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89Ac[227]
Actinidessolid at STPD-block

Actinium

Group: f-block•Period: 7•Standard Atomic Weight: [227] u
Open Full Periodic TableView Bohr Model

Why is Actinium in this position?

Understanding the scientific rationale behind Actinium's position in the periodic table:

Group Assignment
Group Lanthanide / Actinide

Prototype element of the Actinide series (historically Group 3) filling 5f/6d levels.

Period Assignment
Period 7

Belongs to Period 7 because its outermost electrons occupy n=7 (7s²).

Orbital Block
D-block

Electronically a d-block element ([Rn] 6d¹ 7s²), but classified as the first member of the f-block actinide series.

Chemical Category
Actinides

Classified as an actinide radioactive metal with +3 chemistry.

Atomic Structure & Bohr Shell Model

Shell Distribution: [2, 8, 18, 32, 18, 9, 2]

Bohr Atomic Shell Model(2, 8, 18, 32, 18, 9, 2)

Hover or tap any shell orbit ring to inspect electron counts and 2n² capacities.

89 Protons (p⁺)138 Neutrons (n⁰)
KShell K: Electron 1 of 2Shell K: Electron 2 of 2LShell L: Electron 1 of 8Shell L: Electron 2 of 8Shell L: Electron 3 of 8Shell L: Electron 4 of 8Shell L: Electron 5 of 8Shell L: Electron 6 of 8Shell L: Electron 7 of 8Shell L: Electron 8 of 8MShell M: Electron 1 of 18Shell M: Electron 2 of 18Shell M: Electron 3 of 18Shell M: Electron 4 of 18Shell M: Electron 5 of 18Shell M: Electron 6 of 18Shell M: Electron 7 of 18Shell M: Electron 8 of 18Shell M: Electron 9 of 18Shell M: Electron 10 of 18Shell M: Electron 11 of 18Shell M: Electron 12 of 18Shell M: Electron 13 of 18Shell M: Electron 14 of 18Shell M: Electron 15 of 18Shell M: Electron 16 of 18Shell M: Electron 17 of 18Shell M: Electron 18 of 18NShell N: Electron 1 of 32Shell N: Electron 2 of 32Shell N: Electron 3 of 32Shell N: Electron 4 of 32Shell N: Electron 5 of 32Shell N: Electron 6 of 32Shell N: Electron 7 of 32Shell N: Electron 8 of 32Shell N: Electron 9 of 32Shell N: Electron 10 of 32Shell N: Electron 11 of 32Shell N: Electron 12 of 32Shell N: Electron 13 of 32Shell N: Electron 14 of 32Shell N: Electron 15 of 32Shell N: Electron 16 of 32Shell N: Electron 17 of 32Shell N: Electron 18 of 32Shell N: Electron 19 of 32Shell N: Electron 20 of 32Shell N: Electron 21 of 32Shell N: Electron 22 of 32Shell N: Electron 23 of 32Shell N: Electron 24 of 32Shell N: Electron 25 of 32Shell N: Electron 26 of 32Shell N: Electron 27 of 32Shell N: Electron 28 of 32Shell N: Electron 29 of 32Shell N: Electron 30 of 32Shell N: Electron 31 of 32Shell N: Electron 32 of 32OShell O: Electron 1 of 18Shell O: Electron 2 of 18Shell O: Electron 3 of 18Shell O: Electron 4 of 18Shell O: Electron 5 of 18Shell O: Electron 6 of 18Shell O: Electron 7 of 18Shell O: Electron 8 of 18Shell O: Electron 9 of 18Shell O: Electron 10 of 18Shell O: Electron 11 of 18Shell O: Electron 12 of 18Shell O: Electron 13 of 18Shell O: Electron 14 of 18Shell O: Electron 15 of 18Shell O: Electron 16 of 18Shell O: Electron 17 of 18Shell O: Electron 18 of 18PShell P: Electron 1 of 9Shell P: Electron 2 of 9Shell P: Electron 3 of 9Shell P: Electron 4 of 9Shell P: Electron 5 of 9Shell P: Electron 6 of 9Shell P: Electron 7 of 9Shell P: Electron 8 of 9Shell P: Electron 9 of 9QShell Q: Electron 1 of 2Shell Q: Electron 2 of 2AcZ = 89

Educational Note: This Niels Bohr planetary model visually illustrates principal quantum energy shells ($n=1, 2, 3\dots$) and electron counts. In modern quantum mechanics (Schrödinger model), electrons do not orbit in fixed circular planetary tracks, but exist as 3D probability clouds (orbitals: $s, p, d, f$) governed by the Heisenberg uncertainty principle.

Electron Shell Filling Breakdown

Shell K (n=1):2 / 2 electrons (100%)
Shell L (n=2):8 / 8 electrons (100%)
Shell M (n=3):18 / 18 electrons (100%)
Shell N (n=4):32 / 32 electrons (100%)
Shell O (n=5):18 / 50 electrons (36%)
Shell P (n=6):9 / 72 electrons (13%)
Shell Q (n=7):2 / 98 electrons (2%)
Aufbau Electron Configuration
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 6d¹ 7s²

Neutral ground state configuration. Valence electrons: 3.

Atomic & Quantum Properties

Electronegativity (Pauling)1.1 Pauling
1st Ionization Energy499 kJ/mol
Electron Affinity-34 kJ/mol
Atomic Radius (empirical)215 pm
Common Oxidation States+3
Crystal StructureFCC

Physical & Thermal Properties

Density at STP10.07 g/cm³
Melting Point1050 °C (1323 K)
Boiling Point3198 °C (3471 K)
Magnetic OrderingParamagnetic
Discovery Year1899
Discovered ByAndré-Louis Debierne

Real-World Uses, Occurrence & Compounds

Major Industrial & Everyday Uses
  • Targeted Alpha Therapy (Actinium-225 / ²²⁵Ac-PSMA) delivering four lethal alpha particles directly to cancer tumor cells
  • Neutron source for activation analysis when alloyed with beryllium
  • Thermoelectric power source in deep-space probes
Occurrence in Nature

Occurs in trace quantities in uranium ores as a radioactive decay intermediate; produced commercially by irradiating Radium-226 in nuclear reactors.

Etymology & Name Origin

From Greek 'aktis' meaning ray or beam

Important Chemical Compounds
Ac₂O₃ (Actinium oxide)
AcCl₃ (Actinium chloride)
AcF₃ (Actinium fluoride)
Interesting Chemical Facts
  • • Actinium glows with an intense, eerie pale blue light in the dark because its ferocious radioactivity excites surrounding air molecules.
  • • Actinium-225 is called a 'nanoscopic atomic sledgehammer': as it decays through its daughter cascade, it emits four alpha particles that tear apart cancer DNA.
  • • Actinium gives its name to the entire Actinide series (Z=89 to 103).
Safety & Handling Note

Extremely hazardous radioisotope; intense radiation hazard requiring specialized glove-box containment.

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