Periodic Table


Atomic number: 89
Atomic weight: 227
Symbol: Ac
Group number: 3
Electronic configuration: [Rn].6d1.7s2


Actinium is dangerously radioactive. The chemical behaviour of actinium is similar to that of the rare earths, particularly lanthanum. It is found naturally in uranium ores and actinium is 150 times more radioactive than radium.

General information

Discoveror: Andre Debierne
Date discovered: 1899
Discovered at: France
Meaning of name: From the Greek word "aktinos" meaning "ray"

Physical data

Standard state: solid at 298 K
Colour: silvery
Density of solid at ambient temperature/kg m-3: 10070
Molar volume/cm3: 22.55

Radii /pm

Atomic: no data
Covalent (single bond):
Pauling radius for the ion [Ac]-: no data

Valence shell orbital radius maxima (Rmax)
orbital s p d f
radius235.4no data130.8no data


Both values are quoted on the Pauling scale.

Pauling: 1.1
Allred Rochow: 1.00

Crystal Structure

structure: ccp (cubic close-packed)

The following CrystalMaker image represents the solid state structure. For most elements, the most stable allotrope is illustrated. Try WebElements version 2 for interactive virtual reality and CHIME images.


Temperatures (/K)

melting point: 1323
boiling point: 3573

Enthalpies /kJ mol-1

fusion: no data
vaporization: no data

single bond enthalpies:
Ac-F Ac-Cl Ac-Br Ac-I Ac-Ac
no data no data no data no data no data

Ionization enthalpies /kJ mol-1

Number Enthalpy


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/Ac.html