---
title: "Protactinium (Pa): Atomic Mass, Electron Configuration & Properties"
url: https://www.moldraw.com/elements/protactinium.html
description: "Protactinium (Pa), element 91: atomic mass 231.04, electron configuration [Rn] 5f² 6d¹ 7s², group 3, period 7, f-block actinide. Melting and boiling point, density, electronegativity and shells."
lang: en
publisher: Scidart Academy
generated: 2026-11-03
---
91Pa231.04

# Protactinium (Pa) – element 91

Protactinium is a chemical element with the symbol **Pa** and atomic number **91**. It is a actinide in group 3 (actinides), period 7 of the periodic table (f-block), and is a solid at room temperature.

Protactinium (Pa) has atomic number 91, atomic mass 231.04 u and electron configuration [Rn] 5f2 6d1 7s2.

[Download periodic table PDF](https://www.moldraw.com/printables/periodic-table.pdf)[Open interactive periodic table](https://www.moldraw.com/tools/free-chem-tools/interactive-periodic-table.html)

## Protactinium facts

| Symbol | Pa |
| --- | --- |
| Atomic number | 91 |
| Atomic mass | 231.04 u (g/mol) |
| Category | actinide |
| Group | group 3 (actinides) |
| Period | 7 |
| Block | f-block |
| Phase at 20 °C | Solid |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f2 6d1 |
| Noble-gas notation | [Rn] 5f2 6d1 7s2 |
| Electrons per shell | 2, 8, 18, 32, 20, 9, 2 |

## Bohr model and shells

Protactinium has 91 electrons in 7 shells: 2, 8, 18, 32, 20, 9, 2. Full configuration and noble-gas notation are in the table; work out any element or ion with the [electron configuration calculator](https://www.moldraw.com/tools/free-chem-tools/electron-configuration-calculator.html).

## Physical and atomic properties

| Melting point | 1841 K (1567.8 °C) |
| --- | --- |
| Boiling point | 4300 K (4026.8 °C) |
| Density | 15.37 g/cm³ |
| Electronegativity (Pauling) | 1.5 |
| Electron affinity | 53.03 kJ/mol |
| Ionization energies | 568 kJ/mol (1st) |
| Discovered by | William Crookes |

Use the atomic mass in the [molar mass calculator](https://www.moldraw.com/tools/free-chem-tools/molar-mass-calculator.html) for any formula containing Pa.

## Position in the periodic table

Tap any element to open its page.

## Frequently asked questions

### What is the atomic mass of protactinium?

The atomic mass of protactinium is 231.04 u, so one mole of protactinium atoms weighs 231.04 g.

### What is the electron configuration of protactinium?

Protactinium has 91 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f² 6d¹, or [Rn] 5f² 6d¹ 7s² in noble-gas notation. Electrons per shell: 2, 8, 18, 32, 20, 9, 2.

### Is protactinium a metal?

Yes. Protactinium is a actinide.

### Where is protactinium on the periodic table?

Protactinium is in group 3 (actinides), period 7, in the f-block.

Data: [Periodic-Table-JSON](https://github.com/Bowserinator/Periodic-Table-JSON) (CC BY-SA 3.0); standard atomic weights from IUPAC (abridged). Values in brackets are the mass number of the longest-lived isotope.