An Etymological Dictionary of Astronomy and Astrophysics

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.

See also: Protactinium, literally “parent of actinium,” because actinium is a decay product of protactinium, from Gk. protos “first,” → proto-,

• → actinium.