Periodic table of the chemical elements
Hover an element, or Tab into the table and move with the arrow keys. Press an element to open its card.
Tap an element or slide your finger across the table
91 Pa,Protactinium
Actinides · Solid
Atomic mass 231.04
91
Average mass of an atom in unified atomic mass units (1 u is 1/12 of the mass of a carbon-12 atom). Accounts for the natural mix of the element’s isotopes.
Number of the vertical column, 1 to 18 in IUPAC numbering. Elements in a group share the same outer-shell layout, so their properties are alike.
Number of the horizontal row. Usually equal to the number of electron shells in the atom; palladium is the exception, with four shells in period 5.
The subshell (s, p, d or f) that receives the last electrons in the usual filling order; it divides the table into four blocks. Real atoms sometimes differ: lanthanum starts the f-block but has no f electrons.
How electrons are arranged in subshells. The bracket stands for the previous noble gas; the superscript is the number of electrons in a subshell.
Number of electrons in each shell, from the nucleus outwards.
Temperature at which the solid turns liquid at standard pressure. Helium, carbon and arsenic do not melt at normal pressure: helium needs about 25 atm, carbon and arsenic turn straight into vapour.
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
How strongly an atom pulls electrons towards itself in a chemical bond. On the Pauling scale it runs from 0.79 for caesium to 3.98 for fluorine.
Energy needed to remove the most loosely bound electron from an atom. The lower it is, the more easily the element gives up an electron, which is typical of metals.
Energy released when a neutral atom gains an electron. Highest for the halogens. Noble gases, nitrogen and some metals form no stable negative ion.
Half the length of a single covalent bond between two atoms of the element. It shows the size of an atom inside a molecule or crystal: 76 pm for carbon, 132 pm for iron.
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Protactinium (Pa) is a chemical element with atomic number 91. It sits in period 7 of the periodic table, in the f-block. It is an actinide. At room temperature it is a solid. Its standard atomic weight is 231.04.
The name means ‘before actinium’, from the Greek protos, ‘first’: protactinium decays into actinium. In 1913 Kasimir Fajans and Oswald Göhring found a very short-lived form of element 91 and called it brevium, ‘brief’. In 1918 a long-lived form was found independently by Otto Hahn and Lise Meitner in Germany and by Frederick Soddy and John Cranston in Britain, and the element got its present name.
Protactinium occurs in uranium ores in very small amounts. In 1961 British scientists processed about 55 tonnes of ore to obtain 125 grams of it.
Protactinium is rare, highly radioactive and poisonous, and it has no uses outside scientific research.