Protactinium, element 91, was discovered in 1918 by German chemist Otto Hahn and physicist Lise Meitner. They initially dubbed the element proactinium because the element’s radioactive decay converts protactinium into actinium, therefore it “comes before” (pro-) actinium. Shortly after this initial naming, the pair inserted the t to make the name easier to pronounce and it became protactinium.

In English use, the name was sometimes modified to protoactinium, using the prefix proto- instead of pro-, probably out of confusion over the extra t. Both versions of the name were in use in the first half of the 20th century, as evidenced by the following citations, the first from the 1918 Journal of the Chemical Society:

Assuming that 8% of the uranium atoms disintegrating produce “protoactinium,” the quantity in the 73 mg. is that in equilibrium with 86 grams of uranium.

And this from Chemical Abstracts of 1919:

Protactinium is one of the 5 new radioactive elements occupying a place in the periodic table hitherto vacant.

The International Union of Pure and Applied Chemistry (IUPAC) declared protactinium to be the standard spelling in 1949.1

The chemical symbol of protactinium is Pa.

1Oxford English Dictionary, protactinium, 3rd Edition, December 2007, Oxford University Press, accessed 17 November 2009,

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