F elements are the 28 metallic elements that fill the two rows sitting below the main body of the periodic table, split into the lanthanides (elements 57–71, from lanthanum to lutetium) and the actinides (elements 89–103, from actinium to lawrencium). What sets them apart from every other element is that their outermost distinguishing electrons occupy f orbitals, a deeply buried shell that gives these metals a distinctive and sometimes bizarre set of chemical, optical, and magnetic behaviors. They range from cerium, which is about as common in Earth’s crust as copper, to einsteinium, which exists only in vanishingly small quantities inside nuclear reactors. Together they underpin technologies from permanent magnets and nuclear fuel to luminescent medical probes, yet many people never encounter them by name.
Why the F Orbitals Matter
Every element’s chemistry is shaped by where its outermost electrons sit. For most of the periodic table those electrons are in s, p, or d orbitals, which extend relatively far from the nucleus and interact readily with neighboring atoms. F orbitals are different: they are compact, buried beneath outer s and d electrons, and poorly shielded from the pull of the nucleus. That buried quality is the single most important fact about f elements because it explains nearly everything unusual about them.
Because the f electrons are tucked inside, the chemical environment around an f-element ion barely disturbs them. The ion’s bonding is dominated by its size and charge rather than by the shape of its electron cloud. This is why the 15 lanthanides are notoriously difficult to separate from one another: they are all roughly the same size, all strongly prefer the +3 oxidation state, and all bond to other atoms in a largely ionic (charge-driven) fashion. Their similarity frustrated chemists for over a century and still poses a real industrial challenge today.
How Lanthanides and Actinides Differ
At first glance, the actinides look like heavier copies of the lanthanides. Both rows fill f orbitals, both favor the +3 state, and both form large ions that prefer high coordination numbers. But the 5f orbitals of the actinides extend a bit farther from the nucleus than the 4f orbitals of the lanthanides, so they are less perfectly shielded and more available for bonding. Comparisons of their solution chemistry show that while an ionic model describes both series reasonably well, the actinides display slightly greater covalency, meaning their electrons are shared with bonding partners to a modestly larger degree. This difference is especially pronounced in metal-nitrogen bonds.1Journal of the Less Common Metals. Comparison of the solution chemistry of the actinides and lanthanides
That extra covalent character has practical consequences. The early actinides (uranium, neptunium, plutonium) access a much wider range of oxidation states than any lanthanide. Uranium readily cycles between +3 and +6 in the environment, and plutonium is notorious for existing simultaneously in four different oxidation states in the same solution. Quantum-chemical analyses have shown that high spin-orbit splitting in the actinides stabilizes the +2 state at the end of the series, while the +4 state is much more stable across the actinides than the lanthanides.2Russian Chemical Reviews. The Quantum Chemistry of Unusual Oxidation States of the Lanthanides and Actinides These oxidation-state gymnastics are one reason actinide chemistry can be so rich and, when it comes to nuclear waste, so difficult to manage.
Experimental and computational studies of bonding in matched lanthanide-actinide complexes have pinpointed where the extra covalency comes from. When researchers compared uranium and plutonium compounds to lanthanum and cerium analogues with the same ligand frameworks, the actinide-to-ligand bond lengths were consistently shorter than expected from ionic-radius differences alone. The enhancement turns out to have two components: increased participation of metal d orbitals as you go to heavier donor atoms, and increased participation of f orbitals when you switch from a lanthanide to an actinide center.3PubMed. Experimental and theoretical comparison of actinide and lanthanide bonding in MN(EPR(2))(2) complexes (M = U, Pu, La, Ce; E = S, Se, Te; R = Ph, iPr, H)
Luminescence and the Antenna Effect
One of the most commercially useful properties of the lanthanides is their luminescence. Several lanthanide ions emit light at sharply defined wavelengths when excited, producing colors that range from the deep red of europium to the green of terbium to the near-infrared glow of neodymium and ytterbium. These narrow emission bands are remarkably stable: they do not shift with temperature, solvent, or pH the way organic fluorescent dyes do, and lanthanide compounds resist photobleaching, meaning they can be exposed to excitation light for extended periods without fading.4Chimia. Novel Antennae for Luminescent Lanthanide Cations Emitting in the Visible and in the Near-Infrared
There is a catch, though. The f-f electronic transitions responsible for this luminescence are quantum-mechanically “forbidden,” which in practice means lanthanide ions absorb light very weakly on their own. To get useful brightness, chemists surround the lanthanide ion with an organic or inorganic “antenna” molecule that absorbs light efficiently and then transfers that energy to the metal center. This antenna effect has driven the development of lanthanide-based luminescent probes for medical imaging, anti-counterfeiting inks, and fiber-optic amplifiers. The reason your internet signal can travel hundreds of kilometers through glass fiber without dying owes something to erbium ions doped into the fiber, amplifying the near-infrared light that carries data.
Magnetism at the Molecular Scale
F elements also produce some of the most powerful magnets known. At the macroscopic level, alloys like neodymium-iron-boron (NdFeB) are the strongest permanent magnets commercially available, essential for wind turbines, electric vehicle motors, and hard-disk drives. At the molecular level, certain lanthanide complexes behave as single-molecule magnets, retaining their magnetic orientation long enough to store a bit of information in a single molecule. These systems have drawn intense interest for potential use in quantum computing, high-density data storage, and spintronic devices.5Accounts of Chemical Research. Regulating Lanthanide Single-Molecule Magnets with Coordination Geometry and Organometallic Chemistry
The key to these molecular magnets is magnetic anisotropy, a preference for the magnetization to point along one axis rather than any other. Lanthanide ions like dysprosium(III) and terbium(III) have large, asymmetric electron clouds in their f shells that generate extreme anisotropy when placed in the right ligand environment. Researchers have found that the shape of the surrounding ligand field, whether it compresses or elongates the electron density, can be tuned to push operating temperatures higher and relaxation times longer. The current record-holders operate at temperatures that, while still far below room temperature, would have seemed fantastical two decades ago.
Where F Elements Come From
The lighter f elements, up through uranium, formed in stellar processes over billions of years. But the heaviest ones, and a substantial share of the total inventory of elements beyond iron, owe their existence to some of the most violent events in the cosmos. When two neutron stars spiral together and collide, the resulting explosion flings out matter so rich in free neutrons that atomic nuclei can capture them in rapid succession, building up to very heavy masses before the nuclei have time to decay. This is the r-process (rapid neutron capture), and it produces elements all the way up to the actinides.
The first direct proof came in 2017, when gravitational-wave detectors caught the signal of a neutron star merger (GW170817) and telescopes spotted the resulting kilonova, whose light curve matched predictions for the radioactive decay of freshly synthesized r-process elements.6The Astrophysical Journal Letters. Magnetically Driven Baryon Winds from Binary Neutron Star Merger Remnants and the Blue Kilonova of 2017 August Modeling of the ejecta shows that the material thrown off in these mergers is neutron-rich enough to produce a broad range of heavy elements, with characteristic abundance peaks that closely match the pattern seen in our own solar system.7Monthly Notices of the Royal Astronomical Society. Neutron star mergers as the dominant contributor to the production of heavy r-process elements The uranium and thorium in Earth’s crust, the elements whose radioactive decay keeps the planet’s interior warm, were likely forged in one or more such events before our solar system formed.
Supply Chains and Geopolitical Pressure
On Earth, the lanthanides are often called “rare earths,” a misnomer inherited from 18th-century mineralogy. Most are not especially rare in the crust. The real bottleneck is that they seldom concentrate into easily mined deposits, and separating one lanthanide from the next requires tedious, chemical-intensive processes. Production is overwhelmingly concentrated in China, which creates supply-chain fragility for the rest of the world.
The stakes are highest for NdFeB magnets, which are foundational components in wind turbines, electric vehicles, and aerospace systems. Their supply chains face disruption from market volatility, technological challenges, and growing environmental and social scrutiny around mining and processing.8Renewable and Sustainable Energy Transition. Supply chains of NdFeB magnets: a conceptual framework for geopolitics, geoeconomics, technology, and ESG Governments in the United States, European Union, Japan, and Australia have designated multiple lanthanides as critical minerals and are funding efforts to develop domestic mining, alternative magnet chemistries, and recycling infrastructure.
Recycling is one of the more promising avenues. Electronic waste, from hard drives and speakers to hybrid-car batteries, contains meaningful concentrations of neodymium, dysprosium, and other high-value lanthanides. Recovery methods range from conventional hydrometallurgy and pyrometallurgy to newer techniques like bioleaching, ionic-liquid extraction, and approaches inspired by the biology of lanthanide-utilizing bacteria.9PubMed Central. A Review of the Occurrence and Recovery of Rare Earth Elements from Electronic Waste None of these yet operate at a scale that would meaningfully dent dependence on primary mining, but they are advancing quickly.
Nuclear Fuel and Actinide Transmutation
The actinide half of the f-element family occupies center stage in nuclear energy. Uranium-235 and plutonium-239 are the fissile materials that power reactors and, unfortunately, nuclear weapons. Spent nuclear fuel contains a cocktail of actinides known as “minor actinides,” primarily neptunium, americium, and curium, that account for much of the long-lived radioactivity in nuclear waste. A closed fuel cycle, in which these minor actinides are separated from spent fuel and then “transmuted” (converted into shorter-lived or stable isotopes) by irradiation in a fast reactor, could dramatically reduce the time the waste remains hazardous. The international METAPHIX program has been investigating exactly this approach, incorporating minor actinides into metal-alloy fuel pins and irradiating them in a fast neutron spectrum to test both safety and effectiveness.10Progress in Nuclear Energy. Characterization of metallic fuel for minor actinides transmutation in fast reactor
Separating actinides from lanthanides in spent fuel is itself a major chemical challenge, because both groups favor the +3 state and have similar ionic radii. The slight extra covalency of the actinides provides a toehold: organophosphorus extractants can exploit the small differences in how actinides and lanthanides bind to sulfur- or nitrogen-containing ligands. Agents like Cyanex 301, a dithiophosphinic acid, preferentially grab actinides over lanthanides precisely because the actinide f orbitals participate more in bonding to soft donor atoms.11PubMed. Organophosphorus Extractants: A Critical Choice for Actinides/Lanthanides Separation in Nuclear Fuel Cycle
Environmental Behavior of Actinides
When actinides escape into the environment, whether through legacy weapons-production sites, nuclear accidents, or depleted-uranium munitions, their fate depends heavily on speciation: the combination of oxidation state, molecular structure, and the minerals or organic matter they encounter. Uranium, neptunium, and plutonium can interact with common soil minerals, natural organic matter, and even bacteria in ways that either lock them in place or, in some cases, mobilize them on colloidal particles and carry them surprising distances through groundwater.12PubMed. Environmental speciation of actinides
Plutonium is the most complicated case. Its ability to exist in multiple oxidation states simultaneously means that a single soil sample can contain plutonium species with wildly different mobilities. Reduction by iron-containing minerals or by metal-reducing bacteria can convert soluble plutonium(V) to insoluble plutonium(IV), effectively immobilizing it. But attachment to tiny colloid particles can have the opposite effect, carrying plutonium far from its source. Understanding and predicting these competing processes is essential for managing contaminated sites, and the science is still evolving.13PubMed Central. Speciation of Uranium and Plutonium From Nuclear Legacy Sites to the Environment: A Mini Review
Health Hazards of Actinide Exposure
Actinides pose a double threat to living organisms: they are radioactive, and they are chemically toxic heavy metals. Uranium, for instance, is osteotropic, meaning it accumulates in bone. Once lodged in the crystal structure of bone tissue, uranium isotopes sit in close proximity to the pluripotent stem cells that produce blood, raising concern about long-term effects including leukemia.14PubMed Central. Medical effects of internal contamination with actinides: further controversy on depleted uranium and radioactive warfare
Chemical toxicity alone can be significant. Animal studies with thorium nitrate have shown oxidative-stress damage to the liver, with decreased antioxidant enzyme activity and visible tissue injury. Thorium also accumulated in the brain and altered neurobehavioral function. Uranium has been shown to denature proteins in laboratory studies and to interfere with hemoglobin’s oxygen-carrying ability. Even when the radiation dose from a given actinide exposure is low, the chemical burden on the kidneys, liver, and brain may be substantial.15Journal of Radiation and Cancer Research. Mechanism of Carcinogenesis after Exposure of Actinide Radionuclides Disentangling the radiological and chemical contributions to health effects remains an active area of research.
Lanthanides in Biology
For most of the 20th century, lanthanides were considered biologically inert, useful as laboratory tools precisely because they were thought to have no natural role in living systems. That assumption was upended in 2011 with the discovery that certain soil bacteria specifically incorporate early lanthanides, particularly lanthanum, cerium, praseodymium, and neodymium, into enzymes that oxidize methanol. These bacteria use lanthanide-dependent alcohol dehydrogenases in place of the calcium-dependent versions that other organisms rely on, and they do so more efficiently because the lanthanide ion’s higher Lewis acidity makes it a better catalyst for the reaction.16ACS Publications. The Chemistry of Lanthanides in Biology: Recent Discoveries, Emerging Principles, and Technological Applications
The finding has opened an unexpected door for technology. If bacteria can selectively take up specific lanthanides from dilute, mixed solutions, then the biological machinery responsible, including a protein called lanmodulin, could be harnessed for more sustainable extraction and separation of rare earths. Lanmodulin binds lanthanides with extraordinary selectivity and affinity, far outperforming conventional chemical extractants. Researchers are now engineering lanmodulin-derived peptides for use in recycling processes, potentially offering a greener alternative to the acids and organic solvents that dominate current practice.
Exotic Physics Under Extreme Conditions
F elements are a playground for condensed-matter physicists because their f electrons exist on the knife edge between being localized (stuck on one atom) and itinerant (free to roam through the crystal). This duality generates a class of materials called heavy-fermion systems, in which electrons behave as though they have hundreds of times their actual mass. The result is a rich landscape of exotic quantum states: unconventional superconductivity, electronic nematic phases, topological insulators, and phenomena collectively labeled “hidden order” because their underlying symmetry breaking has not yet been identified.17PubMed Central. A microscopic Kondo lattice model for the heavy fermion antiferromagnet CeIn3
Uranium-based compounds have been especially revealing. In the antiferromagnet USb₂, scanning tunneling microscopy has shown that a Kondo lattice, a cooperative entanglement between localized f electrons and mobile conduction electrons, develops inside the magnetically ordered phase at temperatures below about 80 K. This was unexpected because conventional theory predicts that magnetic order and Kondo screening should compete and not coexist.18PubMed Central. Orbital-selective Kondo lattice and enigmatic f electrons emerging from inside the antiferromagnetic phase of a heavy fermion Such discoveries keep forcing theorists to revise their models of how f electrons interact with their surroundings.
Pressure adds another dimension. Several of the heavier actinides undergo dramatic volume collapses when squeezed in a diamond-anvil cell. Americium, curium, berkelium, and californium all show large, abrupt shrinkages at pressures ranging from roughly 110 kbar for americium to 430 kbar for curium, driven by a transition of the f electrons from localized to delocalized states.19Physica B: Condensed Matter. Electronically driven volume collapses of bantam-heavy actinide elements at high pressure Cerium metal shows an analogous collapse at much lower pressure, and the debate over whether that transition involves a change in f-electron count or simply a change in how f electrons hybridize with their neighbors has persisted for decades. High-pressure X-ray emission measurements have provided evidence that the transition involves a genuine change in the f-electron configuration rather than just a smooth crossover.20PubMed. X-ray emission spectroscopy of cerium across the γ-α volume collapse transition These high-pressure experiments offer a window into what happens when the delicate balance between localization and itineracy in f elements is forcibly tipped.