Protactinium – facts and properties
- Symbol
- Pa
- Atomic number
- 91
- Atomic mass
- 231.036 u
- Period
- 7
- Category
- actinide
- Electron configuration
- [Rn] 5f² 6d¹ 7s²
- Electronegativity
- 1.5
- Atomic radius
- 180 pm
- Valence electrons
- 5
- State at standard conditions
- solid
- Melting point
- 1572 °C
- Density
- 15.37 g/cm³
- Discovered
- 1913
Protactinium is a dense, silvery-grey, radioactive metal with atomic number 91. It is one of the rarest elements in nature and very dangerous inside the body. Its name means before actinium, because its main isotope decays into actinium. Kasimir Fajans and Oswald Göhring discovered the element in 1913, and Otto Hahn and Lise Meitner identified it a few years later. It has hardly any applications, yet researchers use it to date deposits on the sea floor.
Position in the periodic table
Occurrence and extraction
Protactinium occurs in nature only in traces. It forms in the uranium-235 decay series, and its main isotope, protactinium-231, has a half-life of more than thirty thousand years. Uranium ore holds less than a gram of it per tonne. Extracting it from ores is hardly worthwhile.
The discovery came in two steps. In 1913 Fajans and Göhring found a very short-lived isotope with a half-life of just over a minute and called the element brevium, from the Latin brevis, short. The long-lived protactinium-231 was identified only in 1917 and 1918 by Otto Hahn and Lise Meitner in Berlin, and independently by Frederick Soddy and John Cranston in Britain. Hahn and Meitner chose the name protactinium, from the Greek protos, first. It says that the element comes before actinium in decay: protactinium-231 turns into actinium-227.
The pure metal was made only decades later. As late as the 1960s, many tonnes of residues from uranium processing yielded just over a hundred grams. Anyone who needs protactinium obtains very small amounts from specialised facilities.
Properties and reactions
Protactinium is in period 7 and is the third actinoid, in the actinoid row of the table between thorium on its left and uranium on its right. Its electrons are arranged as [Rn] 5f² 6d¹ 7s². Added to this are an electronegativity of 1.5 and an atomic radius of 180 pm. The density is 15.37 g/cm³ and the melting point 1572 °C. A boiling point has not been determined so far. Its atomic mass is given as 231.036 u, the mass of protactinium-231.
Protactinium is a shiny, silvery-grey metal that keeps its lustre in air for some time. Chemically it mostly shows the oxidation number plus five, less often plus four. A peculiarity is its behaviour in solution: protactinium tends to settle out as fine particles and to cling to the walls of vessels. This makes chemical study difficult and puts it among the elements hardest to handle.
Protactinium-231 is an alpha emitter, and its decay products, including actinium-227, radiate as well. If protactinium enters the body, it settles in bone, liver and kidneys and irradiates the tissue for a long time. Because of its radiation it counts as highly toxic, even though little is known about its chemical toxicity. Only specialists work with it, in tightly sealed glove boxes.
Research and synthesis
There is no use outside research. The tiny amounts and the danger make the element uninteresting for technology. Protactinium-231 is prepared only for measurements and chemical studies.
It matters in the earth sciences. In seawater, protactinium-231 forms from dissolved uranium-235, and thorium-230 from uranium-234. Thorium sticks more readily to sinking particles than protactinium does. The ratio of the two isotopes in ocean sediments therefore reveals how strongly currents carried protactinium away. From it researchers infer how vigorous the overturning of the Atlantic was in past ice ages and warm periods. The decay of excess protactinium-231 also lets researchers estimate the age of young sediment layers. That helps in understanding the climate of the past.
The element also appears in nuclear technology. In thorium reactors, protactinium-233 forms first and decays with a half-life of about 27 days to fissile uranium-233. It is thus an intermediate of this fuel cycle.
Quick question
What is the atomic number of protactinium (Pa)?
FAQ
Where does the name protactinium come from?
Hahn and Meitner formed it from the Greek protos, first, and actinium. Protactinium-231 decays to actinium-227, so it stands before it in the decay series. The name recalls that relationship.
Is protactinium dangerous?
Yes. It is an alpha emitter that settles in bone, liver and kidneys if taken into the body. Only specialists work with the few samples that exist, under strict protective measures.
What is protactinium used for?
Practically only for research. The ratio of protactinium-231 to thorium-230 in ocean sediments shows how strong the ocean currents once were. There is no technical application.