Halogens

Position of halogens in the periodic table

The halogens from the Greek, former of salts are the chemical elements that form group 17 or group VII (used previously) of the periodic table: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At) and teneso (Ts). The latter is also in the f-block metals.

In their natural state they are found as chemically active diatomic molecules [X₂]. To completely fill their last energy level (s²p⁵) they need one more electron, so they have a tendency to form a mononegative ion, X^-. This ion is called a halide; the salts that contain it are known as halides. They have an electronegativity ≥ 2.5 according to the Pauling scale, fluorine presenting the highest electronegativity, and this decreases when going down in the group. They are oxidizing elements (this characteristic also decreases when going down the group), and fluorine is capable of taking most of the elements to the highest oxidation state.

Many synthetic and some natural organic compounds contain halogens; these compounds are called halogenated compounds. Thyroid hormone contains iodine atoms. Chlorides play an important role in the functioning of the brain through the action of the neurotransmitter inhibitor of the transmission of the GABA neurotransmitter.

Some compounds have properties similar to those of halogens, which is why they are called pseudohalogens. The pseudohalide may exist, but not the corresponding pseudohalogen. Some pseudohalides: cyanide (CN^-), thiocyanate (SCN^-), fulminate (CNO^-), etc.

History

The word "halogen" comes from the Greek words hals, 'salt' and genes, 'origen', which originates salt. The name halogen refers to the property of each of the halogens to form, with sodium, a salt similar to common salt (sodium chloride). All members of the group have a valence of -1 and combine with metals to form halides (also called halides), as well as with metals and nonmetals to form complex ions. The first four elements of the group react easily with hydrocarbons, obtaining alkyl halides.

Halogen elements are those that occupy group 17 of the Periodic System. The molecular forms of the (diatomic) halogens F, Cl, Br, I, and At are volatile elements, whose color intensifies with increasing atomic number. Fluorine is a pale yellow gas, slightly heavier than air, corrosive, and with a pungent, irritating odor. Chlorine is a greenish-yellow gas with a pungent and irritating odor. Bromine at room temperature is a dark red liquid, three times as dense as water, which easily volatilizes producing a poisonous reddish vapor. Iodine is a crystalline solid at room temperature, black and shiny, which sublimes giving a very dense, poisonous violet vapor with a pungent odor like chlorine. Astatine is a very unstable element that exists only in short-lived radioactive forms, and that appears in the decay process of 235U. Table 1 shows some of the physical and atomic properties of the elements in this group.

All atoms have a configuration that differs from that of a noble gas by one electron, so elements tend to form negative species, X¯, or to form single covalent bonds. The chemistry of these elements and their compounds change with the size of the elements.

As expected, the melting and boiling points increase as you go down the group opposite to the right. The ionization energies of the halogens present very high values that decrease with increasing atomic number. Electron affinities are high as a consequence of the tendency to gain an electron and thus achieve the configuration of noble gas(es).

Features

Halogens show trends in their binding energy from top to bottom on the periodic table with fluoride showing minimal deviation. They are shown to have strong binding energy with other atoms but weak interactions with the diatomic F₂ molecule. Which means that as you go down the periodic table the reactivity of the element decreases due to the increase in the size of the atom.

Halogens are highly reactive, so they can be harmful to biological organisms in sufficient quantities. Its high reactivity is due to the high electronegativity that its atoms present due to their highly effective nuclear charges. Halogens have 7 valence electrons in their outer energy shell, so when reacting with another element they satisfy the octet rule. Fluoride is the most reactive of the elements, it attacks inert materials such as glass and forms compounds with inert noble gases. It is a corrosive and highly toxic gas.

Compounds

Halogens form homonuclear diatomic molecules (not tested with astatine). Due to their relatively weak intermolecular forces, chlorine and fluorine are part of the group of "elementary gases".

Among the compounds formed by halogens are hydrogen halides, metal halides, interhalogens.

Hydrogen halides

All hydrogen halides, HX, are gases at 298K and have a strong acid odor. The direct combination of H₂ and X₂ to form HX can only be used in the synthesis of chloride and bromide. Hydrogen fluoride is prepared by treating the appropriate fluorides with concentrated H₂SO₄ and analogous reactions prepare HCl. Analogous reactions with bromides and iodides result in partial oxidation of HBr or HI to Br₂ or I₂ and thus synthesis takes place with PX₃ prepared in place.

Metal halides

All alkali metal halides have a NaCl or CsCl structure and their formation can be considered in terms of the Born-Haber cycle. The reaction of sodium with chloride is:

2Na + Cl₂ → 2NaCl

In the absence of solvent, the energy change associated with the reaction involves:

• The difference in energy terms in C-Cl and C-F link (not dependent on M)
• The difference between the electronic affinity of F and Cl (not dependent on M)

Most of the metallic difluorenes crystallize with the CaF₂ or rutile structure and for most of them a simple ionic model is adequate. Metal trichlorides are crystallographically more complex than difluorides, but they usually occur in symmetrical three-dimensional structures and many contain octahedral (sometimes distorted) metal centers. Most metal tetrahalides are either volatile species (SnCl₄, TiCl₄), or contain rings or bridged M-F-M chains (SnF₄); metal-halogen bridges are longer than terminal bonds. Penta metal halides can have chain or ring structures (NbF₅) or molecular structures (SbCl₅), while hexalides are molecular and octahedral (UF ₆, MoF₆). In general, increased oxidation results in a structural change throughout the three-dimensional ionic series of layers or molecular polymer.

Interhalogen

An interhalogen is a compound that contains two or more different halogen atoms (fluorine, chlorine, bromine, iodine, or astatine).

Most known interhalogens are binary (composed of two different elements). Its general formula is XY_n, where n = 1, 3, 5, or 7, and X is the least electronegative halogen. All of them are prone to hydrolysis, and ionize to give rise to polyhalogen ions.

No interhalogen compounds containing three or more different halogens are definitely known, although some books claim that IFCl₂ and IF₂Cl have been synthesized and theoretical studies indicate that some compounds in the BrClF_n series are barely stable.

Types of interhalogens

Diatomic Interhalogens

Interhalogens of the XY form have intermediate physical properties between the two halogens that compose them. The covalent bond between the two atoms has something of an ionic character. The less electronegative halogen, X, is being oxidized and has a partial positive charge. All combinations of fluorine, chlorine, bromine and iodine having the general formula mentioned above are known, but not all of them are stable. Some combinations of astatine with other halogens are not even known, and those that are are highly unstable.

• Chlorine monofluoride (CIF) is the lightest interhalogen compound. CIF is a colorless gas with a normal boiling point of -100 °C.
• Bromine monofluoride (BrF) has not been obtained as a pure compound, is dissociated in trifluoride and free bromine. It is synthesized according to the following Br reaction_2(l) + F_2(g) → 2BrF_(g)
• iodine monofluoride (IF) is unstable and decomposes to 0 °C, disproportionates in elemental iodine and iodine pentafluoride.
• Bromine monochloride (BrCl) is a red-café gas with a boiling point of 5 °C.
• Yode monochloride (ICl) exists as transparent red crystals that melt to 27.2 °C to form a coffee liquid (similar in appearance and weight the bromine). React with HCl to form a HICl strong acid₂. The crystalline structure of ICl consists of frowned chains in zigzag, with strong interactions between them.
• Astate monochloride (AtCl) is made by the direct combination of gaseous astate with chlorine or by the successive addition of ion dicromato astate to a chloride acid solution.
• Monobromide iodine (IBr) is synthesized by the direct combination of the elements to form dark red solid crystals. It melts at 42 °C and ebules at 116 °C to form a partially dissociated steam.
• Astate monobromide (AtBr) is obtained by the direct combination of astatus with bromine vapor or with an aqueous solution of iodine monobromide.
• Astatus monoyodide (AtI) is synthesized by the direct combination of astatus and iodine. It's the heaviest known interhalogen compound.

Astatine fluoride hasn't been discovered yet. Their absence has been attributed to the extreme reactivity of these compounds, including the reaction of fluoride initially formed on the walls of a glass container to form a non-volatile product. Although synthesis of the compound is thought to be possible, it may require a liquid fluorine solvent.

Tetraatomic Interhalogens

• Chlorine trifluoride is a colorless gas that condenses to a green liquid and solid is white. It is obtained by reacting chlorine with an excess fluoride at 250 °C in a nickel tube. It reacts more violently than the fluoride and is explosive. The molecule is flat and T-shaped. It is used to manufacture uranium hexafluoride.
• Bromine Trifluoride (BrF₃) is a green-yellow liquid that leads electricity. It ionizes to form [BrF₂]⁺ + [BrF₄]⁻.
• Iodine Trifluoride (IF)₃) is a yellow solid that breaks down above -28 °C. It can be synthesized from its elements, but care must be taken to prevent IF formation₅. F₂ attacks the I₂ to form IF₃ a −45 °C in CCl₃F. At low temperatures reaction I₂ + 3XeF₂ → 2IF₃ + 3Xe can be used. It's a very unstable compound.
• Iodine trichloride (ICl₃) forms lemon yellow crystals that melt under pressure to a coffee liquid. It can be obtained from its elements at low temperatures, or from iodine pentoxide and hydrogen chloride. It reacts with several metals to form tetrachloroyoduros and hydrolyzes in water. The molecule is a flat-dimer (ICl)₃)₂with every iodine around the four chlorines.

Hexaatomic Interhalogens

• Chlorine Pentafluoride (ClF₅) is a colorless gas, made by reacting chlorine trifluoride with fluoride at high temperatures and pressures. It reacts violently with water, with most metals and non-metals.
• Bromine Pentafluoride (BrF₅) is a sintetized colorless fluid when reacting bromine trifluroruo with gluor at 200 °C. It is physically stable but reacts violently with water, with most metals and non-metals.
• Iodine Pentafluoride (IF)₅) is a synthesized fluid when reacting iodine pentox with fluoride or iodine with silver (II). It is highly reactive, reacts with elements, oxides and carbon halures. The molecule has a tetragonal pyramid shape.
• Iodine Pentabromide (IBr₅) if there is controversy at this point, it is a dark red-café liquid or a colourless brown-yellow solid, made by reacting iodine with bromine at 60 °C. In its liquid state most of its properties are similar to bromine, in any state, is very toxic. It's unstable to warm it up from the bromine boiling point.

Octaatomic Interhalogens

• iodine heptafluoride (IF)₇) is a colorless gas and a strong fluorizing agent. It is synthesized by reacting iodine pentafluoride with gaseous fluoride. The molecule is a pentagonal pyramid. This compound is the only known interhalogen where the largest atom is linked with seven smaller atoms.

All attempts to synthesize bromine heptafluoride have failed, instead bromine pentafluoride and fluorine are produced in a gaseous state.

Generally most important applications of halogens

Apart from the already mentioned halogen lamps, there are many other applications of halogens.

Fluorine derivatives are of considerable importance in the field of industry. Notable among them are fluorinated hydrocarbons, such as Freon antifreeze and Teflon resin, a lubricant with remarkable mechanical properties.

Chlorine finds its main application as a bleaching agent in the paper and textile industries. Likewise, it is used in the sterilization of drinking water and swimming pools, and in the industries of dyes, medicines and disinfectants.

Bromides act medically as sedatives, and silver bromide is used as a fundamental element in photographic plates. Iodine, whose presence in the human organism is essential and whose defect produces goiter, is used as an antiseptic in the case of wounds and burns.

Diatomic Halogen Molecules

Halogen	Molécula	Structure	Model	d(X−X) / pm (gaseous phase)	d(X−X) / pm (solidated phase)
Fluor	F2			143	149
Clothes	Cl2			199	198
Bro.	Br2			224	227
Yodo	I2			266	272

Toxicity

Halogens tend to decrease in toxicity towards the heavier halogens.

Fluorine gas is extremely toxic: at concentrations of 0.1% it is lethal within minutes. Hydrofluoric acid is also toxic: it is capable of penetrating the skin and causing very deep and painful burns. In addition, fluoride anions are toxic, although not quite as toxic as pure fluoride (which is lethal around 5 to 10 grams). Its prolonged consumption at concentrations of 1.5 mg/L is associated with dental fluorosis, an abnormality in the oral cavity. At concentrations greater than 4 mg/L, the risk of developing osseous fluorosis, hardening of the bones, increases. Currently recommended levels of bloom for drinking water range from 0.7 to 1.2 mg/L to avoid adverse effects of fluoride without wasting its benefits. People with levels between normal and those required to develop bone fluorosis tend to develop arthritis-like symptoms.

Chlorine gas is highly toxic. When inhaled at concentrations of 3 parts per million, a toxic reaction occurs immediately. Inhaling it at 50 parts per million is extremely dangerous and inhaling concentrations of 500 parts per million for even a few minutes is fatal. Hydrochloric acid is a dangerous chemical.

Pure bromine is somewhat toxic but less so than fluorine and chlorine. One hundred milligrams of bromine is lethal. Bromine anions are lethal, but less than bromine which is lethal in 30 gram doses.

Iodine is relatively toxic, capable of irritating the eyes and lungs, with a safety limit of 1 milligram per cubic meter. When ingested orally 3 grams can be lethal. Its ions are mostly non-toxic, but can cause death if ingested in large amounts.

Astatine is very radioactive and therefore very dangerous.