Fluoride. Ions. Electronic configuration. And how water and toothpaste can protect our teeth from decay.
1-Minute NomNom
This NomNom is about the tooth, the whole tooth, and nothing but the tooth about fluorine.
Cities and companies sometimes choose to add fluoride into water and toothpaste. Fluoride, which is the anion (i.e. negatively charged ion) of fluorine and has the symbol F−, helps to protect teeth from decay.
But how does the fluoride do it? Does it not get quickly washed down our throats or rinsed out of our mouths, along with the water we drink or the toothpaste after we have brushed our teeth?
The whole truth lies in the reactivity of fluorine. Fluorine is one of the most reactive elements known to us: it “reacts with almost anything instantly“. When the fluorine ions, in the form of fluoride, meet even briefly with the calcium and phosphate compounds in our tooth’s enamel, they form calcium fluorophosphate – also known as fluorapatite:
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Ca10(PO4)6(OH)2 + 2F− → Ca10(PO4)6F2 + 2OH−
[calcium apatite + fluoride → calcium fluorophosphate (fluorapatite) + OH ions]
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which is readily “incorporated into the dental enamel“.
Tooth decay occurs when the bacteria, bits of food, sugar and saliva in our mouths combine to form plaque. Plaque is a sticky acidic film that can dissolve the hard outer coating of enamel on our tooth, eventually causing cavities that damage the tooth. Fluorapatite is not dissolved by these acids, and thus forms a coating that protects the enamel and tooth from further damage.
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A Little More Science
One of the ways cities add fluoride to water is to use sodium fluoride. Sodium fluoride (NaF) is a great example of ionic bonding, where elements combine by accepting or giving up electrons, thus becoming ions, and in this case giving us Na+ and the F− needed for the phenomenon above.
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Why do the elements do this? They do this to achieve a stable electronic configuration. This stability follows a chemical rule of thumb called the octet rule, where atoms in the first two horizontal periods of the Periodic Table tend to combine such that each atom has eight electrons in its outermost shells (see the animated diagram of NaF again). Each atom thus has the same electronic configuration as a noble gas (the first six elements in the last group 18 of the periodic table), which has low reactivity and is stable.
The fluoridation of water is considered by the United States Centres for Disease Control and Prevention as one of the “Ten Great Public Health Achievements in the 20th Century“. All it took was the simple fluorine ion, ionic bonding, and stable electronic configurations.
And that is the whole tooth.
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photos: in order – depositphotos/lunamarina; dteurope; kninwong; By Wdcf (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons; conceptw; dedukh
