Protactinium

Protactinium is a rare, dense, silvery actinide metal: one of the most problematic elements to work with because it is intensely radioactive and highly toxic. It has virtually no practical uses and was the last naturally occurring element to be isolated in pure form (in 1934).

• Atomic Number9191 protons, 91 electrons
• Atomic Mass231.03588 u91× heavier than hydrogen
• State at Room TempSolidSolid
• Density15.37 g/cm³
• Melting / Boiling1571.8°C
• Discovered1913

What is Protactinium?

Protactinium is an actinide with 91 protons and the oxidation state +5 (unlike most actinides which prefer +3). Its name comes from the Greek protos (first) + aktinos (ray), the "parent of actinium", because Pa-231 decays to actinium-227. Lise Meitner and Otto Hahn discovered the most stable isotope (protactinium-231) in 1917.

Fact Protactinium is element 91, symbol Pa. As an actinide, it is part of the f-block of the periodic table: one of 15 radioactive elements from actinium (89) to lawrencium (103). All actinides are radioactive; none has a stable isotope. They are produced in nuclear reactors and by decay of heavier elements, and their chemistry has been studied using only microgram or nanogram quantities.

Where you find Protactinium

On Earth

Protactinium does not occur in significant natural abundance. It is found in trace quantities in uranium ores as a product of neutron capture and radioactive decay. World production is measured in micrograms or milligrams per year.

How we use Protactinium

Protactinium is an actinide with 91 protons and the oxidation state +5 (unlike most actinides which prefer +3)

How it was discovered

Protactinium is an actinide with 91 protons and the oxidation state +5 (unlike most actinides which prefer +3). Its name comes from the Greek protos (first) + aktinos (ray), the "parent of actinium", because Pa-231 decays to actinium-227. Lise Meitner and Otto Hahn discovered the most stable isotope (protactinium-231) in 1917.

Deeper dive: protactinium and the actinide series

The actinides (elements 89-103) form the lower of the two rows below the main body of the periodic table. They represent the filling of the 5f electron subshell. Unlike the lanthanides (the upper row), the actinides show greater variety in their chemistry because the 5f, 6d and 7s orbitals are close in energy. The early actinides, thorium through neptunium, can show many different oxidation states (e.g. uranium from +3 to +6). The heavier actinides increasingly resemble the lanthanides in preferring the +3 state.

All actinides beyond bismuth (83) are radioactive. The lightest, thorium, protactinium and uranium, have long enough half-lives to survive from the formation of the solar system. Neptunium and beyond are almost entirely synthetic, produced in nuclear reactors or accelerators. The transuranic elements were created at remarkable facilities including Oak Ridge National Laboratory, the Berkeley Cyclotron, the GSI in Darmstadt and JINR in Dubna.

Moving to 92 protons brings us to the next element on the periodic table.