Protactinium (Pa)
Element 91 · Actinide · atomic mass 231.04 u · f-block, period 7
Electron shells: 2, 8, 18, 32, 20, 9, 2
Protactinium is scarce, expensive and highly toxic, with essentially no use outside research. Extracting a tiny amount once required processing tonnes of uranium waste.
The UK Atomic Energy Authority once extracted 125 grams of it from 60 tonnes of waste — still one of the largest batches ever made.
- Atomic number
- 91
- Atomic mass
- 231.04 u
- Electron configuration
- [Rn] 7s² 5f² 6d¹
- Group · period · block
- f-block · 7 · f
- Oxidation states
- +4, +5
- Electronegativity
- 1.5 (Pauling)
- Melting point
- 1,572 °C
- Boiling point
- —
- Density
- 15.37 g/cm³
- State at 20 °C
- Solid
- Appearance
- Silvery-grey metal
- Discovered
- Otto Hahn, Lise Meitner, Frederick Soddy and John Cranston, 1913
91 protons, and 91 electrons when neutral
IUPAC standard atomic weight
Shown in the strip below the table
What Protactinium is used for
- Scientific research
- Ocean-sediment dating
Atomic structure of Protactinium
A neutral protactinium atom has 91 protons and 91 electrons. Its most common isotope carries about 140 neutrons, which is where the mass number 231 comes from. Change the neutron count and you have a different isotope of protactinium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 32, 20, 9, 2. In orbital notation that is [Rn] 7s² 5f² 6d¹. As a f-block element its valence count is variable, which is why it can form compounds in several oxidation states.
3D electron orbitals
See where Protactinium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Protactinium sits in the periodic table
Discovery and naming
Protactinium was discovered by Otto Hahn, Lise Meitner, Frederick Soddy and John Cranston in 1913. Its name comes from Greek protos + actinium, "before actinium", which is also why its symbol (Pa) does not match its English name.
Every isotope of protactinium is radioactive, so it has no standard atomic weight. Periodic tables show the mass number of its longest-lived isotope in square brackets instead.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Protactinium?
The standard atomic weight of Protactinium is 231.04 u, as published by IUPAC's Commission on Isotopic Abundances and Atomic Weights.
How many protons, neutrons and electrons does Protactinium have?
Protactinium has 91 protons, because the atomic number is the proton count and that is what makes an atom Protactinium at all. A neutral Protactinium atom has the same number of electrons, 91. The neutron count varies by isotope; the most common one has about 140, since 231 − 91 = 140.
What is the electron configuration of Protactinium?
The electron configuration of Protactinium is [Rn] 7s² 5f² 6d¹. Its electrons are arranged 2, 8, 18, 32, 20, 9, 2 across 7 shells.
Is Protactinium a metal?
Yes. Protactinium is classified as a actinide, on the metallic side of the table.
What group and period is Protactinium in?
Protactinium sits in the f-block strip below the main table, in period 7, among the actinides. Elements in that strip are not given a group number in the standard 18-column layout.
What is the charge of Protactinium?
Protactinium most commonly forms a 5+ ion. Like most transition metals it has several accessible oxidation states, so the charge depends on the compound. Across all its compounds it shows oxidation states of +4, +5.
What is the melting point of Protactinium?
Protactinium melts at 1,572 °C. That puts it in the solids at everyday temperatures.
What is Protactinium used for?
Scientific research, Ocean-sediment dating. Protactinium is scarce, expensive and highly toxic, with essentially no use outside research.
Who discovered Protactinium, and where does its name come from?
Protactinium (Pa) was discovered by Otto Hahn, Lise Meitner, Frederick Soddy and John Cranston in 1913. The name comes from Greek protos + actinium, "before actinium".
Is Protactinium radioactive?
Yes. Every isotope of Protactinium is radioactive — it has no stable form at all, which is why the periodic table shows its mass in square brackets rather than as a standard atomic weight.