A silver-white radioactive → chemical element; symbol Ac. The first member of the → actinide series of the → periodic table. → Atomic number 89; → atomic weight 227.0278; → melting point about 1,050°C; → boiling point 3,200°C ± 300°C; → specific gravity 10.07; → valence +3. It is found with uranium minerals in pitchblende. Its longest lived → isotope is 227Ac with a → half-life of 21.77 years.
From actin-, variant of actino-, from Gk. aktinos "ray, beam" + → -ium. The discovery of actinium is shared between two chemists who independently found the element. The earlier discovery was made by the French chemist André Debierne (1874-1949) in 1899 in pitchblende residues left after Pierre and Marie Curie had extracted → radium. The element was rediscovered in 1902 by the German chemist Friedrich Otto Giesel (1852-1927), who called it emanium.
A → radioactive → chemical element which is a malleable, shiny silver-gray metal; symbol Pa. → atomic number 91; → mass number of most stable isotope 231; → melting point greater than 1,600°C; → boiling point 4,026°C; calculated → specific gravity 15.37; → valence +4, +5. Protactinium has 24 → isotopes of which only three are found in nature. The most stable is protactinium-231 (→ half-life about 32,500 years); it is also the most common, being found in nature in all uranium ores in about the same abundance as radium. The element was discovered by Otto Hahn and Lise Meitner, who found one of its isotopes in 1917. It was isolated in 1934, by Aristid von Grosse.