Plutonium

Plutonium (Pu) is an actinide with atomic number 94. It has no stable isotope and no characteristic isotopic composition on Earth, so it has no standard atomic weight; its longest-lived isotope has mass number 244. Its electron configuration is [Rn] 5f6 7s2.

Plutonium key data

Symbol	Pu
Atomic number	94
Mass number (longest-lived isotope)	244
Protons	94
Neutrons (longest-lived isotope, plutonium-244)	150
Electrons (neutral atom)	94
Category	Actinide
Period	7
Group	f-block, no group
Block	f-block
Noble gas notation	[Rn] 5f6 7s2
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f6 6s2 6p6 7s2
Electrons per shell	2, 8, 18, 32, 24, 8, 2

Mass number: IUPAC CIAAW radioactive elements, from NUBASE2020.

Measured properties of plutonium

From PubChem, the US National Institutes of Health. Public domain. PubChem’s periodic table gives no value for the rows not shown. The first ionisation energy is the value for plutonium from the NIST Atomic Spectra Database, in place of PubChem’s 6.06 eV.

Melting point	913 K (639.85 °C), 68th of 103 by melting point
Boiling point	3501 K (3227.9 °C), 29th of 93 by boiling point
Density	19.84 g/cm³, 6th of 96 by density
State at room temperature	Solid
Electronegativity	1.28 (Pauling), 72nd of 95 by electronegativity
Van der Waals radius	243 pm, joint 14th of 99 by atomic radius
First ionisation energy	6.0258 eV (581.4 kJ/mol), 75th of 102 by first ionisation energy
Common oxidation states	+6, +5, +4, +3
Discovered	1940

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.

Plutonium electron configuration

The full electron configuration of plutonium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f6 6s2 6p6 7s2. With the noble-gas core collapsed it is [Rn] 5f6 7s2, and the outermost subshell is 7s2.

Bohr model of plutonium (Pu): a nucleus and seven shells holding 2, 8, 18, 32, 24, 8 and 2 electrons from the inside out. — Bohr model of plutonium (Pu). 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 plutonium (Pu), [Rn] 5f6 7s2, with the radon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing six unpaired electrons, in 5f. — Orbital diagram of plutonium (Pu). 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 plutonium 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 = 244 / (6.02214076 × 10²³)
= 4.0517 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 244
= 2.4681 × 10²¹ atoms

Plutonium has no standard atomic weight, because it has neither a stable isotope nor a characteristic isotopic composition on Earth. The figure used above is the mass number of plutonium-244, so treat these as nominal.

Plutonium on the periodic table

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