Actinides

The actinides are a row of 15 chemical elements in the periodic table with atomic numbers 89 (actinium) through 103 (lawrencium). They lie beneath the lanthanides in the f‑block (often shown as a separate bottom row) and their atoms fill the 5f electron subshell as one moves from Ac to Lr. All actinides are radioactive (many highly so) and most are unstable and hard to handle. Only the lighter actinides (up through uranium) occur naturally in appreciable amounts; the heavier ones are made synthetically in reactors or particle accelerators. The actinides are famous for their roles in nuclear chemistry and energy: uranium and plutonium serve as reactor fuels and in nuclear weapons, while other actinides are used as neutron sources, radiation emitters, and in scientific research. Like the lanthanides, actinides typically form +3 oxidation states, but they can exhibit a variety of valences. Their high radioactivity and heavy masses give them unique properties, and their study has been central to advancing nuclear science.

List

Actinium (Ac, Atomic #89) – First of the Actinides

Main article: Periodic table/Actinides/Actinium

Actinium is the namesake of the actinide series, the row of 15 f-block elements from Ac to Lr in period 7. It is a silvery metal that is so intensely radioactive that a chunk of actinium will glow blue in the dark from ionized air. Actinium was discovered in 1899 in pitchblende ore (the same uranium-rich mineral where the Curies found radium). As the first actinide, actinium behaves somewhat like the rare-earth lanthanides above it, but with a twist: it’s about 150 times more radioactive than radium, making it quite dangerous to handle. Actinium has virtually no commercial uses due to its scarcity and radioactivity, but it has seen limited application as a neutron source – mixtures of actinium and beryllium can emit neutrons for research purposes. In recent years, certain isotopes like actinium-225 have gained attention for potential cancer treatment in targeted radiotherapy (using its alpha radiation to destroy cancer cells), but these medical uses are still experimental. Overall, actinium’s main role is as a scientific curiosity and a stepping stone into the actinide series, showcasing the increasing radioactivity that defines period 7.

Thorium (Th, Atomic #90) – A Fertile Nuclear Fuel Candidate

Main article: Periodic table/Actinides/Thorium

Thorium is a naturally occurring actinide metal that, unlike many of its heavier neighbors, is moderately abundant in the Earth’s crust. It is a dense, silver-grey metal and is slightly radioactive (its most stable isotope, thorium-232, has a half-life comparable to the age of the Earth). Historically, thorium found a unique use in gas lantern mantles – tiny fabric nets in camping lanterns were impregnated with thorium oxide, which glows bright white when heated, providing illumination. This was once a common source of light in the era before electric flashlights. Thorium’s ability to emit electrons when heated also made it useful as a material in certain vacuum tubes and welding electrodes.

However, the most significant interest in thorium today is its potential as an alternative nuclear fuel. Thorium is “fertile,” meaning it can be converted inside a reactor into uranium-233, a fissile fuel for nuclear reactors. Some countries have researched thorium-based nuclear reactors as a potentially safer or more abundant option than conventional uranium fuel. While thorium reactors are not yet mainstream, the element remains an intriguing candidate for future nuclear technology. In nature, thorium is the second-most abundant actinide after uranium and is found in minerals like monazite. It decays very slowly, contributing to background radiation. Overall, thorium bridges the gap between the relatively stable heavy elements and the increasingly synthetic realm of heavier actinides.

Protactinium (Pa, Atomic #91) – Rare and Mysterious

Main article: Periodic table/Actinides/Protactinium

Protactinium is one of the rarest and most chemically complex naturally occurring elements. Its name, meaning “parent of actinium,” comes from the fact that actinium is produced when protactinium decays. This element is a member of the actinide series and is vanishingly scarce in nature – found only in trace amounts within uranium ores. Because protactinium is highly radioactive and very hard to isolate (it took until 1934 for scientists to isolate a pure sample), it has no significant practical uses. In the early days of radiochemistry, protactinium had a temporary name “brevium” (for its short-lived isotope), but when a longer-lived isotope was found, it was renamed protactinium.

Chemically, protactinium is notable for exhibiting multiple oxidation states and forming a variety of compounds (showing both +5 and +4 states, among others), which hinted at the broader chemical flexibility of actinides compared to the lanthanides. Yet, due to its scarcity and the extreme precautions needed to handle it safely, protactinium is mainly of interest to scientists. It is often described as one of the more “mysterious” elements. The average person is unlikely to ever encounter protactinium – which is probably for the best, given its high radio-toxicity. Its most important role is essentially to exist as an intermediate in the decay of uranium-235 into actinium-227, helping scientists understand radioactive decay chains in nature.

Uranium (U, Atomic #92) – Heavy Metal Powerhouse

Main article: Periodic table/Actinides/Uranium

Uranium is perhaps the most famous of the actinides and one of the best-known elements of period 7. A dense, hard metal with a silvery appearance (that tarnishes black in air), uranium has been used by humans since long before its radioactivity was understood – for instance, uranium compounds were used to tint glass and ceramics a vivid yellow-green (uranium glass) in the 19th century. In the modern world, uranium’s significance comes from its nuclear properties. Uranium-235 is the primary fuel for nuclear power reactors and was the material used in the first atomic bomb. In a reactor, uranium atoms undergo fission (splitting) to release enormous energy, which is harnessed to generate electricity. Naturally occurring uranium (mostly U-238 with a bit of U-235) is mined from ores like pitchblende and processed into fuel rods for reactors around the world.

Uranium is also key to nuclear weapons: the bomb dropped on Hiroshima in 1945 used highly enriched uranium. Additionally, the decay of uranium in Earth’s crust is the source of radon gas and contributes significantly to background radiation. In terms of abundance, uranium is the last element found in substantial quantities in nature (thorium and uranium are the only actinides that exist in sizable amounts on Earth). Beyond its nuclear uses, uranium’s dense metal has been used in armor-piercing ammunition and counterweights, though its toxicity and radioactivity make such uses controversial. In summary, uranium stands out as a pivot point of period 7 – a bridge between natural chemistry and the realm of man-made elements, and a cornerstone of nuclear science and technology.

Neptunium (Np, Atomic #93) – The First Transuranium Element

Main article: Periodic table/Actinides/Neptunium

Neptunium is the element immediately following uranium on the periodic table, and thus the first element that is strictly synthetic in origin (transuranium means “beyond uranium”). It was discovered in 1940 by scientists who bombarded uranium with neutrons, creating neptunium-239, which quickly beta-decayed to plutonium-239. The element was named after the planet Neptune, in keeping with uranium being named after Uranus – carrying on the tradition of planets for successive elements. Neptunium is a silvery metal that is radioactive and not found free in nature (though minute traces form in uranium ores via rare nuclear reactions).

In nuclear reactors, neptunium is produced as a byproduct when uranium fuel absorbs neutrons. One isotope, neptunium-237, has a long half-life of over 2 million years and is present in spent nuclear fuel; this has prompted studies into how to handle or utilize this waste product. There are few practical uses for neptunium, but one niche application is in specialized detectors – neptunium can be used in devices to detect high-energy neutrons. It has also been used to produce plutonium-238 when bombarded with neutrons (plutonium-238 is valued for space probe power sources). Overall, neptunium is primarily of interest for scientific research on heavy element behavior and for managing nuclear materials. Its chemistry is quite complex, with multiple possible oxidation states in solution (ranging from +3 to +7), demonstrating an increasing departure from simpler periodic trends as we move further into period 7.

Plutonium (Pu, Atomic #94) – Fuel of Bombs and Spacecraft

Main article: Periodic table/Actinides/Plutonium

Plutonium is a heavyweight element with an outsized reputation. Discovered in 1940–41 at the University of California, Berkeley, plutonium was the second transuranium element made (after neptunium). It is named after Pluto (then considered the ninth planet) to follow the planet naming sequence. Plutonium is a silvery metal that tarnishes to yellow or olive green. It is highly radioactive and radiates heat; a chunk of plutonium will warm itself from ongoing radioactive decay. The element is most famous as a key ingredient in nuclear weapons – the “Fat Man” bomb dropped on Nagasaki in 1945 had a core of plutonium-239. Plutonium-239 is fissile, meaning it can sustain a nuclear chain reaction, which also makes it useful as a nuclear reactor fuel (mixed with uranium in MOX fuel, for example).

Beyond weaponry and reactors, plutonium has another important application: powering deep-space missions. The isotope plutonium-238 emits steady heat and has a half-life of 88 years, making it ideal for radioisotope thermoelectric generators (RTGs). These nuclear batteries have powered spacecraft like Voyager, Curiosity, and New Horizons when solar power was impractical. Only a few kilograms of Pu-238 can keep a spacecraft running for decades. On Earth, plutonium’s applications are limited due to its toxicity and radioactivity – it’s a heavy metal poison and emits dangerous alpha radiation. The element has several isotopes with different uses and hazards; for instance, Pu-240’s tendency to undergo spontaneous fission complicates the handling of reactor-grade plutonium. Plutonium’s discovery and utilization changed history, ushering in the nuclear age. It exemplifies the double-edged nature of period 7 elements: powerful energy sources and destructive potential wrapped in an unstable metallic package.

Mushroom cloud over Nagasaki after the plutonium-based “Fat Man” atomic bomb was detonated on August 9, 1945. Plutonium-239, produced synthetically in nuclear reactors, was the core fuel for this bomb and remains a key fissile material in nuclear arsenals

Americium (Am, Atomic #95) – Guardian in Smoke Detectors

Main article: Periodic table/Actinides/Americium

Americium is a synthetic actinide metal that has found its way into millions of households – in the form of tiny specks inside smoke detectors. Discovered in 1944 during the Manhattan Project (by Glenn Seaborg’s team at Chicago), americium was named after the Americas (by analogy to its lanthanide homologue europium). The most common isotope, americium-241, emits alpha particles and a low-energy gamma ray; this property is used in ionization smoke alarms. A small 0.3 microgram Am-241 source in a smoke detector ionizes air in a sensing chamber; if smoke enters, it disrupts the ionization and triggers the alarm. This clever application is by far the most widespread use of any transuranium element.

Outside of smoke detectors, americium has a few specialized uses. Am-241’s gamma emission can be used as a portable source for gauging thickness/density in industrial radiography or quality control devices. Americium’s chemistry is somewhat easier to manage than plutonium’s, and kilogram quantities of Am have been produced by reprocessing nuclear reactor fuel. However, americium is still highly radioactive and toxic, requiring careful handling. It can exist in multiple oxidation states in solution, but generally Am behaves like other actinides, readily forming oxides and halides. Americium’s presence in homes (albeit in infinitesimal amounts) is a fascinating quirk – an element of the atomic age quietly performing a lifesaving task in everyday life.

Curium (Cm, Atomic #96) – Homage to the Curies, and Rover Rock Analyst

Main article: Periodic table/Actinides/Curium

Curium, named in honor of pioneering scientists Marie and Pierre Curie, is another synthetic actinide discovered by Seaborg’s team (in 1944). It is a hard, dense, radioactive metal that glows red in the dark (from its intense radioactivity heating the air). While curium itself doesn’t have everyday uses, one of its isotopes has a notable role in space exploration: Curium-244 is used in the alpha-particle X-ray spectrometers (APXS) on Mars rovers and other landers. These instruments deploy curium’s alpha particles to bombard rocks and soil; the induced X-rays reveal the elemental composition of Martian samples. In essence, curium helps robotic explorers “taste” the chemistry of other worlds.

Curium-244 is useful for this purpose because it emits a lot of alpha particles and has a manageable half-life (~18 years) – long enough to last a mission, short enough to be active. Outside of APXS devices, curium (especially ^242Cm and ^244Cm) can serve as a heat source in radioisotope power units, though plutonium-238 is more commonly used. Curium is highly radioactive; ^244Cm generates about 3 watts of heat per gram. In the lab, curium’s compounds show beautiful red or yellow hues and it exhibits a range of oxidation states (most commonly +3). There are no consumer applications for curium – it is far too rare and dangerous – but its specialized scientific uses make it an interesting example of how even short-lived period 7 elements contribute to technology. The discovery of curium filled in another link of the actinide series, confirming the predictions of the periodic table and further cementing the Curie legacy in elemental form.

Berkelium (Bk, Atomic #97) – A Berkeley Creation

Main article: Periodic table/Actinides/Berkelium

Berkelium is a synthetic actinide named after the University of California, Berkeley, where it was first produced in 1949. By bombarding americium with helium nuclei in a cyclotron, scientists created berkelium – adding another member to the growing transuranium family. Berkelium is a metal with no stable isotopes; its most stable isotope (^247Bk) has a 1,380-year half-life. Only very small amounts of berkelium have ever been made, and it has remained a laboratory curiosity with no practical uses outside scientific research.

One interesting footnote: a sample of berkelium-249 (produced at great expense in a nuclear reactor over many months) was used in 2010 as a target to discover element 117, tennessine. This shows how berkelium and other actinides serve as stepping stones to create even heavier elements in period 7. Chemically, berkelium behaves like a typical actinide metal (primarily exhibiting a +3 oxidation state in compounds). It’s intensely radioactive, emitting beta particles that quickly decay into other elements (for instance, ^249Bk decays to californium). Berkelium’s rarity and short life mean it’s not encountered outside of specialized facilities. Nonetheless, the synthesis and identification of berkelium were significant in the 20th-century race to extend the periodic table – literally putting “Berkeley” into the periodic chart.

Californium (Cf, Atomic #98) – Neutron Shooter

Main article: Periodic table/Actinides/Californium

Californium, named for the state of California (and University of California) where it was discovered in 1950, is one of the more useful transfermium elements. It is best known for a specific isotope, californium-252, which is a powerful neutron emitter. A small quantity of ^252Cf can release an enormous flux of neutrons via spontaneous fission – in fact, microgram amounts of Cf can replace much larger neutron sources. This makes californium very valuable in certain applications: it is used in portable neutron sources for purposes like neutron radiography and activation analysis. For example, californium-based devices can help detect gold or silver ores (by activating elements in a sample and analyzing the gamma rays emitted), or to inspect airline luggage and cargo for explosives. In medicine, californium’s neutron emissions have been used experimentally in neutron therapy to treat certain cancers (such as cervical cancer), where high-energy neutrons can destroy tumor cells that are resistant to other radiation.

Californium must be produced in nuclear reactors or particle accelerators; dedicated facilities can produce a few milligrams per year by bombarding curium targets with neutrons. ^252Cf has a half-life of about 2.6 years, so it needs regular replenishment. Handling californium is challenging – its intense neutron radiation makes it extremely dangerous without heavy shielding. In chemistry, californium behaves like other late actinides (commonly Cf(III) in compounds), but studying its chemistry requires specialized labs. Despite its hazards, californium exemplifies how the far end of period 7 can contribute in niche but critical ways, from scanning materials to saving lives in medicine.

Einsteinium (Es, Atomic #99) – H-Bomb Afterglow

Main article: Periodic table/Actinides/Einsteinium

Einsteinium is a highly radioactive metal named after the famed physicist Albert Einstein. It was first detected in the debris of the Ivy Mike hydrogen bomb test in 1952, when scientists sifted through fallout and found traces of unknown element 99. The difficulty of obtaining einsteinium is extreme – it can only be made in significant amounts by blasting heavy elements with neutrons in a high-powered nuclear reactor, and even then you get only microgram quantities. Einsteinium glows blue in the dark from its own radioactivity (like its cousins californium and curium).

With such scarcity and short half-life (the most stable isotope ^252Es lasts about 472 days), einsteinium has no practical uses outside of scientific study. In recent years, chemists have managed to study einsteinium’s basic chemistry by creating specialized compounds in microscopic quantities, shedding light on how this element forms bonds. These experiments help refine our understanding of the heavy actinides. Einsteinium was crucial historically as a stepping stone: by accumulating einsteinium, researchers in the 1950s were able to synthesize mendelevium (element 101). The discovery of einsteinium and fermium in thermonuclear fallout was a Cold War scientific feat, done secretly and published once security clearances allowed. For the general public, einsteinium is mostly a name – a tribute to Einstein – since one cannot encounter it in everyday life. Its presence underscores both the power of nuclear reactions (creating new elements in bomb blasts) and the dedication of scientists who study these ephemeral atoms.

Fermium (Fm, Atomic #100) – 100th Element from the Atomic Fireball

Main article: Periodic table/Actinides/Fermium

Fermium, element 100, was another product of the 1952 Ivy Mike hydrogen bomb test, discovered alongside einsteinium in the fallout. It is named after Enrico Fermi, the Italian-American physicist who was instrumental in the development of the first nuclear reactor. Fermium is even harder to accumulate than einsteinium; only minuscule amounts have ever been isolated. Its most stable isotope (^257Fm) has a 100-day half-life. Fermium’s existence was kept classified for several years due to Cold War secrecy (since its discovery revealed information about the bomb’s yield).

Like einsteinium, fermium has no uses outside fundamental research. It has been produced in laboratories by intense neutron bombardment (for example, in nuclear reactor “pulse” experiments that create a flurry of heavy elements). Studying fermium is challenging, but its chemistry marks the end of an era – beyond fermium, conventional solution chemistry becomes nearly impossible because available samples are too small and short-lived. Fermium mainly exists to complete the story of the actinides. It demonstrated that the periodic table extended to 100 and beyond, and its discovery in bomb debris highlighted the reach of human technology. For most people, fermium is an obscure element; it doesn’t enter daily life. But hitting the 100th element was a symbolic milestone, and naming it for Fermi honored one of the great minds of nuclear physics.

Mendelevium (Md, Atomic #101) – Honoring the Table’s Architect

Mendelevium is a synthetic actinide named after Dmitri Mendeleev, the Russian chemist who created the periodic table framework. It was first synthesized in 1955 by a team at Berkeley that included Glenn Seaborg and Albert Ghiorso. They made mendelevium by bombarding einsteinium-253 with alpha particles (helium nuclei) in a cyclotron – an experiment that produced a few atoms of element 101. This was a remarkable achievement, as it required new techniques to identify just one atom at a time. The naming of mendelevium for Mendeleev broke the tradition of using place names or mythological references, directly honoring a scientist (one who had died decades earlier).

In terms of properties, mendelevium behaves as a typical actinide chemically (primarily showing a +3 oxidation state in solution). Only trace amounts of mendelevium have ever existed, so like the previous few elements, it has no practical applications outside scientific research. Its main importance was to further fill out the actinide series and to demonstrate the capabilities of mid-20th-century nuclear science. The creation of mendelevium was also a Cold War scientific race milestone (the Americans at Berkeley beat Soviet laboratories to element 101). For a general audience, mendelevium’s significance is largely historical and symbolic: it connected back to the origins of the periodic law (via Mendeleev’s name) and proved that even 90 years after Mendeleev’s initial table, new elements were still being added as predicted.

Nobelium (No, Atomic #102) – A Contested Discovery

Main article: Periodic table/Actinides/Nobelium

Nobelium, element 102, is named after Alfred Nobel (of Nobel Prize fame). Its discovery in the late 1950s was controversial, with teams in Sweden, the United States, and the Soviet Union all contributing to its identification. Eventually, the name Nobelium was adopted to honor Nobel. Like its neighbors, nobelium is an artificial actinide with no stable isotopes (the longest-lived is ^259No with a 58-minute half-life). Producing nobelium requires advanced particle accelerators bombarding heavy targets (such as curium) with ions (like carbon or neon).

Chemists have studied nobelium in tracer experiments and found that it can exhibit a +2 oxidation state in addition to the +3 common to actinides – a subtle but interesting deviation that hints at the changing electron configurations as the actinide series ends. This +2 state makes nobelium somewhat resemble alkaline earth metals in behavior, showing how late actinides start to deviate from the earlier trend. In everyday terms, nobelium has no uses; it exists only as a result of high-energy physics experiments. The saga of its discovery (initially claimed by a Swedish group in 1957, then re-evaluated) and eventual confirmation by Soviet researchers in Dubna in 1966 is a chapter in scientific detective work. For the public, nobelium is mostly known through its connection to the Nobel name – a reminder that even these short-lived elements carry legacies of scientific heritage.

Lawrencium (Lr, Atomic #103) – Last of the Actinides

Main article: Periodic table/Actinides/Lawrencium

Lawrencium concludes the actinide series and thus the entire f-block of the periodic table. It is named after Ernest O. Lawrence, the inventor of the cyclotron particle accelerator (and a key figure in early nuclear science). Discovered in 1961 at Berkeley, lawrencium’s synthesis involved bombarding californium with boron nuclei. Lawrencium is highly unstable (most isotopes have half-lives measured in seconds), and only a handful of atoms have ever been studied. As the last actinide, lawrencium is of interest mainly for completing the series; its placement in the periodic table has sparked some discussion because its electron configuration is unusual. In fact, lawrencium’s ground-state electron arrangement doesn’t fill the f-subshell as one might expect, but instead places an electron in the d-orbital, making it sometimes considered a transition metal. This quirk is an example of how relativistic effects and electron correlation impact super-heavy atoms.

Practically, lawrencium has no applications outside scientific experiments. It’s the capstone of the actinides – after Lr, the periodic table continues with element 104 in the d-block. The discovery of lawrencium was also contested between Berkeley and Dubna (reflecting the Cold War competition in element discovery). Ultimately, the name lawrencium was accepted, immortalizing Lawrence. To the wider world, lawrencium is a very obscure element; it decays so fast that even detecting its chemical properties is challenging. Yet, it marks an important boundary in the periodic table: the end of one series and the start of the next, showing that by element 103, scientists had successfully filled all the slots in period 7’s actinide row.