Americium

Americium (Am) is an actinide with atomic number 95. 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 243. Its electron configuration is [Rn] 5f7 7s2.

Americium key data

Symbol	Am
Atomic number	95
Mass number (longest-lived isotope)	243
Protons	95
Neutrons (longest-lived isotope, americium-243)	148
Electrons (neutral atom)	95
Category	Actinide
Period	7
Group	f-block, no group
Block	f-block
Noble gas notation	[Rn] 5f7 7s2
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f7 6s2 6p6 7s2
Electrons per shell	2, 8, 18, 32, 25, 8, 2

Mass number: IUPAC CIAAW radioactive elements, from NUBASE2020.

Measured properties of americium

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 americium from the NIST Atomic Spectra Database, in place of PubChem’s 5.993 eV.

Melting point	1449 K (1175.9 °C), 40th of 103 by melting point
Boiling point	2284 K (2010.9 °C), 53rd of 93 by boiling point
Density	13.69 g/cm³, 13th of 96 by density
State at room temperature	Solid
Electronegativity	1.3 (Pauling), joint 61st of 95 by electronegativity
Van der Waals radius	244 pm, joint 12th of 99 by atomic radius
First ionisation energy	5.9738 eV (576.38 kJ/mol), 80th of 102 by first ionisation energy
Common oxidation states	+6, +5, +4, +3
Discovered	1944

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.

Americium electron configuration

The full electron configuration of americium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f7 6s2 6p6 7s2. With the noble-gas core collapsed it is [Rn] 5f7 7s2, and the outermost subshell is 7s2.

Bohr model of americium (Am): a nucleus and seven shells holding 2, 8, 18, 32, 25, 8 and 2 electrons from the inside out. — Bohr model of americium (Am). 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 americium (Am), [Rn] 5f7 7s2, with the radon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing seven unpaired electrons, in 5f. — Orbital diagram of americium (Am). 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 americium 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 = 243 / (6.02214076 × 10²³)
= 4.0351 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 243
= 2.4782 × 10²¹ atoms

Americium 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 americium-243, so treat these as nominal.

Americium on the periodic table

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