Nuclear and radiochemistry

Chemistry of Actinides and Superactinides

From Seaborg’s actinide hypothesis to single-atom transactinide chemistry: 5f electronic configurations, oxidation state diversity, actinyl bonding, and relativistic 5g/6f superactinides.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionIntuition: the accessible frontier of 5f electrons

In the 4f lanthanides, the inner f-orbitals are buried deep inside the filled 5s and 5p shells, shielded from the chemical environment and yielding almost universally trivalent chemistry. In the 5f actinides, the radial distribution of early 5f orbitals extends much further outward, with energies very close to the 6d and 7s levels. Consequently, early actinides participate directly in chemical bonding and display a rich spectrum of oxidation states from +3 up to +7.

Interactive spectrum comparing M³⁺ ionic radii contraction between lanthanides and actinides, and displaying the highest established oxidation states across the actinide series (Ac +3 to Lr +3).

SchoolSchool level: electron configurations and oxidation states

Definition: Seaborg’s actinide hypothesis

Proposed by Glenn T. Seaborg in 1944, this hypothesis established that the elements starting from actinium (Z=89) form a second 14-member inner-transition series filling the 5f subshell, parallel to the 4f lanthanides. Prior to this insight, thorium, protactinium, and uranium were mistakenly grouped under titanium, vanadium, and chromium in the main transition d-block.

Actinide electron configurations and oxidation states
Element	Z	Ground-state configuration	Common oxidation states	Highest state (special conditions)
Actinium (Ac)	89	[Rn] $6d^1 7s^2$	+3	+3
Thorium (Th)	90	[Rn] $6d^2 7s^2$	+4	+4
Protactinium (Pa)	91	[Rn] $5f^2 6d^1 7s^2$	+4, +5	+5
Uranium (U)	92	[Rn] $5f^3 6d^1 7s^2$	+3, +4, +5, +6	+6
Neptunium (Np)	93	[Rn] $5f^4 6d^1 7s^2$	+3, +4, +5, +6	+7 (alkaline oxidant)
Plutonium (Pu)	94	[Rn] $5f^6 7s^2$	+3, +4, +5, +6	+7 (alkaline oxidant)
Americium (Am)	95	[Rn] $5f^7 7s^2$	+3 (dominant), +5, +6	+7 (solid/alkaline)
Curium (Cm)	96	[Rn] $5f^7 6d^1 7s^2$	+3	+4 (fluoride, oxide)
Berkelium (Bk)	97	[Rn] $5f^9 7s^2$	+3, +4	+4
Californium (Cf)	98	[Rn] $5f^{10} 7s^2$	+2, +3	+4 (solid matrix)
Einsteinium to Nobelium	99–102	[Rn] $5f^{11-14} 7s^2$	+3 (+2 for Md, No)	+2 dominant in aq No
Lawrencium (Lr)	103	[Rn] $5f^{14} 7s^2 7p^1$	+3	+3

\ce{Pu^4+(aq) + PuO2+(aq) <=> Pu^3+(aq) + PuO2^2+(aq)} \qquad (E^\circ \approx 0.12\text{ V})

Example: Actinide contraction in six-coordinate M3+ cations

The Shannon ionic radius (coordination number 6) decreases from 1.12 Å for Ac3+ to 1.025 Å for U3+, 0.975 Å for Am3+, and 0.95 Å for Cf3+. Calculate the total contraction between Ac3+ and Cf3+, and explain its physical origin compared to the lanthanide contraction.

Solution

The contraction is delta r = 1.12 - 0.95 = 0.17 Å across these 9 elements. As atomic number Z increases, each added proton increases nuclear charge. The corresponding 5f electrons occupy diffuse orbitals with poor radial shielding efficiency toward each other. The effective nuclear charge Z_eff felt by outer shells increases monotonically, pulling all electron shells inward. While similar in magnitude to the lanthanide contraction (~0.17 Å), actinide radii are systematically 0.05 to 0.10 Å larger than their 4f analogues due to the principal quantum number increase (n=5 vs n=4).

UndergraduateUndergraduate level: actinyl bonding, hydrolysis, and solvent extraction

High oxidation states (U(VI), Np(V,VI), Pu(V,VI), Am(V,VI)) exist exclusively as linear dioxo actinyl cations [O=An=O]^(n+) (n = 1 or 2). Strong covalent mixing between oxygen 2p orbitals and actinide 5f_sigma, 5f_pi, and 6d_pi orbitals produces extremely short, unreactive axial bonds (bond order ~3). Coordination chemistry is restricted to the equatorial plane, binding 4 to 6 ligands. In nuclear fuel reprocessing, the PUREX process exploits this geometry: neutral tributyl phosphate (TBP) coordinates equatorially to UO2(NO3)2 and Pu(NO3)4, extracting them into an organic kerosene phase while leaving trivalent fission products in acid.

2\,\ce{PuO2+(aq) + 4H+(aq) <=> Pu^4+(aq) + PuO2^2+(aq) + 2H2O(l)}

Example: Mass and charge balance in Pu(V) disproportionation

Pentavalent plutonium (PuO2+) is thermodynamically unstable in acidic aqueous solution and disproportionates into Pu(IV) and Pu(VI). Write the balanced chemical equation in acid and verify that both atomic mass and ionic charges are rigorously balanced.

Solution

The balanced reaction is: 2 PuO2+ + 4 H+ <=> Pu^4+ + PuO2^2+ + 2 H2O. Let us check atom balance: left has 2 Pu, 4 O (from 2 PuO2+), and 4 H; right has 1 Pu (in Pu^4+) + 1 Pu (in PuO2^2+) = 2 Pu; 2 O (in PuO2^2+) + 2 O (in 2 H2O) = 4 O; and 4 H (in 2 H2O). Now check net charge: left side has 2(+1) + 4(+1) = +6; right side has (+4) + (+2) + 0 = +6. Both charge and atoms are perfectly conserved.

AdvancedAdvanced: covalency, uranocene, and gas-phase transactinide chemistry

While 4f lanthanide bonding is essentially electrostatic, actinides exhibit measurable covalent mixing with ligand orbitals. A landmark demonstration is uranocene, U(eta^8-C8H8)2, synthesized by Streitwieser in 1968. Relativistic density functional theory shows that uranium 5f_xyz and 5f_z(x^2-y^2) orbitals possess the correct e2u symmetry to overlap directly with the pi-molecular orbitals of the cyclooctatetraenide dianions, forming genuine f-orbital covalent bonds. For transactinides (Z >= 104), chemistry is conducted one atom at a time using automated gas chromatography (such as OLGA and COMPACT): rutherfordium forms volatile RfCl4, dubnium yields DbOCl3, and seaborgium forms SgO2Cl2, validating homologous periodic trends modified by relativistic orbital contractions.

\ce{U(IV) + 2 [C8H8]^2- -> U(\eta^8-C8H8)2} \quad (\text{sandwich uranocene, } D_{8h})

ResearchResearch frontier: superactinides and single-atom molecule detection

On the experimental frontier as of 2025, chemical investigation of the heaviest actinides has achieved historic molecular precision. In 2025, researchers at Lawrence Berkeley National Laboratory using the FIONA mass spectrometer reported the first direct identification of gas-phase molecules containing nobelium (Z=102). This breakthrough demonstrates that chemical bond lengths, bond energies, and volatility can now be measured directly on single short-lived actinide atoms, resolving longstanding controversies regarding the periodic boundary between the 5f series and group 3 transition metals.

References

The Chemistry of the Actinide and Transactinide Elements · L. R. Morss, N. M. Edelstein, J. Fuger (eds.), 2010
The Transuranium Elements · G. T. Seaborg, 1946
Covalency in f-element complexes · M. L. Neidig, D. L. Clark, R. L. Martin, 2013