Actinides are the fifteen metallic elements occupying atomic numbers 89 through 103 on the periodic table, from actinium to lawrencium. They include some of the most consequential elements humans have ever worked with: uranium and plutonium fueled both nuclear weapons and civilian power reactors, while thorium is being explored as a next-generation reactor fuel. What sets this family apart from every other row on the periodic table is a set of electron orbitals that blur the line between the predictable behavior of lighter elements and something far stranger, giving rise to chemistry that researchers are still working to pin down.
Why the 5f Orbitals Matter
Every element’s chemistry is shaped by the behavior of its outermost electrons. For the actinides, the defining feature is the gradual filling of what chemists call the 5f orbitals. These orbitals sit at an energy level that makes them neither fully locked away from bonding (as in the lanthanide elements above them on the periodic table) nor fully available for it (as in ordinary transition metals). The result is a kind of chemical split personality that changes as you move across the series.
In the early actinides like uranium and neptunium, the 5f electrons actively participate in forming bonds with surrounding atoms. Spectroscopic studies using advanced X-ray techniques have confirmed this directly: uranium’s 5f orbitals show significant variation in how tightly they are held depending on the bonding environment, meaning those electrons are genuinely engaged in the chemistry.1PubMed Central. The role of the 5f valence orbitals of early actinides in chemical bonding Neptunium behaves similarly. But by the time you reach plutonium, a shift has already begun: plutonium’s 5f electrons appear surprisingly insensitive to different bonding environments, behaving more like localized, inert spectators regardless of whether plutonium sits in a molecule or in a solid crystal.
This distinction between actinides and their lanthanide cousins is not academic trivia. The lanthanides’ equivalent 4f orbitals are tucked so deeply inside the atom that they barely interact with neighboring atoms at all. Experiments comparing uranium compounds with lanthanide compounds of identical structure have demonstrated that uranium’s 5f orbitals participate directly in bonding with surrounding atoms, while the lanthanide 4f orbitals do not.2PubMed. Are 4f-Orbitals Engaged in Covalent Bonding Between Lanthanides and Triphenylphosphine Oxide? An Oxygen K-Edge X-ray Absorption Spectroscopy and Density Functional Theory Study That said, actinide 5f bonding still falls short of the strong overlap seen in ordinary transition metals. Actinide covalency sits in a middle ground, with 5f orbitals that extend far enough to overlap with partner atoms but not to the same degree as the d orbitals in, say, iron or copper.3PubMed. Covalency in Actinide Compounds
The Relativistic Complication
One reason actinide chemistry is so difficult to model is that these are very heavy atoms with large nuclear charges. Electrons near the nucleus of an actinide atom move fast enough that the effects of Einstein’s special relativity become significant. In practice, relativistic effects contract some orbitals and expand others, reshuffling the energy landscape in ways that simpler theories miss entirely.
This has real consequences for how scientists interpret experimental data. A recent quantum chemical study reexamined earlier measurements of bonding in a uranium compound and found that the original analysis had overestimated how much the 5f electrons contributed to the bonds. The discrepancy arose because the earlier work did not account for a relativistic correction to the magnetic properties being measured.4PubMed Central. Relativistic Quantum Chemical Investigation of Actinide Covalency Measured by Electron Paramagnetic Resonance Spectroscopy The takeaway is that even state-of-the-art experiments on actinides can be misread if theorists do not apply relativistic corrections, and much of the older literature likely contains similar overestimates.
Where Actinides Come From
Thorium and uranium exist naturally in Earth’s crust because their half-lives are long enough, billions of years, for them to survive since the solar system formed. Everything heavier than uranium (the “transuranic” actinides) decays far too quickly to persist in nature and must be manufactured in reactors or accelerators. But all of these elements were originally forged in extreme astrophysical events.
The process responsible is the rapid neutron-capture process, commonly called the r-process. In environments where free neutrons are extraordinarily abundant, atomic nuclei can absorb neutrons faster than they can decay, building up to very heavy masses before eventually stabilizing. Neutron star mergers are the leading candidates for where this happens. Simulations of black-hole-neutron-star mergers have shown that the ejected material can reproduce the pattern of heavy-element abundances seen in the Sun, including the actinides.5PubMed. Actinide-Boosting r Process in Black-Hole-Neutron-Star Merger Ejecta
Some ancient stars are observed to have anomalously high actinide abundances relative to lighter heavy elements, a phenomenon called the “actinide boost.” Modeling suggests that actinide production can be so vigorous in the most neutron-rich ejecta that an additional component of lanthanide-rich, actinide-poor material is needed to dilute the actinides down to observed levels.6The Astrophysical Journal. Actinide Production in the Neutron-rich Ejecta of a Neutron Star Merger In other words, the universe seems to overproduce actinides in its most violent moments, and their final cosmic abundance is set by mixing with less neutron-rich material.
Fueling Reactors With Thorium
Most of the world’s nuclear reactors run on uranium, specifically the fissile isotope uranium-235, which makes up less than 1% of natural uranium. Thorium-232, by contrast, is roughly three to four times more abundant in Earth’s crust. It is not itself fissile, but when it absorbs a neutron it eventually converts into uranium-233, which is an excellent nuclear fuel.
Thorium-based fuels have some attractive neutronic properties. Thorium-232’s ability to capture thermal neutrons is about three times greater than that of uranium-238, meaning more of the fertile material gets converted into usable fuel during reactor operation. And because uranium-233 produces more neutrons per neutron absorbed than uranium-235, a thorium fuel cycle can achieve equivalent energy output with a smaller fraction of fissile material to start with.7Nuclear Engineering and Technology. Neutronic optimization of thorium-based fuel configurations for minimizing slightly used nuclear fuel and radiotoxicity in small modular reactors These properties have made thorium a perennial candidate for advanced reactor concepts, especially small modular reactors designed for long core lifetimes without refueling.
The thorium fuel cycle also produces far less plutonium and fewer long-lived transuranic waste products than the conventional uranium cycle, which is one reason it attracts interest from a nonproliferation standpoint. That said, working with thorium introduces its own challenges, including the difficulty of reprocessing thorium-based spent fuel and the intense gamma radiation emitted by uranium-232, an unavoidable byproduct that complicates fuel handling.
The Minor Actinide Waste Problem
When uranium or plutonium fuel is used in a reactor, neutron capture and radioactive decay generate a suite of heavier actinides in the spent fuel. Neptunium, americium, and curium, collectively called the “minor actinides,” are the biggest headaches for long-term waste disposal. They are intensely radioactive, generate significant heat, and have half-lives ranging from hundreds to millions of years. Their presence is a major driver of how large, how hot, and how long-lived a geological waste repository needs to be.
One strategy under active development is partitioning and transmutation: chemically separating the minor actinides from spent fuel and then bombarding them with neutrons in a reactor or accelerator to convert them into shorter-lived or stable isotopes. Research consistently shows that fast-neutron reactors are far more effective at transmuting minor actinides than conventional thermal reactors.8Woodhead Publishing. Developments in the partitioning and transmutation of radioactive waste Pairing transmutation with the deployment of fast reactors for energy production is widely considered the most promising route for reducing the long-term burden of actinide waste.9Nuclear Engineering and Design. Nuclear fuel cycle strategies including Partitioning and Transmutation
Separating actinides from lanthanides in spent fuel is itself a formidable chemical challenge, because the two groups behave similarly in solution. Designing molecules that can selectively grab an actinide ion while ignoring a nearly identical lanthanide ion is an active frontier in coordination chemistry, with researchers pursuing water-soluble ligands that combine selectivity with acid resistance and practical scalability.10ACS Publications. Amine-Terminated Phenanthroline Diimides as Aqueous Masking Agents for Am(III)/Eu(III) Separation: An Alternative Ligand Design Strategy for Water-Soluble Lanthanide/Actinide Chelating Ligands
Actinides in the Environment
Understanding how actinides move through soil and groundwater matters wherever nuclear activities have left a legacy: weapons-production sites, reactor accidents, and underground waste repositories. The behavior of actinides in the environment depends heavily on their chemical form, or “speciation,” which in turn depends on the local chemistry of the water and sediment.
Under oxygen-rich conditions near the surface, uranium and other actinides tend to exist in higher oxidation states and bind to naturally occurring iron and manganese oxide particles in the groundwater. Plutonium is an exception: a significant fraction stays in a lower oxidation state even when oxygen is present. In deeper, oxygen-poor zones near waste repositories, uranium shifts to a lower oxidation state and forms its own tiny colloid particles, which then scavenge other actinides out of solution.11PubMed. Speciation of actinides in groundwater samples collected near deep nuclear waste repositories Whether an actinide stays stuck to a mineral surface or hitches a ride on a mobile colloid particle determines whether it travels centimeters or kilometers from its source.
From a health perspective, ingested or inhaled actinides pose risks beyond their radioactivity. Uranium isotopes are particularly concerning because they accumulate in bone, lodging in both the exchangeable and deep crystalline mineral compartments, placing them in close proximity to the stem cells that produce blood.12PubMed Central. Medical effects of internal contamination with actinides: further controversy on depleted uranium and radioactive warfare This bone-seeking tendency means that even chemically identical but non-radioactive depleted uranium poses a toxic-metal hazard on top of any radiation dose.
Cancer Therapy With Actinium-225
Not all actinide applications involve fuel or weapons. One of the most promising frontiers in cancer treatment uses actinium-225, an actinide isotope that emits alpha particles. Alpha particles are heavy and highly charged, which means they deposit enormous energy over very short distances, just a few cell diameters. If you can attach an actinium-225 atom to a molecule that homes in on tumor cells, you get a potent cell-killing payload that largely spares surrounding healthy tissue.
The concept of targeted alpha therapy with actinium-225 was first proposed in 1993, and two decades of preclinical and clinical work have followed. A growing number of actinium-225-labeled compounds are now in clinical trials.13PubMed Central. Actinium-225 in Targeted Alpha Therapy The most striking clinical results so far have come from treating metastatic castration-resistant prostate cancer, where actinium-225 labeled to a molecule targeting prostate-specific membrane antigen (PSMA) has shown remarkable responses in patients who had exhausted other options.14PubMed Central. An Overview of Targeted Alpha Therapy with 225Actinium and 213Bismuth
The main bottleneck is supply. Actinium-225 is currently produced in very small quantities, primarily from the decay of thorium-229 stockpiles or through proton irradiation of radium-226 targets. Scaling up production to meet potential clinical demand is one of the most pressing challenges in nuclear medicine.
Powering Spacecraft With Americium
Deep-space missions that travel too far from the Sun for solar panels rely on radioisotope thermoelectric generators (RTGs), which convert the heat from radioactive decay directly into electricity. For over half a century, the standard fuel for these generators has been plutonium-238. The problem is that global supplies of plutonium-238 are limited and expensive to produce.
Americium-241, a byproduct of plutonium aging in nuclear weapons stockpiles, is being developed as an alternative. It produces less heat per gram than plutonium-238, so generators using it need to be larger, but americium-241 is far more available. Analyses comparing potential replacement isotopes have concluded that americium-241 has the greatest overall potential as a substitute, factoring in heat output, long-term stability, safety, and supply.15Nuclear Engineering and Design. Modeling of Am-241 as an alternative fuel source in a radioisotope thermoelectric generator The European Space Agency has been particularly active in this area, developing a new generator design based on americium-241 heat sources intended for lunar surface rovers and other mobility systems.16Acta Astronautica. Americium fuelled radioisotope stirling generator for lunar surface mobility systems
Dating Rocks With Uranium Decay
Some of the most practically important actinides are ones most people never think of in that context. Uranium-238 and uranium-235 decay through long chains of intermediate steps, ultimately ending at stable isotopes of lead. Because these decay rates are known with high precision, measuring the ratio of uranium to lead in a mineral like zircon reveals when that mineral crystallized, which in turn dates the rock it came from.
Two main uranium-lead ages can be extracted from a single zircon grain: one based on the decay of uranium-238 to lead-206, and another based on the ratio of lead-207 to lead-206 (which reflects the decay of uranium-235). Standard practice uses the lead-207/lead-206 age for very old zircons, roughly those older than 1.2 billion years, and the uranium-238/lead-206 age for younger ones, because the precision of each method differs depending on how much decay has occurred.17Geoscience Frontiers. Strategies towards statistically robust interpretations of in situ U–Pb zircon geochronology This uranium-lead system is the gold standard for dating events in deep geological time and has been central to establishing the age of the Earth itself.
Nuclear Forensics and the End of the Series
When authorities intercept unknown nuclear materials, identifying where the material came from is critical. Nuclear forensics relies on measuring the isotopic fingerprints of actinide elements within a sample. Even trace amounts of uranium, plutonium, americium, and curium can serve as signatures that distinguish one production source from another. A recent analysis of a bulk neptunium sample demonstrated that the trace actinide constituents present could provide unique identifiers useful for flagging material that has slipped out of regulatory control.18Analytical Chemistry. Trace Actinide Signatures of a Bulk Neptunium Sample
At the far end of the actinide series sits lawrencium, element 103, produced only in particle accelerators and available just atoms at a time. Researchers studying lawrencium have had to develop techniques capable of measuring the properties of individual atoms. One such experiment measured the rate at which doubly charged lawrencium-255 ions exchanged charge with oxygen gas, establishing a lower limit for lawrencium’s second ionization energy. The results were consistent with lawrencium occupying the final position in the actinide series, providing experimental support for where the f-block of the periodic table actually ends.19PubMed. Assessment of the Second-Ionization Potential of Lawrencium: Investigating the End of the Actinide Series with a One-Atom-at-a-Time Gas-Phase Ion Chemistry Technique
How Scientists Actually Study These Elements
Working with actinides is not like working with ordinary chemicals. Many are intensely radioactive, some exist only fleetingly in accelerator experiments, and even the more stable ones require specialized facilities with heavy shielding, gloveboxes, and strict contamination controls. This means the tools scientists use to probe actinide chemistry have had to evolve alongside the questions being asked.
Synchrotron radiation facilities, essentially particle accelerators that produce extremely intense beams of X-rays, have become indispensable. Techniques including X-ray absorption spectroscopy and resonant inelastic X-ray scattering allow researchers to determine the oxidation state of an actinide atom, the arrangement of its neighbors, and even how its electrons participate in bonding, all without needing large quantities of material.20Chemistry–Methods. Frontiers of Synchrotron Methods for Actinide Science These methods have been particularly transformative for studying actinides in environmental samples, spent nuclear fuel, and novel materials where the element of interest may be present at very low concentrations or in complex mixtures that resist simpler analytical approaches.
The combination of synchrotron experiments with relativistic quantum chemical calculations has become the modern standard for understanding actinide bonding. Neither approach alone is sufficient: experiments can be misinterpreted without relativistic theory, and theory without experimental benchmarks risks drifting into speculation. Advancing both in tandem is what continues to push the field forward, revealing an increasingly detailed picture of how these uniquely positioned elements actually behave.