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Periodic table / Actinoids

91Protactinium

Protactinium is an actinoid in period 7, in the actinoid series. At 25 °C it is a solid.

Pa
Shells: 2, 8, 18, 32, 20, 9, 2. A simple picture: electrons do not move in circles.
Symbol
Pa
Atomic number
91 protons in the nucleus
Standard atomic weight
231.04 ± 0.01
Electron configuration
[Rn] 5f2 6d1 7s2
Electrons in each shell
2, 8, 18, 32, 20, 9, 2
Group, period, block
–, 7, f
Family
Actinoids
State at 25 °C
solid
Electronegativity
1.5 (Pauling scale)
First ionisation energy
568.3 kJ/mol (5.89 eV) (estimated by NIST)
Where these numbers come from

Atomic weight: IUPAC CIAAW, abridged standard atomic weights 2024. Electron configuration and ionisation energy: NIST Atomic Spectra Database. Family, state and electronegativity: PubChem. See Sources. Melting points, boiling points and densities are left out for now: published values differ between sources, and we will add them once each has been checked.