Actinium longest lived isotope

Although in a strict sense not a member of the actinide series, actinium is included here for completeness as lanthanum is frequently included in the lanthanide series. The longest lived isotope of actinium is with a half-life of 21.8 years. It occurs in nature from the decay... 

The failure to discover francium earlier is easy to understand when it is remembered that the half-life of the longest lived isotope is only 21 minutes. This gives the element the distinction of being the most unstable to radioactive disintegration of all elements up to number 98 (38). It is also noteworthy that this is the only element in the group discussed in this chapter which was not discovered by artificial preparation in the laboratory. Nevertheless, the rarity of actinium in nature is so great that this element is best prepared artificially when its properties or those of its daughter elements are to be studied. 

Pure and Applied Chemistry (lUPAC), the governing body that officially confirms and names any new elements. The lUPAC lists the atomic weight in brackets when an element does not have any stable nuclides. For example, the atomic weight of actinium is listed as [227]. This represents the mass of its longest-lived isotope. 

The last discovery of an alkali metal occurred almost 80 years later. In 1939, Parisian physicist Marguerite Perey (1909-75) observed an unusual rate of radioactive decay in a sample of a salt of actinium (element 89). She managed to isolate the new element, showed that it was an alkali metal, and named it francium in honor of her native country, France. Because francium s longest-lived isotope has a half-life of only 21 minutes, francium is the rarest element below element 98 in the periodic table, which explains why francium was discovered much later than the other radioactive elements in that part of the table. 

Table 24.4 Half-lives and decay modes of the longest-lived isotopes of actinium and the actinoids.

The 4 + 3 chain is the only one that (barely) includes every element from uranium to thallium. Although included, francium and especially astatine are very minor components. The parent, 704 X 10 year has a half-life considerably smaller than the 4,500 x 10 year age of the Earth. It is long enough for significant amounts (0.720% of the atoms in natural uranium) to remain. The chain includes by far the longest-lived isotopes of protactinium (32.8 x 10 year Pa) and actinium (21.77 year Ac). 

The actinium decay series consists of a group of nuclides whose mass number divided by 4 leaves a remainder of 3 (the 4n + 3 series). This series begins with the uranium isotope which has a half-life of 7.04 X 10 y and a specific activity of 8 X 10 MBq/kg. The stable end product of the series is ° Pb, which is formed after 7 a- and 4 /3-decays. The actinium series includes the most important isotopes of the elements protactinium, actinium, ftancium, and astatine. Inasmuch as U is a conqx>nmt of natural uranium, these elem ts can be isolated in the processing of uranium minerals. The longest-lived protactinium isotope, Pa (ti 3.28 X 10 y) has been isolated on the 100 g scale, and is the main isotope for the study of protactinium chemistry. Ac (t 21.8 y) is the longest-lived actinium isotope. 

There is only one beta decay event per 5 000 alpha decays of polonium-218. Things are even sadder for polonium-216 (1 per 7 000) and polonium-215 (1 per 200 000). The situation speaks for itself. The amount of natural francium on Earth is larger. It is produced by the longest-lived actinium isotope Ac (a half-life of 21 years) and its content is, of course, much higher than that of the extremely rare polonium isotopes capable of producing astatine. 

Actinium was discovered by A. Debierne in 1899. Its name is derived from the Greek word for beam or ray, referring to its radioactivity. The natural occurrence of the longest lived actinium isotope Ac, with a half-life of 21.77 years, is entirely dependent on that of its primordial ancestor, U. The natural abundance of Ac is estimated to be 5.7 10 ppm. The most concentrated actinium sample ever prepared from a natural raw material consisted of about 7 fig of Ac in less than 0.1 mg of... 

All isotopes of actinium are radioactive and exist in aqueous solution only in the trivalent state. All of the isotopes of actinium are relatively short lived, with the longest half-life being 21.6 years for Ac. Consequently, it is difficult to measure the stability of its aqueous species or the solubility of its phases. 

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