The halogens are not metallic elements belonging to Group VIIA or 17 of the periodic table. They have electronegativities and high electronic affinities, which greatly affect the ionic character of their bonds with metals. The word ‘halogens’ is of Greek origin and means ‘salt formers’.
But what are these halogens? Fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and the radioactive and ephemeral element astate (At). They are so reactive that they react with each other to form diatomic molecules: F 2 , Cl 2 , Br 2 , I 2, and At 2 . These molecules are characterized by having similar structural properties (linear molecules), although with different physical states.
What is the appearance of halogens?
Three halogens are shown in the lower image. From left to right: chlorine, bromine and iodine. Neither fluorine nor astatine can be stored in glass containers, since the latter do not resist its corrosivity. Note how the organoleptic properties of halogens change as one moves down their group to the element iodine.
Fluorine is a gas with yellowish hues; chlorine too, but greenish-yellow; bromine is a dark reddish liquid; iodine, a black solid with violet overtones; and astatine, a dark, shiny metallic solid.
Halogens are capable of reacting with almost all the elements of the periodic table, even some noble gases (such as xenon and krypton). When they do, they can oxidize atoms to their most positive oxidation states, turning them into powerful oxidizing agents.
Likewise, they confer specific properties to molecules when they bind or replace some of their atoms. These types of compounds are called halides. In fact, halides are the main natural source of halogens, and many of them are dissolved in the sea or are part of a mineral; such is the case of fluorite (CaF 2 ).
Both halogens and halides have a wide range of uses; from industrial or technological, to simply highlighting the flavor of certain foods such as rock salt (sodium chloride).
Physical and chemical properties of halogens
Fluorine (F) 18.99 g / mol; Chlorine (Cl) 35.45 g / mol; Bromine (Br) 79.90 g / mol; Iodine (I) 126.9 g / mol and Astate (At) 210 g / mol,
Gaseous phase; Cl gas; Br liquid; I solid and At solid.
F, pale yellow-brown; Cl, pale green; Br, reddish-brown; I, violet and At, metallic black * * (assumed)
F -219.6 ° C; Cl -101.5 ° C; Br -7.3 ° C; I 113.7º C and At 302º C.
F -118.12 ° C; Cl -34.04 ° C; Br 58.8 ° C; I 184.3º C and? At 337º C.
Density at 25º C
F- 0.0017 g / cm 3 ; Cl- 0.0032 g / cm 3 ; Br- 3.102 g / cm 3 ; I- 4.93 g / cm 3 and At- 6.2-6.5 g / cm 3
Cl- 0.091 mmol / cm 3 ; Br- 0.21 mmol / cm 3 and I- 0.0013 mmol / cm 3 .
F- 1,681 kJ / mol; Cl- 1,251 kJ / mol; Br- 1,140 kJ / mol; I- 1.008 kJ / mol and At- 890 kJ / mol.
F- 4.0; Cl- 3.0; Br- 2.8; I- 2.5 and At- 2.2.
Halogens have 7 electrons in their valence shell, hence their great eagerness to gain an electron. Also, halogens have high electronegativity due to their small atomic radii and the great attraction that the nucleus exerts on valence electrons.
Halogens are highly reactive, which would then explain their toxicity. In addition, they are oxidizing agents.
The decreasing order of reactivity is: F> Cl> Br> I> At.
State in nature
Due to their great reactivity, halogen atoms are not free in nature; rather, they are found in aggregates or as diatomic molecules linked by covalent bonds.
Halogens do not exist in nature as elemental atoms, but as diatomic molecules. However, they all have in common that they have a linear molecular structure, and the only difference lies in the length of their bonds and in their intermolecular interactions.
Linear molecules XX (X 2 ) are characterized by being unstable, because both atoms strongly attract the pair of electrons towards them. Why? Because its outer electrons experience a very high effective nuclear charge, Zef. The higher Zef, the smaller the distance of the link XX.
As you go down the group, Zef becomes weaker and the stability of these molecules increases. Thus, the decreasing order of reactivity is: F 2 > Cl 2 > Br 2 > I 2 . However, it is incongruous to compare astatine with fluorine, since sufficiently stable isotopes are unknown due to their radioactivity.
On the other hand, its molecules lack a dipole moment, being apolar. This fact is responsible for its weak intermolecular interactions, whose only latent force is the scattering or London force, which is proportional to the atomic mass and the molecular area.
Thus, the small molecule of F 2 does not have enough mass or electrons to form a solid. Unlike I 2 , the iodine molecule, which nevertheless remains a solid that gives off purple vapors.
Bromine represents an intermediate example between the two extremes: Br 2 molecules interact enough to appear in a liquid state .
The astatine probably, due to its increasing metallic character, does not appear as At 2 but as At atoms forming metallic bonds.
Regarding its colors (yellow-greenish-yellow-red-purple-black), the most appropriate explanation is based on the molecular orbital theory (TOM). The energetic distance between the last full molecular orbital, and the next one with the highest energy (anti-bond), is overcome by absorbing a photon with increasing wavelengths.
Halogens react to form halides, either inorganic or organic. The best known are the hydrogen halides: hydrogen fluoride (HF), hydrogen chloride (HCl), hydrogen bromide (HBr) and hydrogen iodide (HI).
All of them dissolved in water generate acid solutions; so acidic that HF can degrade any glass container. Furthermore, they are considered starting materials for the synthesis of extremely strong acids.
There are also so-called metal halides, which have chemical formulas that depend on the valence of the metal. For example, the alkali metal halides have the formula MX, and among them are: NaCl, sodium chloride; KBr, potassium bromide; CsF, cesium fluoride; and LiI, lithium iodide.
The halides of the alkaline earth metals, transition metals or metals of the p block have the formula MX n , where n is the positive charge of the metal. Thus, some examples of them are: FeCl 3 , ferric trichloride; MgBr 2 , magnesium bromide; AlF 3 , aluminum trifluoride; and CuI 2 , cupric iodide.
However, halogens can also form bonds with carbon atoms; therefore, they can intrude into the complex world of organic chemistry and biochemistry. These compounds are called organic halides, and have the general chemical formula RX, X being any of the halogens.
Uses / applications of halogens
-Bromine and chlorine are used in the textile industry to bleach and treat wool, thus preventing its shrinkage when wet.
-It is used as a disinfectant of ditritus and for the purification of drinking water and swimming pools. In addition, compounds derived from chlorine are used in laundries and in the paper industry.
-Find use in the manufacture of special batteries and chlorinated hydrocarbons. It is also used in the processing of meat, vegetables, fish and fruits. Also, chlorine works as a bactericidal agent.
-It is used to clean and detanify leather, and to bleach cellulose. Nitrogen trichloride was formerly used as a bleach and conditioner for flour.
-Phosphene gas (COCl 2 ) is used in numerous industrial synthesis processes, as well as in the manufacture of military gases. Phosphene is very toxic and is responsible for numerous deaths in WWI, where the gas was used.
-This gas is also found in insecticides and fumigants.
-NaCl is a very abundant salt that is used to season food and in the preservation of livestock and poultry. In addition, it is used in body rehydration fluids, both orally and intravenously.
-Halogen atoms that bind to drugs make them more lipophilic. This allows drugs to more easily cross cell membranes, dissolving in the lipids that make it up.
-Chlorine diffuses into the neurons of the central nervous system through ion channels linked to the receptors of the neurotransmitter GABA, thus producing a sedative effect. This is the mechanism of action of several anxiolytics.
-HCl is present in the stomach, where it intervenes by creating a reducing environment that favors food processing. Furthermore, HCl activates pepsin, an enzyme that initiates the hydrolysis of proteins, a stage prior to intestinal absorption of the protein material.
-Hydrochloric acid (HCl) is used in the cleaning of bathrooms, in teaching and research laboratories and in many industries.
-PVC (polyvinyl chloride) is a vinyl chloride polymer that is used in clothing, floor tiles, electrical cables, flexible tubes, pipes, inflatable structures and roof tiles. In addition, chlorine is used as an intermediate in the manufacture of other plastic materials.
-Chlorine is used in the extraction of bromine.
-Methyl chloride acts as an anesthetic. It is also used in the manufacture of certain silicone polymers and in the extraction of fats, oils and resins.
-Chloroform (CHCl 3 ) is a solvent used in many laboratories, especially in organic chemistry and biochemistry laboratories, from teaching to research.
-And finally with regard to chlorine, trichlorethylene is used to degrease metal parts.
-Bromine is used in the gold mining process and in the drilling of oil and gas wells. It is used as a flame retardant in the plastics and gas industries. The bromine isolates the fire from the oxygen causing it to go out.
-It is an intermediary in the manufacture of hydraulic fluids, cooling and dehumidifying agents and preparations for shaping hair. Potassium bromide is used in the manufacture of photographic plates and papers.
-Potassium bromide is also used as an anticonvulsant, but due to the possibility that salt can cause neurological dysfunctions its use has been reduced. Additionally, another of its common uses is as a chip for measurements of solid samples by infrared spectroscopy.
-Bromine compounds are present in medicines used to treat pneumonia. Also, bromine compounds are incorporated into drugs used in trials carried out in the treatment of Alzheimer’s disease.
-Bromine is used to reduce mercury pollution in power plants that use coal as fuel. It is also used in the textile industry to create different color dyes.
-Methyl bromine was used as a pesticide for fumigation of soil and homes, but its damaging effect on ozone has limited its use.
-Halogen lamps are incandescent and the addition of small amounts of bromine and iodine allows a reduction in the size of the bulbs.
-Iodine is involved in the functioning of the thyroid gland, a regulatory hormone of the body’s metabolism. The thyroid gland secretes the hormones T3 and T4, which act on its target organs.For example, hormonal action on the heart muscle causes an increase in blood pressure and heart rate.
-Also, iodine is used to identify the presence of starch. Silver iodide is a reagent used in the development of photographs.
-Some fluoride compounds are added to toothpastes in order to prevent cavities. Derivatives of fluoride are present in various anesthetics. In the pharmaceutical industry, they incorporate fluoride into drugs to study possible improvements in their effects on the body.
-Hydrofluoric acid is used to etch glass. Also in the production of halons (fire extinguishing gases, such as freon). A fluorine compound is used in the electrolysis of aluminum to achieve its purification.
-Anti-reflective coatings contain a fluorine compound. This is used in the manufacture of plasma screens, flat screens and microelectromechanical systems. Fluorine is also present in the clay used in some ceramics.
It is thought that astatine may help iodine in regulating the functioning of the thyroid gland. Also, its radioactive isotope ( 210 At) has been used in cancer studies in mice.
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