Actinium (Ac), atomic number (Z) 89, has 26 isotopes with mass numbers 209–234 and half lives between 8 ns and 22 years. The electron configurations of the several ionic forms of this metal have been determined from the analysis of spectroscopic data obtained in connection with the measurement of the emission lines from neutral and charged gaseous atoms: M0(g):[Rn]6d17s2; M+(g)[Rn]7s2, M+2(g)[Rn]7s1, M+3(g)[Rn]. The crystal structure ionic radius for the +3 ion with a coordination number of six is 0.112 nm. The melting point of the metal is 1100 ± 50°C. Actinium is a group III element, with chemical and physical properties analogous to lanthanum.
The longest-lived isotope of the element, 227Ac, has a 22 year half-life. Consequently, actinium is considered a transient element; its only terrestrial occurrence arises from the decay of 235U. The silvery white metal is found in trace quantities in uranium minerals (ng/g of pitchblende). It was discovered in 1899 by A. Debierne and...
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Palmer, C.E.A. (1998). Actinium . In: Geochemistry. Encyclopedia of Earth Science. Springer, Dordrecht. https://doi.org/10.1007/1-4020-4496-8_3
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