Protactinium is a radioactive metal found in small amounts in uranium ores and in nuclear-reactor materials. It is studied mainly in research. Its assigned standard atomic weight illustrates that radioactivity alone does not determine whether CIAAW can provide a standard atomic-weight value.
FROM ELEMENT TO APPLICATION
AN ELEMENT IN CONTEXT
Why is a sediment isotope ratio not a direct current-speed meter?
Protactinium-231 and thorium-230 in marine sediments
FOLLOW THIS RELATIONSHIP
Identify two isotope records
Marine sediments can preserve protactinium-231 and thorium-230 evidence. Their ratio combines information about two elements rather than measuring the motion of a water parcel directly.
Explore the model (optional)
PaFORM → INTERACTION → CONSEQUENCE
Follow the explanation in either direction. These are authored steps, not a physical clock.
Optional: check your understanding
USE THE EXPLANATION
What should accompany a circulation interpretation of the ratio?
Atomic identity, reference properties and structure models
INTERACTIVE 3D91 visible sites
An electron censusDrag to rotate · scroll or pinch to zoom
[Rn] 5f2 6d1 7s2. Grouping electrons by principal shell connects the neutral-atom configuration to periodic-table patterns.
What this representation means
A three-dimensional counting diagram. Dots are count markers, not measured electron positions; spherical placement and spacing are arbitrary. There are no electron trajectories here. Orbitals describe quantum states.
Kasimir Fajans and Oswald Göhring identified a short-lived of element 91 in 1913. Work by Lise Meitner and Otto Hahn subsequently established the longer-lived isotope associated with the name protactinium. The name refers to its position before actinium in a sequence. Different isotopes explain why discovery accounts can contain more than one date without describing different chemical elements.
1913
First isotope identified
Fajans and Göhring identify a short-lived form of element 91.
1918
Longer-lived isotope reported
Meitner and Hahn report work establishing the isotope now called protactinium-231.
Protactinium occurs in minute amounts in uranium as part of natural radioactive decay series. In seawater, uranium decay supplies protactinium that can attach to particles and enter sediments. This dispersed occurrence creates useful geochemical signals, but it does not make protactinium an industrial raw material.
Protactinium sits where the roles of 5f and 6d are changing across the early actinides. Its common +5 chemistry does not simply duplicate uranium chemistry. Researchers investigate its bonds and chemical forms because small differences in electronic structure can produce substantial differences in how an actinide interacts with surrounding molecules.
Each section links its supporting references. Descriptions and diagrams were authored for this atlas. Source checks completed. Source checks dated 2026-09-06.
CITE THIS PAGE
Family labels and the group-3 layout are teaching conventions. A table position does not establish an unknown bulk property. Electronic configurations refer to neutral atoms; ions and material environments require their own account.
Read each reference property together with its units, conditions and the reference information below.
Not yet recorded: electron affinity, second ionization energy, ground-state term symbol, van der waals radius, thermal conductivity, specific heat capacity, electrical resistivity, crystal structure, lattice parameter a, space group, abundance in seawater, abundance in the universe, annual production, supply-risk listing, hazard classification, biological role, cas registry number, pubchem cid, wikidata item. These families are declared and awaiting an ingestion run against the source each names; they are listed here rather than shown as empty rows.
Physical
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Phase at 20 °C
solid
293.15 K, 101.325 kPa
Melting point in K
1841 K
at standard pressure unless noted
Boiling point in K
4300 K
101.325 kPa
Density in g/cm³
15.37 g/cm³
solids and liquids at 293 K; gases at 273 K and 101.325 kPa
Atomic and electronic
Intended reference: Pauling electronegativity scale · Widely tabulated; original in The Nature of the Chemical Bond
Electronegativity in Pauling
1.5 Pauling
Pauling scale
First ionization energy in kJ/mol
568 kJ/mol
gas-phase neutral atom, ground state
Covalent radius in pm
200 pm
single-bond covalent radius
Common oxidation states
+5
states met in ordinary compounds; not exhaustive
Occurrence and identity
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Abundance in the crust
Not recorded
upper continental crust, mass fraction
Has stable isotopes
Yesderived
Discovery year in CE
1913 CEderived
WHERE IT SITS AMONG ITS NEIGHBORS
Electronegativity across period 7
Pa 1.5 Pauling · Protactinium
Electronegativity across period 7. Exact values.
Ra
0.9
Ac
1.1
Th
1.3
Pa
1.5
U
1.38
Np
1.36
Pu
1.28
Am
1.13
Cm
1.28
Bk
1.3
Cf
1.3
Es
1.3
Fm
1.3
Md
1.3
No
1.3
Lr
1.3
The trend uses the same reference-property values as the periodic table. Elements with no recorded value are left out of the line rather than drawn at zero.
