Fluorine

2008/9 Schools Wikipedia Selection. Related subjects: Chemical elements

9 oxygen ← fluorine → neon - ↑ F ↓ Cl Periodic table - Extended periodic table
General
Name, symbol, number	fluorine, F, 9
Chemical series	halogens
Group, period, block	17,  2, p
Appearance	Yellowish brown gas
Standard atomic weight	18.9984032 (5) g·mol−1
Electron configuration	1s2 2s2 2p5
Electrons per shell	2, 7
Physical properties
Phase	gas
Density	(0 °C, 101.325 kPa) 1.7 g/L
Melting point	53.53  K (−219.62 ° C, −363.32 ° F)
Boiling point	85.03 K (−188.12 ° C, −306.62 ° F)
Critical point	144.13 K, 5.172 MPa
Heat of fusion	(F2) 0.510 kJ·mol−1
Heat of vaporization	(F2) 6.62 kJ·mol−1
Specific heat capacity	(25 °C) (F2) 31.304 J·mol−1·K−1
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 38 44 50 58 69 85
Atomic properties
Crystal structure	cubic
Oxidation states	−1 (strongly acidic oxide)
Electronegativity	3.98 (Pauling scale)
Ionization energies ( more)	1st: 1681.0 kJ·mol−1
	2nd: 3374.2 kJ·mol−1
	3rd: 6050.4 kJ·mol−1
Atomic radius	50 pm
Atomic radius (calc.)	42 pm
Covalent radius	71 pm (see covalent radius of fluorine)
Van der Waals radius	147 pm
Miscellaneous
Magnetic ordering	nonmagnetic
Thermal conductivity	(300 K) 27.7 m W·m−1·K−1
CAS registry number	7782-41-4
Selected isotopes
Main article: Isotopes of fluorine iso NA half-life DM DE ( MeV) DP 18F syn 109.77 min ε 1.656 18O 19F 100% 19F is stable with 10 neutrons
References

Fluorine (pronounced /ˈflʊəriːn/, Latin: fluere, meaning "to flow"), is the chemical element with the symbol F and atomic number 9. Atomic fluorine is univalent and is the most chemically reactive and electronegative of all the elements. In its elementally isolated (pure) form, fluorine is a poisonous, pale, yellowish brown gas, with chemical formula F₂. Like other halogens, molecular fluorine is highly dangerous; it causes severe chemical burns on contact with skin.

Fluorine's large electronegativity and small atomic radius gives it interesting bonding characteristics, particularly in conjunction with carbon, with which it forms stable compounds with a wide range of industrial applications. See covalent radius of fluorine, fluorocarbon, Perfluorocarbon, and fluoropolymer.

Notable characteristics

Pure fluorine (F₂) is a corrosive pale yellow or brown gas that is a powerful oxidizing agent. It is the most reactive and most electronegative of all the elements (4.0), and readily forms compounds with most other elements. It has an oxidation number -1, except when bonded to another fluorine in F₂ which gives it an oxidation number of 0. Fluorine even combines with argon, krypton, xenon, and radon. Even in dark, cool conditions, fluorine reacts explosively with hydrogen. The reaction with hydrogen occurs even at extremely low temperatures, using liquid hydrogen and solid fluorine. It is so reactive that metals, and even water, as well as other substances, burn with a bright flame in a jet of fluorine gas. It is far too reactive to be found in elemental form. In moist air it reacts with water to form also-dangerous hydrofluoric acid.

Fluorides are compounds that combine fluorine with some positively charged counterpart. They often consist of crystalline ionic salts. Fluorine compounds with metals are among the most stable of salts.

Hydrogen fluoride is a weak acid when dissolved in water. Consequently, fluorides of alkali metals produce basic solutions.

Applications

Chemical uses:

• Atomic fluorine and molecular fluorine are used for plasma etching in semiconductor manufacturing, flat panel display production and MEMS (microelectromechanical systems) fabrication. Xenon difluoride is also used for this last purpose.
• Hydrofluoric acid (chemical formula HF) is used to etch glass in light bulbs and other products.
• Fluorine is indirectly used in the production of low friction plastics such as Teflon (or polytetrafluoroethylene), and in halons such as freon.
• Along with some of its compounds, fluorine is used in the production of pure uranium from uranium hexafluoride and in the synthesis of numerous commercial fluorochemicals, including vitally important pharmaceuticals, agrochemical compounds, lubricants, and textiles.
• Fluorochlorohydrocarbons are used extensively in air conditioning and in refrigeration. Chlorofluorocarbons have been banned for these applications because they contribute to ozone destruction and the ozone hole. Interestingly, since it is chlorine and bromine radicals which harm the ozone layer, not fluorine, compounds which do not contain chlorine or bromine but contain only fluorine, carbon and hydrogen (called hydrofluorocarbons) are not on the EPA list of ozone-depleting substances, and have been widely used as replacements for the chlorine- and bromine-containing fluorocarbons. Hydrofluorocarbons do have a greenhouse effect, but a small one compared with carbon dioxide and methane.
• Sulfur hexafluoride is an extremely inert and nontoxic gas, very useful as an insulator in high-voltage electrical equipment. It does not occur in nature, so it is a useful tracer gas, though as an exceptionally potent greenhouse gas its use in unenclosed systems is inadvisable.
• Sodium hexafluoroaluminate ( cryolite), is used in the electrolysis of aluminium.
• In much higher concentrations, sodium fluoride has been used as an insecticide, especially against cockroaches.
• Fluorides have been used in the past to help molten metal flow, hence the name.
• Some researchers including US space scientists in the early 1960s have studied elemental fluorine gas as a possible rocket propellant due to its exceptionally high specific impulse. The experiments failed because fluorine proved difficult to handle, and its combustion products proved extremely toxic and corrosive.
• Compounds of fluorine such as fluoropolymers, potassium fluoride and cryolite are utilized in applications such as anti- reflective coatings and dichroic mirrors on account of their unusually low refractive index.

Dental and medical uses:

• Compounds of fluorine, including sodium fluoride (NaF), stannous fluoride (SnF₂) and sodium MFP, are used in toothpaste to prevent dental cavities. These compounds are also added to municipal water supplies, a process called water fluoridation, though a number of health concerns has sometimes led to controversy.
• Many important agents for general anesthesia such as sevoflurane, desflurane, and isoflurane are hydrofluorocarbon derivatives.
• The fluorinated antiinflammatories dexamethasone and triamcinolone are among the most potent of the synthetic corticosteroids class of drugs.
• Fludrocortisone ("Florinef") is one of the most common mineralocorticoids, a class of drugs which mimics the actions of aldosterone.
• Fluconazole is a triazole antifungal drug used in the treatment and prevention of superficial and systemic fungal infections.
• Fluoroquinolones are a family of broad-spectrum antibiotics.
• SSRI antidepressants, except in a few instances, are fluorinated molecules. These include citalopram, escitalopram oxalate, fluoxetine, fluvoxamine maleate, and paroxetine. A notable exception is sertraline. Because of the difficulty of biological systems in dealing with metabolism of fluorinated molecules, fluorinated antibiotics and antidepressants are among the major fluorinated organics found in treated city sewage and wastewater.

• ¹⁸F, a radioactive isotope that emits positrons, is often used in positron emission tomography, because its half-life of 110 minutes is long by the standards of positron-emitters.

Compounds

Fluorine forms a variety of very different compounds, owing to its small atomic size and covalent behaviour, and on the other hand, its oxidizing ability and extreme electronegativity. For example, hydrofluoric acid is extremely dangerous, while in synthetic drugs incorporating an aromatic ring (e.g. flumazenil), fluorine is used to prevent toxication or to delay metabolism.

The fluoride ion is basic, therefore hydrofluoric acid is a weak acid in water solution. However, water is not an inert solvent in this case: when less basic solvents such as anhydrous acetic acid are used, hydrofluoric acid is the strongest of the hydrohalogenic acids. Also, owing to the basicity of the fluoride ion, soluble fluorides give basic water solutions. The fluoride ion is a Lewis base, and has a high affinity to certain elements such as calcium and silicon. For example, deprotection of silicon protecting groups is achieved with a fluoride. The fluoride ion is poisonous.

Fluorine as a freely reacting oxidant gives the strongest oxidants known. Chlorine trifluoride, for example, can burn water and sand, both compounds of a weaker oxidant, oxygen.

Fluorine compounds involving noble gases were first synthesised by Neil Bartlett in 1962—xenon hexafluoroplatinate, XePtF₆, being the first. Fluorides of krypton and radon have also been prepared. Also argon fluorohydride has been prepared, although it is only stable at cryogenic temperatures.

The carbon-fluoride bond is covalent and very stable. The use of a fluorocarbon polymer, poly(tetrafluoroethene) or Teflon, is an example: it is thermostable and waterproof enough to be used in frying pans. Organofluorines may be safely used in applications such as drugs, without the risk of release of toxic fluoride. In synthetic drugs, toxication can be prevented. For example, an aromatic ring is useful but presents a safety problem: enzymes in the body metabolize some of them into poisonous epoxides. When the para position is substituted with fluorine, the aromatic ring is protected and epoxide is no longer produced.

The substitution of hydrogen for fluorine in organic compounds offers a very large number of compounds. An estimated fifth of pharmaceutical compounds and 30% of agrochemical compounds contain fluorine. The -CF₃ and -OCF₃ moieties provide further variation, and more recently the -SF₅ group.

Fluorite (CaF₂) crystals

This element is recovered from fluorite, cryolite, and fluorapatite.

For a list of fluorine compounds, see here.

History

Fluorine in the form of fluorspar (also called fluorite, calcium fluoride) was described in 1530 by Georgius Agricola for its use as a flux, which is a substance that is used to promote the fusion of metals or minerals. In 1670 Schwanhard found that glass was etched when it was exposed to fluorspar that was treated with acid. Carl Wilhelm Scheele and many later researchers, including Humphry Davy, Caroline Menard, Gay-Lussac, Antoine Lavoisier, and Louis Thenard all would experiment with hydrofluoric acid, easily obtained by treating calcium fluoride ( fluorspar) with concentrated sulfuric acid.

It was eventually realized that hydrofluoric acid contained a previously unknown element. This element was not isolated for many years after this, due to its extreme reactivity; fluorine can only be prepared from its compounds electrolytically, and then it immediately attacks any susceptible materials in the area. Finally, in 1886, elemental fluorine was isolated by Henri Moissan after almost 74 years of continuous effort by other chemists. The derivation of elemental fluorine from hydrofluoric acid is exceptionally dangerous, killing or blinding several scientists who attempted early experiments on this halogen. These men came to be referred to as "fluorine martyrs". For Moissan, it earned him the 1906 Nobel Prize in chemistry (Moissan himself lived to be 54, and it is not clear whether his fluorine work shortened his life).

The first large-scale production of fluorine was needed for the atomic bomb Manhattan project in World War II where the compound uranium hexafluoride (UF₆) was needed as a gaseous carrier of uranium to separate the ²³⁵U and ²³⁸U isotopes of uranium. Today both the gaseous diffusion process and the gas centrifuge process use gaseous UF₆ to produce enriched uranium for nuclear power applications. In the Manhattan Project, it was found that elemental fluorine was present whenever UF₆ was, due to the spontaneous decomposition of this compound into UF₄ and F₂. The corrosion problem due to the F₂ was eventually solved by electrolytically coating all UF₆ carrying piping with nickel metal, which resists fluorine's attack. Joints and flexible parts were made from teflon, then a very recently discovered fluorocarbon plastic which was not attacked by F₂.

Preparation

Fluorine cell room at F2 Chemicals Ltd, Preston, UK

Industrial fluorine production starts with fluorspar (CaF₂), which is heated with sulfuric acid (H₂SO₄) to produce anhydrous hydrogen fluoride (HF). The hydrogen fluoride is added to potassium fluoride (KF) to make potassium bifluoride (KHF₂). Electrolysis of potassium bifluoride produces fluorine gas at the anode, and hydrogen gas at the cathode. This is essentially the same method employed by Moissan in 1886; the use of potassium bifluoride rather than hydrogen fluoride itself aids electrolysis by greatly increasing the conductivity.

2 CaF₂ + H₂SO₄ → 2 HF + CaSO₄

HF + KF → KHF₂

2 KHF₂ → 2 HF + H₂ + F₂

In 1986, when preparing for a conference to celebrate the 100th anniversary of the discovery of fluorine, Karl Christe discovered a purely chemical preparation involving the reaction of solutions in anhydrous HF, K₂MnF₆, and SbF₅ at 150 °C:

K₂MnF₆ + 2SbF₅ → 2KSbF₆ + MnF₃ + ½F₂

Though not a practical synthesis, it demonstrates that electrolysis is not essential.

Safety

Elemental fluorine

Elemental fluorine (fluorine gas) is a highly toxic, corrosive oxidant, which can cause organic material, combustibles, or other flammable materials to ignite. It must be handled with great care and any contact with skin and eyes should be strictly avoided. Fluorine gas has a characteristic pungent odour that is detectable in concentrations as low as 20 ppb. As it is so reactive, all materials of construction must be carefully selected. All metal surfaces must be passivated before exposure to fluorine.

Fluoride ion

Fluoride ions are also highly toxic and must also be handled with great care and any contact with skin and eyes should be strictly avoided.

Hydrogen fluoride and hydrofluoric acid

Contact of exposed skin with hydrofluoric acid solutions poses one of the most extreme and insidious industrial threats—one which is exacerbated by the fact that hydrofluoric acid damages nerves in such a way as to make such burns initially painless. The HF molecule is a weaker acid which is significantly non-dissociated in water, and the intact molecule is capable of rapidly migrating through lipid layers of cells which would ordinarily stop an ion or partly ionized acid, and the burns it produces are typically deep. HF may react with calcium, permanently damaging the bone . More seriously, HF reaction with the body's calcium inside cells can cause cardiac arrhythmias, followed by cardiac arrest brought on by sudden chemical changes within the body ( hypocalcaemia). These cannot always be prevented with local or intravenous injection of calcium salts. Hydrofluoric acid spills over just 2.5% of the body's surface area (about 75 in² or 5 dm²), despite copious immediate washing, have been fatal. If the patient survives, hydrofluoric acid burns typically produce open wounds of an especially slow-healing nature.

Anhydrous hydrogen fluoride will rapidly form hydrofluoric acid on contact with moisture; its physiological effects are then the same.

Organic fluorides

Perfluorocarbons are generally inert and nontoxic, but there are many other fluorine compounds that have physiological effects, both good and bad. For example, fluoroacetic acid (one of the very few natural fluorine compounds) is very poisonous, while fluorouracil is an anti-cancer drug.