What Is Actinium?
Actinium is a chemical element with the symbol Ac and atomic number 89, part of the radioactive actinide series.
The first actinide. A silvery-white, highly radioactive metal that glows blue in the dark.
As a actinide in Period 7 and Group 3 (Transition Metals), actinium's placement on the table reflects its electron configuration of [Rn] 6d¹ 7s², which governs its bonding behavior and the type of compounds it typically forms.
Position on the Periodic Table
Actinium is located in Period 7, the bottom row of the standard periodic table, consisting largely of radioactive and synthetic elements.
Within that row, actinium sits in Group 3 (Transition Metals), alongside elements that share a similar number of valence electrons and, by extension, similar chemical behavior.
This position is not arbitrary — the periodic table was deliberately organized so that an element's row and column reveal its expected reactivity, bonding pattern, and physical state at a glance.
Physical & Chemical Properties
Actinium has an atomic mass of (227) u and a density of 10.07 g/cm³. Its melting point is 1050°C and its boiling point is 3198°C.
Actinium is radioactive, like every other member of the actinide series, with a nucleus prone to spontaneous decay.
Electron Configuration Explained
Actinium's electron configuration is [Rn] 6d¹ 7s², describing how its electrons are distributed across shells and subshells around the nucleus.
Reading the configuration in order shows electrons filling shells of increasing energy, following the standard Aufbau principle that explains how atomic structure builds up across the periodic table.
The outermost (valence) electrons shown in this configuration are what ultimately determine actinium's chemical reactivity and the type of bonds it forms with other elements.
History & Discovery
Actinium was formally discovered in 1899, A.L. Debierne, identified through the careful isolation and analytical techniques typical of that era's chemical research.
Many elements from this period were anticipated by Dmitri Mendeleev's periodic table, which successfully predicted gaps corresponding to undiscovered elements well before laboratory instruments existed to confirm them directly.
What Is Actinium Used For?
Actinium's specific properties make it useful in several applications, including:
- Cancer treatment (Ac-225)
- Neutron sources
- Research
- Targeted alpha therapy
- Scientific studies
These uses reflect actinium's underlying classification as a actinide, since the same properties that define its category in chemistry also determine where it becomes practically useful.
Fun Facts About Actinium
- Actinium glows faint blue in the dark due to ionization of surrounding air by its radioactivity.
- Actinium is element number 89 on the periodic table, in Period 7 and Group 3.
- Its standard atomic mass is (227) u, calculated from the natural abundance of its isotopes.
Frequently Asked Questions
What is Actinium's atomic number?
Actinium has atomic number 89.
What is Actinium's chemical symbol?
Actinium's symbol is Ac.
What group and period is Actinium in?
Actinium is in Period 7, Group 3 (Transition Metals).
What is Actinium's atomic mass?
Actinium's standard atomic mass is (227) u.