Actinium

Actinium (Ac) is an actinide with atomic number 89. 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 227. Its electron configuration is [Rn] 6d1 7s2.

Actinium key data

Symbol	Ac
Atomic number	89
Mass number (longest-lived isotope)	227
Protons	89
Neutrons (longest-lived isotope, actinium-227)	138
Electrons (neutral atom)	89
Category	Actinide
Period	7
Group	3 or none, by convention
Block	d-block or f-block, by convention
Noble gas notation	[Rn] 6d1 7s2
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 6d1 7s2
Electrons per shell	2, 8, 18, 32, 18, 9, 2

Mass number: IUPAC CIAAW radioactive elements, from NUBASE2020.

Measured properties of actinium

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 actinium from the NIST Atomic Spectra Database, in place of PubChem’s 5.17 eV.

Melting point	1324 K (1050.9 °C), 45th of 103 by melting point
Boiling point	3471 K (3197.9 °C), 30th of 93 by boiling point
Density	10.07 g/cm³, 26th of 96 by density
State at room temperature	Solid
Electronegativity	1.1 (Pauling), joint 84th of 95 by electronegativity
Van der Waals radius	260 pm, 7th of 99 by atomic radius
First ionisation energy	5.3802 eV (519.11 kJ/mol), 95th of 102 by first ionisation energy
Common oxidation states	+3
Discovered	1899

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.

Actinium electron configuration

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

Actinium 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 actinium (Ac): a nucleus and seven shells holding 2, 8, 18, 32, 18, 9 and 2 electrons from the inside out. — Bohr model of actinium (Ac). 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 actinium (Ac), [Rn] 6d1 7s2, with the radon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing one unpaired electron, in 6d. — Orbital diagram of actinium (Ac). 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 actinium 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 = 227 / (6.02214076 × 10²³)
= 3.7694 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 227
= 2.6529 × 10²¹ atoms

Actinium 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 actinium-227, so treat these as nominal.

Actinium on the periodic table

Actinium is in period 7. Tables differ on its group: some place it in group 3, in the d-block, and others in the f-block, outside the numbered groups.