Pa Protactinium

Quick answerProtactinium (Pa) is an actinide with atomic number 91 and an atomic mass of 231.04 (2023), in period 7. It was discovered by Kasimir Fajans and Oswald Helmuth Göhring in 1913. It is used for nuclear research and studying decay chains. Its electron configuration is [Rn] 5f2 6d1 7s2.

Protactinium is a scarce, highly radioactive actinide known almost entirely to researchers studying the nuclear fuel cycle.

Key facts

Symbol	Pa
Atomic number	91
Atomic mass	231.04 (IUPAC standard atomic weight, 2023)
Category	actinide
Group	— (f-block element, no group number)
Period	7
Block	f-block
Electron configuration	[Rn] 5f2 6d1 7s2
Discovered	Kasimir Fajans and Oswald Helmuth Göhring, 1913
Uses	nuclear research, studying decay chains, superconductor research

Quick quiz

Three questions from the facts on this page. No sign-up, no score is saved.

1. What is the chemical symbol of Protactinium?

U Th Ac Pa

2. What is the atomic number of Protactinium?

90 92 91 93

3. Which category does Protactinium belong to?

alkali metal halogen noble gas actinide

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Keep exploring

• Periodic table — all 118 elements
• Actinium (Ac) — another actinide, atomic number 89
• Thorium (Th) — another actinide, atomic number 90
• Uranium (U) — another actinide, atomic number 92
• Neptunium (Np) — another actinide, atomic number 93
• Plutonium (Pu) — another actinide, atomic number 94
• Americium (Am) — another actinide, atomic number 95
• Curium (Cm) — another actinide, atomic number 96
• Berkelium (Bk) — another actinide, atomic number 97
• Californium (Cf) — another actinide, atomic number 98
• Einsteinium (Es) — another actinide, atomic number 99
• All actinide elements — on the periodic table

Sources & last updated

• CIAAW — Standard Atomic Weights
• PubChem Periodic Table
• Wikipedia — Timeline of chemical element discoveries

Last updated: 2026-11-12. Figures are shown with their year; where sources disagree, the value is left out rather than estimated.