A half-century journey into the synthetic elements that expanded our periodic table
For centuries, uranium stood as the heaviest known element on the periodic table, a seemingly impenetrable frontier in the landscape of matter. Then, in the middle of the 20th century, a revolution began in scientific laboratories. For over 150 years, uranium was the heaviest known element, but the development of nuclear science opened the door to a strange new world—the world of the transuranium elements 4 . These are the elements beyond uranium, with atomic numbers greater than 92, which are unstable and rarely found in nature 1 3 . Their synthesis over the past half-century has not only expanded the periodic table but has also fundamentally reshaped our understanding of the atom and the forces that hold it together.
The journey into transuranium territory began not with a success, but with a misinterpretation. In 1934, Enrico Fermi and his team in Rome bombarded uranium with neutrons, believing they had created the first element beyond uranium. Instead, they had inadvertently discovered nuclear fission—the splitting of the atom 3 . It wasn't until 1940 that the first transuranium element was positively identified. At the University of California, Berkeley, physicists Edwin McMillan and Philip Abelson bombarded uranium oxide with neutrons from a cyclotron 4 . They produced a new element with atomic number 93, which they named neptunium, after the planet Neptune, which lies beyond Uranus in our solar system 1 3 .
The first transuranium element discovered in 1940 by McMillan and Abelson at Berkeley.
Discovered later in 1940 by Glenn T. Seaborg's team, with considerable practical applications.
The discovery of the next element followed rapidly. Later in 1940, a team led by Glenn T. Seaborg created plutonium (element 94) by bombarding uranium with deuterons (the nuclei of deuterium atoms) 4 . This established a pattern of naming these new elements after the planets, a tradition that would soon evolve. Plutonium-239 would soon prove to have considerable practical application, as its ability to undergo a fission chain reaction made it a powerful energy source for both nuclear weapons and nuclear power reactors 3 .
A pivotal moment in this journey was Seaborg's "actinide hypothesis." He proposed that a new series of elements, akin to the lanthanide series (elements 58-71), was being produced. This new actinide series (elements 89-103) included thorium, uranium, and the new transuranium elements 4 . This conceptual breakthrough provided a much-needed framework for the periodic table and guided the search for new elements.
"The actinide concept fundamentally reshaped our understanding of the periodic table and guided the discovery of new elements."
The neutron-capture path hit a wall at fermium (element 100). The short half-life of fermium-258—a mere 370 microseconds—precludes the production of heavier elements by this method in reactors 3 . To venture further, scientists turned to particle accelerators and new techniques 4 .
First transuranium element discovered
McMillan & AbelsonKey element for nuclear applications
Seaborg's teamFirst post-war discoveries
Seaborg's teamNamed with regional pride
Berkeley LabScientists began using accelerators to fire light, charged particles (like helium nuclei, or alpha particles) at heavy element targets. For elements heavier than mendelevium, this evolved into using "heavy ions" (ions of atoms heavier than helium) as projectiles 3 . This presented a monumental challenge: the resulting elements were produced one atom at a time and often had half-lives of only minutes, seconds, or even milliseconds.
The 1955 synthesis of mendelevium (element 101) perfectly illustrates the ingenuity required for this new era of discovery. A team at Berkeley Lab led by Albert Ghiorso aimed to produce a new element by bombarding a target of einsteinium-253 with alpha particles (helium-4 ions) 4 .
The experimental procedure was a masterstroke of innovation 4 :
Atoms Detected: 17
Significance: First element identified on an "atom-at-a-time" basis
Team: Ghiorso, Harvey, Choppin, Thompson, Seaborg
The experiment was a success, but it underscored the painstaking nature of the work. The team detected seventeen atoms of mendelevium in total 4 . This was the first element to be identified on an "atom-at-a-time" basis and marked the last time the heaviest element was first identified by traditional chemical separation 4 . The successful use of the recoil method provided a powerful new tool that would be essential for discovering all subsequent transuranium elements.
Venturing into the transuranium realm requires a specialized arsenal of equipment and methods. The following details the essential "research reagents" and tools that have made these discoveries possible.
Machines that accelerate charged particles (ions) to high speeds, providing enough energy to overcome the repulsion between the projectile and the target nucleus, allowing them to fuse 4 .
Targets made of elements like curium or berkelium. Their thinness allows newly formed atoms to recoil out for collection 4 .
Methods like gas jets or conveyor belts that swiftly separate the newly synthesized, recoiling atoms from the target material, allowing their rapid study before they decay 4 .
Highly sensitive detectors that measure the energy of alpha particles emitted by the new elements. Each element and isotope has a characteristic alpha decay energy, serving as a fingerprint for identification 4 .
Chemical processes tailored to separate elements based on their predicted properties, often performed incredibly quickly to study short-lived atoms 4 .
The relentless push into heavier territory has added 26 named elements to the periodic table since uranium. The work, once dominated by Berkeley Lab, has expanded to include major laboratories worldwide, including the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, the GSI Helmholtz Centre in Germany, and RIKEN in Japan 1 .
This international effort has not been without controversy. The naming of new elements has often been a source of dispute, with different labs claiming priority for discoveries. For years, elements 104 and 105 were known by different names (rutherfordium and kurchatovium, hahnium and nielsbohrium) by American and Russian teams, respectively. IUPAC (International Union of Pure and Applied Chemistry) now adjudicates these disputes, but the debates can be intense, as seen with seaborgium (element 106), which was named for the still-living Glenn Seaborg 1 .
| Atomic Number | Name and Symbol | Year Discovered | Discoverers (Laboratory) | Longest-Lived Isotope Half-Life |
|---|---|---|---|---|
| 93 | Neptunium (Np) | 1940 | McMillan, Abelson (Berkeley) 4 | 2.1 million years 3 |
| 94 | Plutonium (Pu) | 1940 | Seaborg, Kennedy, Wahl, McMillan (Berkeley) 4 | 8.3×107 years 2 |
| 95 | Americium (Am) | 1944 | Seaborg, James, Morgan, Ghiorso (Berkeley) 4 | 7,400 years 2 |
| 96 | Curium (Cm) | 1944 | Seaborg, James, Ghiorso (Berkeley) 4 | 1.6×107 years 2 |
| 97 | Berkelium (Bk) | 1949 | Thompson, Ghiorso, Seaborg (Berkeley) 4 | 1,400 years 2 |
| 98 | Californium (Cf) | 1950 | Thompson, Street, Ghiorso, Seaborg (Berkeley) 4 | 900 years 2 |
| 99 | Einsteinium (Es) | 1952 | Ghiorso et al. (Berkeley/Argonne) 4 | 471.7 days 2 |
| 100 | Fermium (Fm) | 1952 | Ghiorso et al. (Berkeley/Argonne) 4 | 100.5 days 2 |
| 101 | Mendelevium (Md) | 1955 | Ghiorso, Harvey, Choppin, Thompson, Seaborg (Berkeley) 4 | 51.5 days 2 |
| 102 | Nobelium (No) | 1958 | Ghiorso, Sikkeland (Berkeley) 4 | 58 minutes 2 |
| 103 | Lawrencium (Lr) | 1961 | Ghiorso et al. (Berkeley) 4 | 10 hours 2 |
| 104 | Rutherfordium (Rf) | 1969 | Ghiorso et al. (Berkeley) 1 4 | 1.3 hours 2 |
| 105 | Dubnium (Db) | 1970 | Ghiorso et al. (Berkeley) 1 4 | 28 hours 2 |
| 106 | Seaborgium (Sg) | 1974 | Ghiorso et al. (Berkeley/Livermore) 1 4 | 3.1 minutes 2 |
As the atomic numbers increased, a general trend of decreasing half-lives emerged. However, nuclear theory predicts a fascinating possibility: an "island of stability." This theory suggests that nuclei with certain "magic" numbers of protons and neutrons (e.g., Z=114, 120, or 126 and N=184) would be exceptionally stable, with half-lives potentially reaching minutes, days, or even millions of years 1 2 . This prediction, first calculated by scientists at Berkeley Lab in the mid-1960s, has guided the search for new elements ever since and represents the next great frontier in this field 4 .
The "island of stability" is a theoretical region in the chart of nuclides where superheavy elements may have significantly longer half-lives than their neighbors. This stability is predicted to occur at specific "magic numbers" of protons and neutrons where nuclear shells are filled.
114, 120, 126
184
Minutes to Millions of Years
"The quest for the transuranium elements over the past half-century is more than a story of filling boxes on the periodic table. It is a testament to human curiosity and ingenuity..."
The quest for the transuranium elements over the past half-century is more than a story of filling boxes on the periodic table. It is a testament to human curiosity and ingenuity, involving groundbreaking theoretical insights, the development of powerful new technologies, and the performance of exquisite, atom-scale experiments. From the first traces of neptunium to the ongoing hunt for the island of stability, this journey has profoundly expanded our understanding of the fundamental building blocks of our universe.