Fluorine

2007 Schools Wikipedia Selection. Related subjects: Chemical elements

Distinguish from fluorene and fluorone.

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
Atomic mass	18.9984032 (5) g/mol
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
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 mW·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% F is stable with 10 neutrons
References

Fluorine ( IPA: /ˈflʊərɪːn, -ɔːrɪːn/, 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, yellow- green gas, with chemical formula F₂. Like other halogens, molecular fluorine is highly dangerous; it causes severe chemical burns on contact with skin.

Fluorine's relatively large electronegativity and small atomic radius gives it interesting bonding characteristics, particularly in conjunction with carbon. See covalent radius of fluorine.

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 electronegative of all the elements (4.0), and readily forms compounds with most other elements. Fluorine even combines with the noble gases, krypton, xenon, and radon. Even in dark, cool conditions, fluorine reacts explosively with hydrogen. It is so reactive that glass, 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 and has such an affinity for most elements, including silicon, that it can neither be prepared nor be kept in ordinary glass vessels. Instead, it must be kept in specialized quartz tubes lined with a very thin layer of fluorocarbons. In moist air it reacts with water to form also-dangerous hydrofluoric acid.

In aqueous solution, fluorine commonly occurs as the fluoride ion F^-, although HF is such a weak acid that substantial amounts of it are present in any water solution of fluoride at near neutral pH. Other forms are fluoro- complexes, such as [FeF₄]^-, or H₂F⁺.

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.

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. Karl Scheele and many later researchers, including Humphry Davy, 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. It was an effort which cost several researchers their health or even their lives. 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 fluorine-containing plastic which was not attacked by F₂.

Safety

Both elemental fluorine and fluoride ions are highly toxic and must be handled with great care and any contact with skin and eyes should be strictly avoided. When it is a free element, fluorine has a characteristic pungent odour that is detectable in concentrations as low as 20 nL/L. Its MAC-value is 1 1 µL/L. All equipment must be passivated before exposure to fluorine. For more information consult an MSDS.

Contact of exposed skin with HF solutions posses one of the most extreme and insidious industrial threats-- one which is exacerbated by the fact that HF damages nerves in such a way as to make such burns initially painless. The HF molecule is capable of rapidly migrating through lipid layers of cells which would ordinarily stop an ionized acid, and the burns are typically deep. HF may react with calcium, permanently damaging the bone. More seriously, reaction with the body's calcium can cause cardiac arrhythmias, followed by cardiac arrest brought on by sudden chemical changes within the body. These cannot always be prevented with local or intravenous injection of calcium salts. HF spills over just 2.5% of the body's surface area, despite copious immediate washing, have been fatal (this corresponds with an area of about 9 in² or 23 cm ²). If the patient survives, HF burns typically produce open wounds of an especially slow-healing nature.

Elemental fluorine is a powerful oxidizer which can cause organic material, combustibles, or other flammable materials to ignite.

Preparation

Elemental fluorine is prepared industrially by Moissan's original process: electrolysis of anhydrous HF in which KHF₂ has been dissolved to provide enough ions for conduction to take place.

In 1986, preparing for a conference to celebrate the 100th anniversary of the discovery of fluorine, Karl Christe discovered a purely-chemical preparation by reacting together at 150 °C solutions in anhydrous HF of K₂MnF₆ and of SbF₅. The reaction is: 2K₂MnF₆ + 4SbF₅ → 4KSbF₆ + MnF₂ + F₂ This is not a practical synthesis, but demonstrates that electrolysis is not essential.

Compounds

Fluorine can often be substituted for hydrogen when it occurs in organic compounds. Through this mechanism, fluorine can have a very large number of compounds. 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.

Fluorite (CaF₂) crystals

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

For a list of fluorine compounds, see here.

Applications

Atomic fluorine and molecular fluorine are used for plasma etching in semiconductor manufacturing, flat panel display production and MEMS fabrication. Other uses:

• 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, 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 have chlorine or bromine and contain only fluorine, carbon and hydrogen (called hydrofluorocarbons), are not on the E.P.A. 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, and a member of a class of compounds that are potent greenhouse gases.
• Many important agents for general anesthesia such as sevoflurane, desflurane, and isoflurane are hydrofluorocarbon derivatives.
• 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.
• Sodium hexafluoroaluminate ( cryolite), is used in the electrolysis of aluminium.
• 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 combination of health concerns and urban legends has sometimes led to controversy.
• 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.
• 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.

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 of course proved extremely toxic and corrosive.