Actinium

Actinium is element 89 (Ac), located in period 7, group — and the f block. At room temperature its reference phase is solid. Its electron configuration is [Rn] 6d1 7s2.

Atomic number89

Standard atomic weightNo standard atomic weight

Group—

Period7

Blockf

Electron configuration[Rn] 6d1 7s2

Electronic structure

What controls its chemistry

Actinide chemistry involves the 5f, 6d and 7s orbitals. The early actinides can access several oxidation states; for the heavier members, +3 becomes increasingly important.

Every actinide is radioactive. Nuclear stability and isotope half-life are therefore inseparable from the chemistry, especially for elements that exist only in trace quantities or must be produced artificially.

Chemical forms

Compounds, ions and materials

Actinides form oxides, halides and coordination compounds, with several oxidation states accessible for the early members. The chemistry cannot be separated from radiochemistry because every isotope is radioactive and sample composition can change through decay.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The listed density is 10 g/cm³. The melting point is 1 324 K (1 051 °C). The boiling point is 3 473K (3 227 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where actinium is found and used

In practice, actinium is encountered in scientific research and neutron sources.

In practice

Examples of real uses

Scientific research

Actinium is produced, isolated or measured mainly to test atomic, nuclear and periodic trends rather than for bulk commercial use.

Neutron sources

Certain isotopes of actinium provide neutrons for research, analysis, calibration or specialised industrial work.

Periodic position

What its neighbours tell us

Actinium is in period 7. In the f-block sequence shown here, it appears between radium and thorium. This sequence allows a direct comparison with the neighbouring f-block elements.

Reference data

Physical and atomic properties

Classificationactinide

Phase at room temperatureSolid

Electron shells2 · 8 · 18 · 32 · 18 · 9 · 2

Density10 g/cm³

Melting point1 324 K (1 051 °C)

Boiling point3 473K (3 227 °C)

Pauling electronegativity1.1

First ionisation energy519kJ/mol

Electron affinity33.77 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of actinium is associated with Friedrich Oskar Giesel. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

Actinium has no standard atomic weight. There is no characteristic terrestrial isotopic mixture from which a stable abundance-weighted average can be assigned.

Actinium has no stable isotope. Its isotopes are radioactive, and their half-lives and decay modes become important whenever the element is measured, handled or used.

Safety

Handling and exposure

Actinium is radioactive. Work with it requires radiological controls appropriate to the isotope, activity, radiation type and chemical form.