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actinide series, mass number, periodic table, specific gravity, half-life

Actinium, symbol Ac, radioactive metallic element found in all uranium ores. The atomic number of actinium is 89; the element is in the actinide series of the periodic table.

Actinium was discovered in 1899 by the French chemist Andre Louis Debierne. The element’s name comes from the Greek word aktinos, which means “ray.” The element is found in uranium ores to the extent of 2 parts to every 10 billion parts of uranium. Actinium-227, a naturally-occurring isotope, is a member of the actinium series, called the actinium decay series, resulting from the radioactive decay of uranium-235. It has a half-life of 21.8 years. Another naturally-occurring isotope, actinium-228, is a member of the thorium series resulting from the decay of thorium-232. This isotope, known also as mesothorium-2, has a half-life of 6.13 hours. Isotopes ranging in mass number from 209 to 234 are known. Actinium melts at about 1050°C (about 1922°F), boils at about 3200°C (about 5800°F), and has a specific gravity of about 10.1.

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actinide series, mass number, periodic table, specific gravity, half-life, extent, element, ray, hours, member, years

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