Protactinium (Pa)

Protactinium properties

Atomic number

91

Atomic mass

231.04 u

Category

Actinide

Group

—

Period

7

Block

f-block

State at room temp.

Solid

Electronegativity

1.5

Melting point

1,841 K (1,567.8 °C)

Boiling point

4,300 K (4,026.8 °C)

Density

15.37 g/cm³

Valence electrons

2

Common charges

+4, +5

Oxidation states

+5, +4

Atomic radius

243 pm

Ionization energy

5.89 eV

Electron affinity

—

Radioactive

Yes, no stable isotopes

Discovered

1913

Protactinium electron configuration

The electron configuration of Protactinium (Pa) is [Rn] 5f2 6d1 7s2. Its 91 electrons are arranged in shells as 2, 8, 18, 32, 20, 9, 2.

Protons, neutrons and electrons in Protactinium

A neutral Protactinium atom has 91 protons and 91 electrons. Most Protactinium atoms have about 140 neutrons: the atomic mass rounded to 231, minus the atomic number 91.

Neutrons are approximate: atoms of the same element can have different numbers of neutrons (isotopes). Atomic number and mass

How many valence electrons does Protactinium have?

2

Protactinium has 2 valence electrons in its outermost shell. As an f-block element, its 4f or 5f electrons can also take part in bonding.

See every element: Periodic table with valence electrons

Protactinium orbital diagram

Each box is an orbital that holds up to two electrons with opposite spins (↑↓). The inner shells are the same as the noble gas in brackets. How to write electron configurations

Molar mass of Protactinium

231.04 g/mol

One mole of Protactinium atoms (6.022 × 10²³ atoms) has a mass of 231.04 grams. Use this value to convert between grams and moles.

Protactinium facts

• In 1961 British scientists extracted about 125 grams of protactinium from 60 metric tons of nuclear waste, long the world's main supply.
• Its longest-lived isotope, protactinium-231, has a half-life of about 33,000 years.
• Protactinium (Pa) has atomic number 91 and is a solid at room temperature. It was discovered in 1913.

Position in the periodic table

Protactinium is element 91, in period 7 of the periodic table. It is one of the Actinides in the f-block, shown in the rows below the main table.

All actinides

Uses of Protactinium

Protactinium has no practical uses outside scientific research because it is rare, highly radioactive and toxic. Geologists and oceanographers measure the ratio of protactinium-231 to thorium-230 in ocean sediments. This ratio helps date the sediments and shows how the strength of ocean currents has changed over the past tens of thousands of years.

Who discovered Protactinium?

Kasimir Fajans and Oswald Göhring identified a short-lived isotope of protactinium in Germany in 1913 and called it brevium; in 1917 and 1918 Otto Hahn and Lise Meitner in Berlin, and independently Frederick Soddy and John Cranston in Britain, found the longer-lived protactinium-231. The name means 'before actinium', because it decays into actinium.

Where does the name come from?

Protactinium comes from the Greek protos, 'first', combined with actinium, because it decays into actinium and so comes before it in the decay chain. The original form, protoactinium, was shortened in 1949.

Discovery timeline