Actinium is a radioactive element found in uranium ores and produced for research through nuclear transformations. Its radiation can excite the surrounding air. With no assigned standard atomic weight, actinium is best understood through identified isotopes and their measured behavior rather than a single generic atomic mass.
FROM ELEMENT TO APPLICATION
AN ELEMENT IN CONTEXT
Why is actinium targeting a chemistry problem as well as a nuclear one?
Actinium-225 in molecular targeting systems
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Locate the radioactive component
Actinium-225 supplies the radioactive component in targeting research. Its role must be distinguished from the molecule intended to carry that component through a biological system.
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AcFORM → INTERACTION → CONSEQUENCE
Follow the explanation in either direction. These are authored steps, not a physical clock.
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Why cannot the starting actinium label describe all later chemistry?
Atomic identity, reference properties and structure models
INTERACTIVE 3D89 visible sites
An electron censusDrag to rotate · scroll or pinch to zoom
[Rn] 6d1 7s2. Grouping electrons by principal shell connects the neutral-atom configuration to periodic-table patterns.
What this representation means
A three-dimensional counting diagram. Dots are count markers, not measured electron positions; spherical placement and spacing are arbitrary. There are no electron trajectories here. Orbitals describe quantum states.
André-Louis Debierne reported actinium in 1899 during work on pitchblende residues. Friedrich Giesel independently obtained a strongly radioactive substance in 1902 that was later identified with the same element. Early samples were complicated mixtures, so the history is less tidy than a single isolated specimen suggests. Actinium eventually lent its name to the actinide series, connecting an early discovery with the chemistry of many heavier elements.
1899
Actinium reported
Debierne announces a radioactive element from pitchblende residues.
1902
Independent investigation
Giesel studies the material later recognized as the same element.
Natural actinium occurs in trace quantities within uranium chains, principally as actinium-227. Research and applications require purpose-produced material rather than mining a separate actinium . The isotope label matters: naturally encountered actinium and an isotope selected for a laboratory study need not have the same nuclear behavior.
Actinium commonly forms +3 , an important reference point for actinide chemistry. Its relatively large ion interacts with surrounding molecules in ways that matter when designing binding agents. Nuclear decay remains independent of this chemical classification, so a successful molecular bond cannot remove the radioactivity of an actinium isotope.
Each section links its supporting references. Descriptions and diagrams were authored for this atlas. Source checks completed. Source checks dated 2026-09-06.
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Family labels and the group-3 layout are teaching conventions. A table position does not establish an unknown bulk property. Electronic configurations refer to neutral atoms; ions and material environments require their own account.
Read each reference property together with its units, conditions and the reference information below.
Not yet recorded: electron affinity, second ionization energy, ground-state term symbol, van der waals radius, thermal conductivity, specific heat capacity, electrical resistivity, crystal structure, lattice parameter a, space group, abundance in seawater, abundance in the universe, annual production, supply-risk listing, hazard classification, biological role, cas registry number, pubchem cid, wikidata item. These families are declared and awaiting an ingestion run against the source each names; they are listed here rather than shown as empty rows.
Physical
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Phase at 20 °C
solid
293.15 K, 101.325 kPa
Melting point in K
1323 K
at standard pressure unless noted
Boiling point in K
3471 K
101.325 kPa
Density in g/cm³
10.07 g/cm³
solids and liquids at 293 K; gases at 273 K and 101.325 kPa
Atomic and electronic
Intended reference: Pauling electronegativity scale · Widely tabulated; original in The Nature of the Chemical Bond
Electronegativity in Pauling
1.1 Pauling
Pauling scale
First ionization energy in kJ/mol
499 kJ/mol
gas-phase neutral atom, ground state
Covalent radius in pm
215 pm
single-bond covalent radius
Common oxidation states
+3
states met in ordinary compounds; not exhaustive
Occurrence and identity
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Abundance in the crust
Not recorded
upper continental crust, mass fraction
Has stable isotopes
Noderived
Discovery year in CE
1899 CEderived
WHERE IT SITS AMONG ITS NEIGHBORS
Electronegativity across period 7
Ac 1.1 Pauling · Actinium
Electronegativity across period 7. Exact values.
Ra
0.9
Ac
1.1
Th
1.3
Pa
1.5
U
1.38
Np
1.36
Pu
1.28
Am
1.13
Cm
1.28
Bk
1.3
Cf
1.3
Es
1.3
Fm
1.3
Md
1.3
No
1.3
Lr
1.3
Electronegativity down group 3
Ac 1.1 Pauling · Actinium
Electronegativity down group 3. Exact values.
Sc
1.36
Y
1.22
La
1.1
Ac
1.1
The trend uses the same reference-property values as the periodic table. Elements with no recorded value are left out of the line rather than drawn at zero.
