Protactinium

Protactinium (Pa) is an actinide with atomic number 91 and an atomic mass of 231.04. Its electron configuration is [Rn] 5f2 6d1 7s2.

Protactinium key data

Symbol	Pa
Atomic number	91
Atomic mass (standard atomic weight)	231.04
Molar mass	231.04 g/mol
Protons	91
Neutrons (most abundant isotope, protactinium-231)	140
Electrons (neutral atom)	91
Category	Actinide
Period	7
Group	f-block, no group
Block	f-block
Noble gas notation	[Rn] 5f2 6d1 7s2
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f2 6s2 6p6 6d1 7s2
Electrons per shell	2, 8, 18, 32, 20, 9, 2

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Properties of protactinium

From PubChem, the US National Institutes of Health. Public domain. PubChem’s periodic table gives no value for the rows not shown.

Melting point	1845 K (1571.9 °C), 22nd of 103 by melting point
Density	15.37 g/cm³, 11th of 96 by density
State at room temperature	Solid
Electronegativity	1.5 (Pauling), joint 54th of 95 by electronegativity
Van der Waals radius	243 pm, joint 14th of 99 by atomic radius
Predicted first ionisation energy	5.89 eV (568 kJ/mol), 82nd of 102 by first ionisation energy
Common oxidation states	+5, +4
Discovered	1913

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Protactinium electron configuration

The full electron configuration of protactinium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f2 6s2 6p6 6d1 7s2. With the noble-gas core collapsed it is [Rn] 5f2 6d1 7s2, and the outermost subshell is 7s2.

Protactinium does not follow the simple filling order. Its measured configuration differs from the one the aufbau rule predicts, because at this point in the table the 5f and 6d subshells lie so close in energy that one electron goes into 6d rather than 5f. This is a measured fact about the atom rather than an exception to be derived.

Bohr model of protactinium (Pa): a nucleus and seven shells holding 2, 8, 18, 32, 20, 9 and 2 electrons from the inside out. — Bohr model of protactinium (Pa). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of protactinium (Pa), [Rn] 5f2 6d1 7s2, with the radon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing three unpaired electrons, in 5f and 6d. — Orbital diagram of protactinium (Pa). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one protactinium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 231.04 / (6.02214076 × 10²³)
= 3.8365 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 231.04
= 2.6065 × 10²¹ atoms

Protactinium on the periodic table

Protactinium is in period 7, outside the numbered groups, in the f-block.