Estimates vary from 18 to 25 ppm, which is comparable to the abundance of cobalt (20–30 ppm). The primary decay products at atomic weights below 45Sc are calcium isotopes and the primary products from higher atomic weights are titanium isotopes. The primary decay mode at masses lower than the only stable isotope, 45Sc, is electron capture, and the primary mode at masses above it is beta emission. The known isotopes of scandium range from 36Sc to 60Sc. This element also has five nuclear isomers, with the most stable being 44m2Sc ( t 1/2 = 58.6 h). All of the remaining radioactive isotopes have half-lives less than 4 hours, and the majority of these have half-lives less than 2 minutes. Twenty-five radioisotopes have been characterized with the most stable being 46Sc, which has a half-life of 83.8 days 47Sc, 3.35 days the positron emitter 44Sc, 4 hours and 48Sc, 43.7 hours. In nature, scandium is found exclusively as the isotope 45Sc, which has a nuclear spin of 7/2 this is its only stable isotope. Scandium turnings ignite in the air with a brilliant yellow flame to form scandium oxide. It does not react with a 1:1 mixture of nitric acid ( HNO 3) and 48.0% hydrofluoric acid ( HF), possibly due to the formation of an impermeable passive layer. It is susceptible to weathering and dissolves slowly in most dilute acids. It develops a slightly yellowish or pinkish cast when oxidized by air. Scandium is a soft metal with a silvery appearance. In the chemical compounds of the elements in group 3, the predominant oxidation state is +3. A diagonal relationship exists between the behavior of magnesium and scandium, just as there is between beryllium and aluminium. The properties of scandium compounds are intermediate between those of aluminium and yttrium. The global trade of scandium oxide is 15–20 tonnes per year. To this day, its use in such alloys remains its only major application. Because of the low availability and difficulties in the preparation of metallic scandium, which was first done in 1937, applications for scandium were not developed until the 1970s, when the positive effects of scandium on aluminium alloys were discovered. Scandium is present in most of the deposits of rare-earth and uranium compounds, but it is extracted from these ores in only a few mines worldwide. It was discovered in 1879 by spectral analysis of the minerals euxenite and gadolinite from Scandinavia. Historically, it has been classified as a rare-earth element, together with yttrium and the lanthanides. It is a silvery-white metallic d-block element. Scandium is a chemical element with the symbol Sc and atomic number 21.
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