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Protactinium

Element 91 · Actinide

Specimen

Atomic number
91
Atomic mass
231.04
State at room temperature
Solid
Melting point
1,572 °C
Boiling point
Unknown
Density
15.37 g/cm³
Discovered
1913

Atomic mass from IUPAC.1 Other physical data from PubChem.2

Can I eat it?

In food and water
Radioactive. All protactinium is radioactive. Uranium ores contain small amounts of it. Its radiation is the main danger. 3

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The atom

Pa
ShellElectrons
1 (K) 2
2 (L) 8
3 (M) 18
4 (N) 32
5 (O) 20
6 (P) 9
7 (Q) outermost 2
Total91

Configuration [Rn] 5f² 6d¹ 7s²

5f ↑ ↑
6d ↑
7s ↑ ↓
A shell diagram: each ring is an electron shell and each dot is one electron. The nucleus holds 91 protons. The outermost shell is shown in red. Electrons do not travel on rings: they fill orbitals, regions where an electron is likely to be found. The boxes are those orbitals for the electrons outside the Rn core, with one arrow per electron.

Related elements

Actinides
Ac Actinium Th Thorium U Uranium Np Neptunium Pu Plutonium Am Americium Cm Curium Bk Berkelium Cf Californium Es Einsteinium Fm Fermium Md Mendelevium No Nobelium Lr Lawrencium

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Sources

  1. 1

    IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), 2024. Abridged Standard Atomic Weights 2024 .

    The atomic mass shown for the 84 elements that have a standard atomic weight.

  2. 2

    National Center for Biotechnology Information (PubChem), 2026. Periodic Table of Elements .

    Electron configuration, melting and boiling points, density and discovery year for all 118 elements.

  3. 3

    Royal Society of Chemistry, Periodic Table, 2026. Protactinium - Element information, properties and uses .

    Reference page; its Biological role and Natural abundance sections are quoted.

    • Protactinium has no known biological role. It is toxic due to its radioactivity. Natural abundance Small amounts of protactinium are found naturally in uranium ores.
  4. 4

    National Institute of Standards and Technology (NIST), 2026. Atomic Weights and Isotopic Compositions for All Elements .

    Isotopic composition of each element; the most abundant isotope sets the neutron count in the nucleus view.