomic number increases the elements darken in colour, becoming less volatile and less reactive. Fluorine is a pale yellow gas at room tem-perature, lighter in colour than chlorine which is a greenish-yellow gas. Bromine is a dark red liquid which gives off a dense red vapour, while iodine is a shiny grey crystalline solid, giving a violet vapour on heating. Fluorine, chlorine and the vapour of bromine are extremely irritant to nose and throat passages and are very poisonous. Chlorine was discovered by Scheele in 1774 but not recognized as an element. The gas was formed in a reaction between hydrochloric acid, or muriatic acid as it was known (Latin, muria = brine), and manganese(rv) oxide (pyrolusite). Because aqueous solutions of the new substance evolved oxygen in sunlight and because it was formed by oxidation of muriatic acid, Berthollet called the gas oxymuriatic acid in 1785. In a later series of experiments, Davy failed to show the presence of oxygen, concluding that the gas was an element. In 1810, he called it chlorine (Greek, chloros = greenish-yellow).
eBook - PDF- Leo M.L. Nollet, Leen S. P. De Gelder, Leo M.L. Nollet, Leen S. P. De Gelder(Authors)
- 2013(Publication Date)
- CRC Press(Publisher)
They can form a very large number of inorganic and organic compounds. Most of the halides can be classified into two categories. The fluorides and chlorides of many metallic elements, especially those belonging to the alkali metal and alkaline earth metal (except beryllium) families, are ionic compounds. Most of the halides of nonmetals such as sulfur and phospho-rus are covalent compounds. Fluorine, being the most electronegative of all reactive elements, occurs only with only 0 and − 1 oxidation numbers. Chlorine, bromine, and iodine can have oxidation numbers of − 1, 0, + 1, + 3, + 5, and + 7 in ions or molecules. Table 8.1 summarizes some of the most important char-acteristics of the stable Halogens. In nature, because of their high reactivity, the Halogens are always found combined with other ele-ments. Chlorine, bromine, and iodine occur most often as halides in sea water, in soil, and in minerals like halite (NaCl), sylvite (KCl), iodargite (AgI), and bromargyrite (AgBr). Chloride is a major anionic component of the biomass. Fluorine occurs in sparingly soluble mineral deposits such as fluorite and fluorspar (CaF 2 ), cryolite (Na 3 AlF 6 ), and fluorapatite (Ca 5 (PO 4 ) 3 F). The most easily oxidized halogen element, the iodine, is also found in iodates. The Halogens are toxic materials. Their toxicity together with their reactivity decreases from fluorine to iodine. Except for astatine, the Halogens are produced on an industrial scale and are used as reagents or oxidizing agents. Chlorine production is by far the largest. It is accomplished by electrochemical oxida-tion of aqueous sodium chloride solutions. Fluorine, however, cannot be obtained by electrochemical oxidation of aqueous solutions. Water decomposition would come at lower potential than the oxidation of fluoride. Even if an electrode with high overpotential could be found, the evolved fluorine would react immediately with the water content of the electrolysis cell.
eBook - PDFExperimental Inorganic/Physical Chemistry
An Investigative, Integrated Approach to Practical Project Work
- M A Malati(Author)
- 1999(Publication Date)
- Woodhead Publishing(Publisher)
8 The Halogens 8.1 INTRODUCTION The Halogens (F, CI, Br, I and At) occupy group 17 of the Periodic Table. The atoms have 7 outer electrons and hence share a pair of electrons between two atoms forming a σ bond in the molecules Xj. The bonds become longer down the group. The bond dissociation enthalpy decreases from chlorine down the group. However, fluorine has a value close to that of iodine. As the Van der Waals forces increase with the size of the molecules, the boiling points of the elements increase. Thus fluorine and chlorine are gases whereas bromine is a liquid and iodine is a solid. These facts and the trend in the dissociation enthalpy explain the high reactivity of fluorine and the decrease in reactivity down the group. The halogen atom requires one electron to complete the stable configuration of the next noble gas. Hence the anions X' represent the most stable oxidation state of -I. The electron attachment energy becomes less negative from CI to I as expected from their atomic radii. However, F has a less negative value than CI. This behaviour and the anomalous dissociation enthalpy of F2 are ascribed to interelectronic repulsions in the compact 2p sub-shell of F. The Halogens tend to form σ bonds with other non-metals as well as between themselves. E-X bonds (where Ε is a non-metal and X is a halogen) increase in length but decrease in strength from F to I. Because F cannot expand its valence shell beyond 8, it forms one E-F bond whereas the lower Halogens have available d orbitals. Hence they can reach a maximum covalence of seven. Fluorine is also unique because of its high electronegativity (the highest in the Periodic Table) and its small radius. Thus it forms strong hydrogen bonds: F-H-E where E=F, Ο or N. The positive oxidation numbers of the Halogens from +1 to +7 increase by two units.
eBook - PDFThe Chemistry of Chlorine, Bromine, Iodine and Astatine
Pergamon Texts in Inorganic Chemistry, Volume 7
- A. J. Downs, C. J. Adams(Authors)
- 2016(Publication Date)
- Pergamon(Publisher)
26. CHLORINE, BROMINE, IODINE AND ASTATINE A. J. DOWNS and C. J. ADAMS University of Oxford 1. I N T R O D U C T I O N 1.1. GENERAL ATOMIC PROPERTIES^ The properties peculiar to non-metals can be attributed more or less directly to the relatively large effective nuclear charge experienced by the valence electrons of the atom, a characteristic reflected in the high ionization potential and electron affinity and the relatively small size of such an atom. Because electrons in the same shell shield one another relatively inefficiently from the nucleus, the incidence of non-metallic properties is a periodic function of atomic number, increase of which in a given period is accompanied by the transition from metallic to non-metallic behaviour. The noble gases, with their unique qualities of electron localization, are the culmination of this development. However, it is the preceding group of typical elements—the Halogens—which provides, in physical and chemical terms, the best defined and most homogeneous family of non-metals, described elsewhere 1 as the most perfect sçries we have. The magnitudes of the effective nuclear charges associated with the halogen atoms are indicated by the ionization potentials, which are only 1-4 eV short of those of the corre-sponding noble gas atoms, and by the relatively small sizes of the atoms. Further, the electron configuration of the halogen atoms in their ground states, ns 2 np 5 , just one electron short of the corresponding noble gas configuration, causes the atoms to be unusually powerful electron-acceptors, with electron affinities higher than those known for any other atomic species.
eBook - PDFChemistry of the Non-Metallic Elements
The Commonwealth and International Library: Intermediate Chemistry Division
- E. Sherwin, G. J. Weston, J. E. Spice(Authors)
- 2016(Publication Date)
- Pergamon(Publisher)
CHAPTER IV THE Halogens THE elements in this group form a well-defined family of extreme interest and importance. Some of the data which help in the understanding of the chemistry of these elements are presented in Table 4.1. Astatine is not included in the table but is con-sidered separately (p. 63). Electronic Structure Each element has one electron less than the nearest noble gas, so that the chemistry is largely dominated by the formation of univalent negative ions. It is in this form (together with the elemental form) that these elements show the most marked resemblances to each other. Fluorine shows no other oxidation numberf than — 1 in its compounds whereas the other Halogens all show positive oxidation numbers, of which the odd numbers + 1, +3, +5 and +7 are most important. In these states there are quite marked differences to be seen between the elements. For example, there are no perbromates known (XO4). This fact is matched by similar facts for adjacent elements in the first long period. Thus PC1 5 exists but no AsCl 5 , S0 3 is easily made but Se0 3 is difficult to produce. The values for electron affinities would seem to indicate that chlorine should be the most vigorous oxidizing agent by reason of the reaction, Cl + e -» Cl However, the electron affinities are measured from the free atom whereas the elements themselves are diatomic molecules X 2 . t See p. 73 for a discussion of this concept. 31 B* 32 CHEMISTRY OF THE NON-METALLIC ELEMENTS Before the negative ion can be formed the molecule must be dis-sociated into atoms and the energy required for this is much smaller for fluorine than for chlorine so that this more than compensates for the lower value of the electron affinity.
eBook - PDFAnalytical Chemistry of Organic Halogen Compounds
International Series in Analytical Chemistry
- L. Mázor, R. Belcher, H. Freiser(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
In these instances a compact, continuous layer of metal halide protects the surface against further attack. Thus, for example, dry chlorine gas can be stored in iron containers. Some important properties of halogen elements are given in Table I. TABLE I. SOME IMPORTANT PROPERTIES OF HALOGEN ELEMENTS Fluorine Chlorine Bromine Iodine Atomic weight 18.9984 35.453 79.909 126.9044 Atomic number 9 17 35 53 Electron configura-tion [He] 2s 2 2p 5 [Ne] 3s 2 3p 5 [Ar] 3d 1 0 4s 2 4p 5 [Kr] 4d 1 0 5s 2 5p 5 Boiling point, °C -188.2 - 34.7 58.78 184.35 Melting point, °C -219.6 -1 0 1 . 0 - 7.2 113.7 Density, g m l -1 1.11 1.56 3.12 4.97 Atomic volume, ml 17.1 18.7 23.5 25.7 Covalent radius, Â 0.72 0.99 1.14 1.33 Ion radius, (—1) Â 1.36 1.81 1.95 2.16 Ion radius, (—7) Â 0.07 0.26 0.39 0.50 1st ionization energy Kcal/g-atom 402 300 273 241 Electronegativity 4.0 3.0 2.8 2.5 Standard electrode, potential 2.85 1.36 1.08 0.58 Energy of C-halo-gen bond, Kcal, 25°C 103.8 66.6 53.0 38.7 Length of C-halo-gen bond, Â 1.317 1.77 1.91 2.12 Atomic refraction 1.25 5.967 8.865 13.900 I I . M A I N C H A R A C T E R I S T I C S O F O R G A N I C H A L O G E N C O M P O U N D S There are few naturally occurring organic halogen compounds. This is to be expected, because organic compounds take up Halogens mainly by reaction with elemental Halogens or reactive halogen compounds (e.g., phosphorus halides) which do not exist to any significant extent in nature. Compounds with double or triple bonds, as well as some alcohols, are exceptions as these can be halogenated much more readily, for example with hydrogen halides. 22 ANALYTICAL CHEMISTRY OF ORGANIC HALOGEN COMPOUNDS The Halogens and halide ions have no specific role in the operation of living cells. Only iodine is extracted from sea-water by some Laminaria and Fucus species; it is accumulated as covalently bound iodine.
- Robert Burke(Author)
- 2013(Publication Date)
- CRC Press(Publisher)
Some elements are so reactive that they do not exist naturally as single atoms. They chemically bond with another atom of that same element to form “diatomic” molecules. The diatomic elements are hydrogen, oxygen, nitrogen, chlorine, bromine, iodine, and flu-orine. One way to remember the diatomic elements is by using the acronym HONClBrIF, pronounced honk-le-brif, which includes the symbols for all of the diatomic elements. Oxygen is commonly referred to as O 2 , primarily because oxygen is a diatomic element. Two oxygen atoms have covalently bonded together and act as one unit. Much can be learned about a compound by looking at its elemental composition. Generally speaking, chemicals that contain chlorine in their formula may be toxic to some degree because chlorine is toxic. There are exceptions, such as sodium chloride, with the formula NaCl. Sodium chloride is table salt. The toxicity is low, but even if you did not know sodium chloride was table salt and treated it as a toxic material because of the chlo-rine, your error would be on the side of safety. If you are going to make errors when 112 Hazardous materials chemistry for emergency responders © 2010 Taylor & Francis Group, LLC dealing with hazardous materials, always attempt to err on the side of safety. You may have “egg on your face” afterward and take some ribbing, but no one has ever died from embarrassment. On the other hand, if you are not cautious and your error is not on the side of safety, it could be fatal! Atom An atom is the smallest particle of an element that retains all of its elemental characteris-tics. The word atom comes from the Greek, meaning “not cut.” For example, take a sheet of paper and tear it in half. Keep tearing the paper in half until you cannot tear it any more. You could then take scissors and cut the paper into smaller pieces. Eventually, you will not be able to cut the paper into a smaller piece.
eBook - ePubChemistry and Analysis of Radionuclides
Laboratory Techniques and Methodology
- Jukka Lehto, Xiaolin Hou(Authors)
- 2011(Publication Date)
- Wiley-VCH(Publisher)
, require radiochemical separations before measurement.Table 11.1 Important halogen radionuclides11.2 Physical and Chemical Properties of the HalogensHalogens are nonmetals and are located in group 17 of the periodic table with the electron structure [Ng]ns2 np5 . There are seven electrons in the outermost shell, that is they have the octet structure of the noble gases but with one electron missing. As a result, they readily form negative ions with a single charge and have the oxidation state of −I (F− , Cl− , Br− , I− , At− ). In the case of fluorine, −I is, in practice, the only oxidation state. The Halogens are extremely electronegative, fluorine being the most electronegative element of all. Salts formed with these negative ions of single charge, the halides, are highly soluble, and solubility increases with the atomic number. In higher oxidation states, the Halogens also form oxoacids. For example, in the oxidation state +V, iodine forms iodic acid, whose anion is iodate (IO3 − ), and in the oxidation state +VII it forms periodic acid, whose anion is periodate (IO4 − ). As seen from Figure 11.1 , iodide is the predominant chemical form. Iodate is present in oxidizing conditions in alkaline solutions. Formation of periodate requires oxidizing conditions more extreme than any found in natural systems. Chlorine most commonly appears in solution as the chloride ion, Cl− , and the perchlorate ion, ClO4 − , can only be found in very oxidizing conditions, again not found in natural systems (Figure 11.1 ). The Halogens can also be in the form of gases (F2 , Cl2 , Br2 , I2 , At2
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