Flourine

There is much debate about both the science and the ethics of adding fluorine to drinking water supplied to people's homes. For more about this see Do you want fluorine added to your drinking water? and Legality of Fluoridation of Tap Water in Southampton (UK), 19 Jan 2011.

Flourine, which should be spelt "Fluorine" is an element in Group 7 of the Periodic Table. Members Group 7 are also known as Halogens.
(The other halogens - that is, members of the same group are Chlorine, Bromine, Iodine and Astatine. These have some properties in common with fluorine and there are also certain "trends" or patterns in the variation of other properties across this group of elements.)

Each fluorine atom consists of 9 protons, 9 electrons and usually* 10 neutrons.
*The most common isotope of fluorine is fluorine-19.

The name 'fluorine' is derived from the Latin noun 'fluo' which means a stream or flow of water.

History and Discovery of Fluorine

The mineral fluorite, which is a natural form of calcium fluoride (and therefore contains the element fluorine), was discussed in a printed article written in 1530 by the German scholar and scientist Georgius Agricola. He called it 'fluorspar' and commented on it's usefulness as a flux, which is a chemical cleaning agent used to prepare metal surfaces by removing oxidation from them before they are joined together by soldering, brazing or welding. The earliest known and documented preparation of hydrofluoric acid (HF) was by an unknown English glassworker in 1720. Then in 1771, Swedish chemist Carl Wilhelm Scheele obtained impure hydrofluoric acid by heating fluorite with sulfuric acid. Almost anhydrous acid was reported in 1809. In 1811 French physicist André-Marie Ampère suggested that it was a compound of hydrogen and 'an unknown element, analogous to chlorine', for which he proposed the name fluorine. Due to the extreme reactivity of fluorine (meaning that it has a high tendency to form compounds with other elements incl. e.g. by chemical reactions that etch any glass with which it comes into contact), the element fluorine was not isolated for another 74 years. In 1886 the French chemist Ferdinand Frederick Henri Moissan (who is often referred to as simply 'Henri Moissan') reported success in isolating elemental fluorine gas (F₂), for which he received the Nobel Prize for Chemistry in 1906. Sadly other chemists had been blinded or killed in their attempts to isolate this highly reactive and dangerous element.

Fluorine at standard temperature and pressure ('standard conditions')

Elemental fluorine is a pale yellow diatomic gas (F₂) that is highly toxic, corrosive, oxidising, and can cause ignition of some organic materials.

Fluorine gas is not found in nature but fluorine is present in certain naturally-occuring minerals, e.g. Cryolite (Na₃AlF₆), Fluorite (CaF₂), and Fluorapatite 3Ca₃(PO₄)₂.Ca(ClF). The reason fluorine is not found in its elemental form in nature is its extreme reactivity: Fluorine is the most reactive of all the elements.

Health effects of Fluorine

Humans can detect the smell of fluorine gas at extremely low concentrations - from only one part in 20 billions.

Not only is elemental fluorine highly toxic but soluble fluorides are also moderately toxic.

Fluoride in Toothpaste and Dentistry

Some inorganic fluoride compounds, e.g. sodium fluoride (NaF), tin(II) fluoride (SnF₂) and sodium monofluorophosphate (Na₂PO₃F) are used in toothpastes to prevent dental cavities. These are said to protect tooth enamel from attack by bacteria that cause dental caries (cavities).

Fluoride Poisoning

There have been some cases of fluoride poisoning due to accidental ingestion of insecticides containing inorganic fluoride or rodenticides containing sodium fluoroacetate. There have also been reports of fluoride poisonings due to the ingestion of fluoride-containing toothpaste, and concerns expressed about the possible malfunction of water fluoridation equipment.

The fluoride ion (F^-) is readily absorbed by the stomach, intestines and excreted through urine.
Ingested fluoride - which means fluoride that enters the body via the mouth - initially acts locally on the intestinal mucosa, where it forms hydrofluoric acid in the stomach. Then it binds calcium and interferes with the action of some enzymes that normally aid the digestive process.

Other Health and Medical Uses of fluorine-based compounds

Sodium monofluorophosphate (Na₂PO₃F), which is mentioned above because it is included in some toothpastes, is also used in some treatments for the osteoporosis.

Compounds of Fluorine (in general)

Fluorine is highly reactive and combines with almost all elements, in may cases forming fluorides.

In many cases alkali metal fluorides resemble the corresponding chloride in terms of structure and solubilities.

The fluoride ion (F^-) is basic hence hydrofluoric acid (HF) is a weak acid in water solution. It is also highly corrosive and known to "attack" (react with surfaces of) glass. The fluorides of alkali metals produce basic solutions. The fluoride ion is a Lewis base, and has a high affinity to some elements such as calcium and silicon.

Noble gas compounds

Fluorine is unusual in its ability to react and form stable compounds with the (mostly unreactive) noble gases. Examples of such compounds include xenon difluoride, xenon tetrafluoride, xenon hexafluoride and xenon oxytetrafluoride, also compounds of krypton, radon, argon and helium. Even neon forms a very short-lived fluoride.

Organofluorine compounds

Organofluorine compounds are chemical compounds that include at least one carbon–fluorine covalent bond. This is the strongest covalent bond in organic chemistry and is very stable. Fluorine replaces hydrogen in hydrocarbons even at room temperature without significant change of molecular size so the range of organofluorine compounds is considerable. The most industrially important compounds of fluorine include Teflon and chlorofluorocarbons.

Isotopes of Fluorine

Naturally occuring fluorine is monoisotopic, which means that there is only one isotope of natural fluorine. That isotope is fluorine-19. As the atomic number of fluorine is 9, there are 9 protons and 19 - 9 = 10 neutrons in each atom of fluorine-19.

Compounds of the radioactive isotope fluorine-18 emit positrons and are sometimes used in positron emission tomography ('PET scanning'), because its half-life of about 110 minutes is long by the standards of positron-emitters. Examples of use include assessment of glucose metabolism in the brain and for imaging tumors in oncology. Certain compounds of fluorine-18 may be used for diagnosis, staging, and monitoring treatment of some cancers such as Hodgkin's disease, lung cancer, breast cancer, and others.

Uses of Elemental Fluorine

• Plasma Etching in semiconductor manufacturing.
• Flat panel display production and microelectromechanical systems fabrication

Uses of Fluorine (including fluorine combined with other elements to form compounds of fluorine)

• Organic Chemistry: Fluorine is used in the synthesis of organic fluorine compounds (for definitions see 'What is Organic Chemistry ?' and 'What is a compound ?').
• Haloalkanes such as chlorofluorocarbons (CFCs) used to be produced from fluorine-based compounds for extensive use in air conditioning and in refrigeration applications. However, many CFCs are no-longer allowed to be used for those purposes due to concerns about their contribution to the destruction of the ozone layer around the earth.

There are many uses of specific well-known compounds of fluorine.

Examples include:

• Freon (also known as dichlorodifluoromethane, CCl₂F₂) is used as a refrigerant.
• Hydrofluoric acid and soume fluoride-containing salts are used to etch glass.
• Sulphur Hexafluoride is used as an inert dielectric medium in high voltage switching.
• Teflon was invented in 1938 and is in widespread use as a non-stick surface for domestic kitchen equipment such as pans.
• Uranium Hexafluoride (UF₆) is used for the separation of the isotopes of uranium in centrifuges. This is convenient due to the monoisotopic occurrence of fluorine - hence uranium hexafluoride molecules differ in mass only due to differences in the uranium atom isotope mass differences, i.e. uranium-235 and uranium-238.
• Xenon Difluoride (together with elemental fluorine) it is used for fabricating microelectromechanical systems.