Currently about 50% of the US population drinks water that has some fluorine added (in the form of sodium fluoride, NaF) to reduce tooth decay. We know now that fluorine combines with hydroxyapatite to make fluorapatite, which is more resistant to acidic decay than hydroxyapatite is. Tooth enamel consists mostly of a mineral called hydroxyapatite, which is composed of calcium, phosphorus, oxygen, and hydrogen. The protective effects of fluorine have a simple chemical explanation. Research continued, and in the 1930s, the US Public Health Service found that low levels of fluorine in water would provide the benefit of resisting decay without discoloring teeth. After years of study, excess fluorine compounds in the drinking water were discovered to be the cause of both these effects. In the early 1900s, a dentist in Colorado Springs, Colorado, noted that many people who lived in the area had brown-stained teeth that, while unsightly, were surprisingly resistant to decay. Unprotected by enamel, a tooth will start to decay, thus developing cavities and other dental problems. Acids found in some foods or made by bacteria that feed on food residues on our teeth are capable of dissolving enamel. It has to be hard so that our teeth can serve us for a lifetime of biting and chewing however, tough as it is, tooth enamel is susceptible to chemical attack. For definitions of ionic radius and further information, follow the hypertext link.The hardest material in the human body is tooth enamel. Further information is available in inorganic chemistry textbooks, usually at Level 1 or First Year University level. The terms low spin and high spin refer to the electronic configurations of particular geomtries of certain d-block metal ions. For electronic configurations, where it matters, the values given for octahedral species are low spin unless stated to be high spin. Size does depend upon geometry and environment. In this table, geometry refers to the arrangment of the ion's nearest neighbours. Hartree-Fock wave functions and radial expectation values: hydrogen to lawrencium, LA-3691, Los Alamos Scientific Laboratory, USA, 1968. The R max values for neutral gaseous element valence orbitals are abstracted from reference 1. Image showing periodicity of valence s-orbital radius for the chemical elements as size-coded balls on a periodic table grid. Table: valence shell orbital radii for potassium. Two values are given here, one is based upon calculations and the other upon observation - follow the appropriate link for further details. The problem is its meaning, which is clearly very different in different sources and books. The term "atomic radius" is not particularly helpful although its use is widespread. Follow the appropriate hyperlinks for definitions of each radius type. The size of neutral atoms depends upon the way in which the measurement is made and the environment. All values of radii are given in picometres (pm). Follow the appropriate hyperlinks for literature references and definitions of each type of radius. There are several other ways ways to define radius for atoms and ions. It is not always easy to make sensible comparisons between the elements however as some bonds are quite short because of multiple bonding (for instance the O=O distance in O 2 is short because of the the double bond connecting the two atoms. One measure of size is the element-element distance within the element.
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