Actinides: actinides periodic table facts

What are actinides? the radioactive metals at the bottom of the table

The actinides are a family of 15 heavy radioactive metals with atomic numbers 89 through 103. Every one of them is unstable: none has a single stable isotope, so each undergoes radioactive decay, giving off energy as alpha particles, beta particles, gamma rays or, in the heaviest members, by splitting apart. That shared radioactivity is the defining trait of the whole actinide group, and it is why most of them were unknown until the nuclear age. Concerns about drinking water limits and groundwater contamination are the focus of the arsenic atomic mass article on this topic.

Chemists also use the informal symbol An for "any actinide" in equations. The IUPAC naming rules prefer the term actinoid, because the suffix -ide usually signals a negative ion such as chloride, but the older name is still accepted and far more common in textbooks and research papers.

Only four members occur naturally in measurable amounts. The other eleven are synthetic actinides, made in reactors or accelerators.

Their study, often called actinide science, sits where chemistry meets physics: you can't understand how these metals bond without understanding how their nuclei fall apart. The early members (thorium to uranium) even behave a little like transition metals, while the later ones look much more like the lanthanides one row above.

Why the actinide series is named after actinium

The whole actinide series is named after its first member, actinium, which André-Louis Debierne discovered in 1899 in the residue left after Marie and Pierre Curie extracted radium from uranium ore. The element's own name comes from the Greek aktis, meaning "ray" or "beam", a nod to the radiation that gave it away. The label for the full series came later, in the 1930s and 1940s, once scientists realised that actinium and the elements after it form one family rather than a set of unrelated heavy metals.

Are actinides metals?

Yes. All 15 actinides are metals, and the ones made in large enough amounts to see are silvery and tarnish quickly in air.

They are strongly electropositive, giving up electrons more readily than most of the table, and finely divided uranium, plutonium and thorium are pyrophoric: a fine powder can ignite on its own in air. The heaviest members (from fermium, atomic number 100, upward) have never been made in visible amounts, so their metallic nature is predicted from theory and from single-atom chemistry rather than observed in a lump of metal.

Where the actinide series sits on the periodic table

On the periodic table, the actinides belong to period 7, directly after radium. They are usually printed as the second of two additional rows beneath the main body, with the lanthanides above them. That placement is a matter of page layout: a fully stretched 32-column table would slot both series straight into the sixth and seventh row, but it would be too wide to print comfortably. For chemistry notes on the alkaline earth metal, data on atomic number 20, electron configuration and melting point are gathered in the calcium element for quick lookup.

Together with the lanthanides they make up the f-block elements. The position mirrors lanthanum and its row: actinium sits under lanthanum.

An element's atomic number is simply the count of protons in its nucleus, so each step from 89 to 103 adds one proton and, roughly, one electron to the 5f subshell. Because of that, the actinides are sometimes simply called the 5f elements.

Electron configuration and the 5f orbital

The electron configuration of each actinide starts from the radon core, [Rn], and then fills the 5f orbital, the 6d orbital and the 7s orbital. The order is less tidy than the textbook rule suggests.

Thorium has no 5f electrons at all as a free atom ([Rn] 6d² 7s²), while uranium is [Rn] 5f³ 6d¹ 7s². Lawrencium ends the row as [Rn] 5f¹⁴ 7s² 7p¹, which is why some chemists place it in the d-block instead.

The irregularities come from interelectronic repulsion and from how close in energy the 5f and 6d levels are. In the early actinides the 5f electrons reach far enough from the nucleus to take part in bonding, a big difference from the 4f electrons of the lanthanides, which stay tucked inside the atom. Heavy atoms like these also need relativistic corrections, because their inner electrons travel at a large fraction of the speed of light, which shrinks the s orbitals and lets the f orbitals spread outward.

List of the 15 actinide elements

Here is every element in the series with its symbol, how you would encounter it, and the longest-lived of its known isotopes. Long half-lives mean a sample stays around; short ones mean researchers have to work within minutes or hours.

Read down the last column and you can see the pattern that governs the whole series: past curium, half-lives collapse from millions of years to days and then minutes, which is why so little is known about the chemistry of fermium, mendelevium and nobelium.

Naturally occurring actinides: actinium to uranium

Thorium and uranium have isotopes with half-lives long enough to have survived since Earth formed, so you'll find them throughout the Earth's crust; thorium is roughly three times more abundant than uranium. Actinium and protactinium exist only because uranium and thorium keep decaying into them, so they turn up in trace quantities in uranium ore. Natural abundance drops off a cliff after that: tiny amounts of neptunium and plutonium form in uranium minerals when stray neutrons are captured, but almost all the neptunium and plutonium in existence was made by people.

Transuranium elements: neptunium to lawrencium

Every actinide element with an atomic number above 92 is one of the transuranium elements, also called transuranic elements. Those beyond plutonium form a further subgroup, the transplutonium elements. All of them were first synthesized in a reactor, an accelerator or a weapons test, usually by bombarding a heavy target with neutrons or light nuclei, rather than found in rock.

Neptunium and plutonium are now made by the tonne in power reactors, americium and curium by the kilogram, californium by the gram, and the last four only an atom at a time in a laboratory. That steep fall in quantity follows the half-lives in the table above: each step up the series is harder to make and disappears faster.

Discovery and synthesis of the actinoids

The history of the series splits neatly in two. Thorium, uranium, actinium and protactinium were found by chemists picking apart minerals between 1789 and 1913.

Everything after uranium came from deliberate synthesis, starting with Enrico Fermi's neutron experiments of 1934, which first hinted that elements beyond uranium could be made. Much of the research that followed took place under wartime secrecy.

From pitchblende to protactinium

Martin Heinrich Klaproth identified uranium in 1789 in pitchblende, the black ore now called uraninite, and named it after the newly discovered planet Uranus. What he isolated was actually an oxide; Eugène-Melchior Péligot produced the first uranium metal in 1841. Jöns Jacob Berzelius found thorium in a Norwegian mineral in 1829 and named it after Thor, the Norse god of thunder; the purer mineral thorianite was described later.

Actinium followed in 1899. Protactinium came last among the natural members: Kasimir Fajans and Oswald Göhring found a short-lived isotope in 1913, and Lise Meitner and Otto Hahn identified the long-lived protactinium-231 in 1917–1918. Its name means "before actinium", because it decays into actinium.

Glenn Seaborg and the actinide hypothesis

Edwin McMillan and Philip Abelson made neptunium at Berkeley in 1940 by hitting uranium with neutrons, and Glenn Seaborg's team made plutonium early the next year. Plutonium soon became the core of the Manhattan Project. In 1944 Seaborg proposed what came to be known as the actinide hypothesis: that the elements from actinium onward form a second inner-transition series under the lanthanides, rather than continuing the transition-metal columns.

Moving them to their own row correctly predicted the chemistry of americium and curium, and Seaborg's group synthesized berkelium (1949) and californium (1950) next. Seaborg shared the 1951 Nobel Prize in Chemistry, and element 106, seaborgium, was named in his honour while he was still alive.

Einsteinium and fermium turned up in the fallout of Ivy Mike, the first full-scale hydrogen bomb test in November 1952, where the nuclear explosion packed many neutrons into uranium nuclei within a fraction of a second. Mendelevium (1955) was the first element made one atom at a time, by bombarding einsteinium with alpha particles in a 60-inch cyclotron at what is now Lawrence Berkeley National Laboratory. Nobelium and lawrencium followed in the 1950s and 1960s, with priority split between Berkeley and Dubna after decades of dispute, and both were made in a particle accelerator by firing light nuclei such as neon, boron or oxygen at heavy targets.

How synthetic actinides build up inside a reactor

Inside running nuclear reactors, uranium nuclei soak up stray neutrons all the time, and that is where most synthetic actinides are born. Each neutron capture adds one unit of mass, and each beta decay converts a neutron into a proton, moving the nucleus one place up the table. The best-known chain turns uranium-238 into plutonium-239:

$$^{238}_{92}\text{U} + {}^{1}_{0}n \rightarrow {}^{239}_{92}\text{U} \xrightarrow{\beta^-} {}^{239}_{93}\text{Np} \xrightarrow{\beta^-} {}^{239}_{94}\text{Pu}$$

The two beta steps take about 23 minutes and about 2.4 days. Longer neutron irradiation keeps climbing the ladder through americium and curium toward californium, which is how high-flux research reactors produce isotopes of the heavier members.

Actinide isotopes and their half-lives

More than 200 isotopes of these elements are known, and their half-lives span an almost absurd range, from fractions of a second to longer than the age of the universe. A half-life is the time it takes for half the atoms in a sample to decay. It decides almost everything about an isotope: whether it exists in nature, how intensely radioactive it is per gram, and whether it is useful or simply a waste problem.

Most of these isotopes lose energy by alpha decay, throwing out a helium nucleus. The heavier ones increasingly favour spontaneous fission instead, where the nucleus simply splits in two without being hit by anything; californium-252 does this in about 3% of its decays, which makes it a powerful source of free neutrons.

Natural uranium is 99.27% uranium-238, 0.72% uranium-235 and a trace of uranium-234. Each primordial isotope heads a long decay chain that passes through radium and radon before ending at a stable lead isotope, and that steady trickle of daughters is the only reason actinium and protactinium exist in nature.

Physical properties of actinides

As metals, the actinides are dense, reactive and unusually varied for elements in one series. Their atoms and ions are large: the ionic radii of the +3 ions are some of the biggest of any metal. Yet they show more structural oddities than any other group, with plutonium alone taking six different crystal forms between room temperature and its melting point.

Density and the actinide contraction

Moving from actinium to lawrencium, the atomic radius shrinks a little with each step, a trend known as the actinide contraction. The 5f electrons shield the outer electrons poorly, so the growing nuclear charge pulls the whole atom tighter.

Smaller atomic size and heavy nuclei combine to give very high density: neptunium, at about 20.4 g/cm³, is the densest of the series, and uranium (19.1 g/cm³) is nearly as dense as gold. Thorium is the light one at roughly 11.7 g/cm³.

Malleability, ductility and melting point

Thorium and uranium show good malleability and ductility: they can be rolled, drawn into wire and machined, though uranium has to be protected from oxidation while hot. Each has a high melting point, about 1,750 °C for thorium and 1,132 °C for uranium. Neptunium and plutonium are oddly low, near 640 °C.

Plutonium is also brittle in its room-temperature form, and it shrinks rather than expands when it melts. Uranium's boiling point is above 4,100 °C, one of the widest liquid ranges of any element.

Magnetic behavior and radioactivity

With unpaired 5f electrons, most of these metals and their ions are paramagnetic, drawn weakly into a magnetic field. Their radioactivity affects the bulk metal too: plutonium is warm to the touch because the alpha particles it emits deposit their energy inside the sample, and that steady bombardment slowly damages the crystal lattice of stored plutonium over the years.

Actinide chemistry and oxidation states

The feature that most separates actinide chemistry from lanthanide chemistry is the spread of oxidation states. Lanthanides sit almost entirely at +3.

The early actinides, whose 5f electrons can join in bonding, reach far higher. This wide valence range peaks in the middle of the series and then narrows again.

Plutonium is the extreme case: it can hold four different oxidation state values in the same solution at once. The high states don't exist as bare ions. Uranium(VI) appears as the linear uranyl ion, UO₂²⁺, and neptunium and plutonium form similar "-yl" ions.

From americium onward, the late actinides settle at a valence of +3 and behave almost exactly like lanthanides, which makes them hard to separate from one another. Nobelium is the odd one out with its preferred +2 oxidation state, the result of a completely filled 5f shell.

Reactivity and complex formation

In bulk, the actinides react with air, hot water, dilute acids and hydrogen, and they act as strong reducing agents. In solution their chemistry is dominated by complex formation: the small, highly charged ions bind ligands such as fluoride, carbonate, phosphate and organic chelators very tightly. Stability generally follows charge density, with An⁴⁺ forming the strongest complexes, then AnO₂²⁺, An³⁺ and AnO₂⁺.

Actinide compounds: oxides, hydroxides and salts

The most important actinide compounds are the oxides. Uranium dioxide (UO₂), pressed into ceramic pellets, is the fuel in most of the world's reactors, and thorium dioxide has one of the highest melting points of any oxide, about 3,350 °C.

The hydroxides are poorly soluble and drop out of solution as the pH rises, which is one way to pull these metals out of a mixture. Halides matter just as much: the fluorides are insoluble in water and are reduced to metal with calcium or magnesium, while uranium hexafluoride, a gas just above room temperature, is what enrichment plants spin in their centrifuges.

Actinides vs lanthanides

The two f-block rows look like twins on the chart, but they behave quite differently. The lanthanides are mostly stable, found together in ore, and keep an almost fixed valence of +3. The actinides are all radioactive and mostly synthetic, and their chemistry ranges far wider.

Among the lanthanides, only promethium lacks a stable isotope. The lanthanides plus scandium and yttrium are known as the rare earth metals.

The actinides are not normally counted in that group, even though thorium and uranium often share the same ores. Both series show a contraction across the row, but the lanthanides are separated with difficulty only because they are alike, while separating actinides is also complicated by their radioactivity.

Where actinide ores occur and how the metals are extracted

Uranium's main mineral is uraninite, an oxide that also forms the pitchblende variety, and it is mined mostly in Kazakhstan, Canada, Namibia and Australia. Thorium comes chiefly from monazite, a phosphate mineral in beach sands that is mined primarily for its lanthanides. Most uranium and thorium minerals contain a little of each other, so the two actinides are often recovered together.

Getting the metals out of these ores is a multi-step process. Uranium ore is crushed and leached with sulfuric acid or, when the rock is rich in carbonates, with sodium carbonate solution. The dissolved uranium is then purified by solvent extraction or ion exchange and precipitated as "yellowcake".

Thorium is dissolved from monazite with hot acid or caustic soda and separated from the rare earths by careful pH control and extraction. The pure oxides or fluorides are finally reduced to metal with calcium or magnesium at high temperature.

Recovering uranium and plutonium from spent nuclear fuel

Reactor fuel that has finished its run still holds about 95% of its uranium, roughly 1% plutonium and smaller amounts of neptunium, americium and curium. The PUREX process (plutonium uranium reduction extraction) dissolves the used pellets in nitric acid and pulls the uranium and plutonium into tributyl phosphate dissolved in kerosene, leaving most fission products in the water.

Adjusting the plutonium oxidation state then splits the plutonium from the uranium. The "minor" actinides left behind dominate the long-term heat and radiotoxicity of high-level waste.

Uses of actinides in energy, medicine and industry

Most actinides are too scarce or short-lived to be useful outside nuclear chemistry labs, but a handful do important work. Uranium, thorium and plutonium release energy on a scale no chemical reaction can match, and a few isotopes of americium, curium, californium and actinium have niche jobs that nothing else can do as well.

Nuclear reactors, power and nuclear weapons

Uranium-235 is the main fuel of nuclear reactors. When a slow neutron strikes it, the nucleus undergoes nuclear fission, splitting into two lighter atoms, releasing about 200 MeV and two or three new neutrons that keep the chain reaction going. One kilogram of uranium-235 releases as much heat as burning roughly 2,700 tonnes of coal, which is why nuclear power supplies close to a tenth of the world's electricity with very little greenhouse gas.

Plutonium-239, bred from uranium-238 inside every reactor, is also fissile and is recycled as mixed-oxide nuclear fuel in some countries. Thorium can't sustain a chain reaction itself, but it breeds fissile uranium-233, so it is studied as a future source of nuclear energy. The same fission physics lies behind nuclear weapons, which use highly enriched uranium-235 or plutonium-239, and that dual use is why the trade in these metals is so tightly controlled.

Actinides in nuclear medicine

In nuclear medicine, actinium-225 is one of the most promising isotopes for targeted alpha therapy. Attached to a molecule that seeks out cancer cells, it releases a series of alpha particles that travel only a few cell widths, killing the tumour while sparing nearby tissue.

This form of radiation therapy is already in clinical trials for prostate cancer and neuroendocrine tumours, where it could make cancer treatment more precise. Californium-252 neutrons have also been used to treat some cervical and brain cancers.

Smoke detectors, spacecraft and industrial uses

The actinide you are most likely to have at home is americium-241: a tiny sealed source of a fraction of a microgram ionizes the air inside most household smoke detectors, and smoke particles interrupt that current. Plutonium-238 gives off steady heat for decades, so it powers radioisotope thermoelectric generators on deep-space probes such as Voyager and New Horizons and on the Curiosity and Perseverance rovers; small cells of it even ran some early cardiac pacemakers in the 1970s. Curium-244 supplied the alpha particles in the X-ray spectrometers aboard several Mars rovers.

Californium-252 is the standard compact neutron source, used to start up reactors, find oil and water in boreholes with neutron moisture gauges and inspect parts by industrial radiography, since neutrons pass through metal that blocks X-rays. Thorium oxide once raised the refractive index of high-end camera lenses, and uranium oxide coloured decorative glass and ceramics yellow-green before both uses faded out.

Choosing between actinides for a job

Picking among actinides comes down to the half-life and decay type of their isotopes, then to what you can legally obtain. Each use above is really a half-life decision.

Matching half-life to the task

Americium-241's 432-year half-life means a smoke detector source barely weakens over the device's ten-year life. Plutonium-238 loses only about 0.8% of its heat output per year, enough to keep a probe running for decades, yet it is short-lived enough to give off far more heat per gram than the long-lived isotopes.

Actinium-225 is useful in medicine precisely because its 9.9-day half-life ends the dose within weeks. A reactor needs the opposite: a fissile nucleus that is long-lived enough to store and ship for years.

Licensing limits on actinides

Bomb-grade material is accounted for gram by gram under IAEA safeguards, so you can't simply buy them. A household smoke detector's source is the rare exception: regulators exempt it from individual licensing because the sealed source is so small.

Actinide toxicity and radiation safety

Actinide toxicity comes from two directions. Chemically, actinides bind to proteins and bone. Radiologically, most of them are alpha emitters, and although alpha particles can't penetrate skin or even a sheet of paper, they do intense damage to cells when an atom is inhaled or swallowed.

That combination of radiation and chemical harm is what scientists mean by radiotoxicity. Plutonium and americium that reach the bloodstream lodge in bone and the liver and stay for decades.

That is why actinides are handled in glove boxes and hot cells, with safety rules focused on keeping particles out of the body rather than on external shielding. Over the long term, the environment is the larger concern: waste management has to isolate neptunium, plutonium and americium for tens of thousands of years, and their movement through soil and water, and any bioaccumulation in plants or animals, is studied closely at former weapons sites. Some soil bacteria help here by reducing soluble uranium(VI) to insoluble uranium(IV), locking these actinides in place underground.

Are actinides more toxic than heavy metals?

Weight for weight, the alpha-emitting isotopes are usually far more dangerous than ordinary heavy metals such as lead, because the radiation does damage that chemistry alone does not. Natural uranium is an exception: its radioactivity is so weak that its main hazard is chemical damage to the kidneys rather than radiation. An alpha emitter that settles in bone stays for decades, which is why a single inhaled particle matters more than its mass suggests.