Exploring the superheavy, human-made inhabitants of the periodic table created through decades of scientific innovation
Look at the periodic table hanging on the wall of any science classroom. For most, it ends with element 92, Uranium, a heavy metal known for its radioactivity. But for nearly a century, scientists have been voyaging into uncharted territory, beyond uranium, creating new elements that are not found in nature.
These are the transuranium elements—the superheavy, human-made inhabitants of the periodic table. This half-century-long (and counting) quest is more than just a race for the next number; it's a fundamental test of our understanding of matter, pushing the very limits of the chemical universe.
The story begins in 1934, when physicist Enrico Fermi attempted to create element 93 by bombarding uranium with neutrons. While he was on the right track, the true discovery came six years later. In 1940, a team at the University of California, Berkeley, led by Edwin McMillan and Philip Abelson, definitively proved the existence of Neptunium (Element 93), named after the planet beyond Uranus . This was the first proof that we could extend the periodic table.
Why aren't these elements found in nature? The answer lies in nuclear stability. As atomic nuclei get larger and more packed with protons (which repel each other), they become increasingly unstable. All elements heavier than Plutonium (Element 94) have half-lives so short—some lasting only milliseconds—that any that may have been formed during the birth of the solar system have long since decayed away. To study them, we must create them ourselves.
Proton repulsion makes heavy nuclei unstable, limiting natural occurrence of transuranium elements.
Scientists create these elements through nuclear reactions in particle accelerators and reactors.
While Neptunium was the first step, the discovery of Plutonium (Element 94) was the giant leap that truly opened the door to the transuranic world. The key experiment was performed by a team led by Glenn T. Seaborg at Berkeley in late 1940-1941 .
A sample of Uranium-238 was placed inside the cyclotron.
The cyclotron fired a nucleus of Deuterium (a hydrogen isotope with one proton and one neutron) at the uranium target.
When a deuteron struck a U-238 nucleus, a nuclear reaction occurred. The uranium nucleus captured the deuteron, creating a new, highly unstable nucleus.
This unstable nucleus almost immediately emitted a beta particle (a high-energy electron), transforming a neutron into a proton. This process increased the atomic number from 92 (Uranium) to 94, creating a new element.
The team used chemical separation techniques to isolate the new substance. They proved it was chemically distinct from all known elements, particularly uranium and neptunium, confirming the creation of Plutonium.
The successful creation and identification of Plutonium-238 (and later the fissile Pu-239) was a monumental achievement. It proved that the "island" of transuranium elements was not just a single new element, but an entire new archipelago waiting to be discovered.
More pragmatically, the fissionable properties of Pu-239 immediately made it a crucial material for nuclear technology, both for energy and, at the time, for weapons. For his pivotal role in the discovery of multiple transuranium elements, Glenn T. Seaborg was awarded the Nobel Prize in Chemistry in 1951.
| Element Name & Number | Year Discovered | Discoverers | How it was Made |
|---|---|---|---|
| Neptunium (93) | 1940 | McMillan & Abelson | U-238 absorbed a neutron to become U-239, which decayed into Np-239. |
| Plutonium (94) | 1940-41 | Seaborg, McMillan, et al. | U-238 was bombarded with deuterons in a cyclotron. |
| Americium (95) | 1944 | Seaborg, James, et al. | Pu-239 absorbed two neutrons in a nuclear reactor to become Pu-241, which decayed into Am-241. |
| Curium (96) | 1944 | Seaborg, James, et al. | Pu-239 was bombarded with alpha particles in a cyclotron. |
Creating the first few transuranium elements was difficult, but making elements beyond 100 (Fermium) requires even more powerful tools and clever strategies.
Modern heavy-ion accelerators are the primary forges. They can accelerate ions of medium-weight elements to velocities up to 10% the speed of light.
Thin, stable foils of heavy elements like Lead (Pb) or Californium (Cf) are used as stationary targets.
Ions of lighter, neutron-rich elements like Calcium-48 or Titanium-50 are used as the "bullets" to be fired at the target.
After the violent collision, a complex mixture of particles flies out. Magnetic and physical separators are used to isolate the one or two atoms of the new element.
Ultra-sensitive detectors surround the target area. They can measure the unique decay pattern—the "fingerprint"—of the new element.
| Element Name & Number | Year Discovered | Synthesis Reaction |
|---|---|---|
| Mendelevium (101) | 1955 | Es-253 + α (alpha particle) → Md-256 |
| Nobelium (102) | 1966 | Cm-246 + C-13 → No-254 |
| Oganesson (118) | 2002/2006 | Cf-249 + Ca-48 → Og-294 |
As scientists pushed further, a fascinating theoretical prediction emerged: the Island of Stability. This is a hypothesized region of the table of nuclides where superheavy elements may have half-lives much longer than their neighbors—perhaps minutes, days, or even millions of years.
This stability is predicted to come from "magic numbers" of protons and neutrons that form complete nuclear shells, making the nucleus remarkably robust. The quest to reach this island drives much of modern heavy element research, promising new insights into the fundamental forces that hold matter together.
Illustrates the challenge of reaching the Island of Stability
| Element | Atomic Number | Most Stable Isotope | Half-Life |
|---|---|---|---|
| Plutonium | 94 | Pu-244 | 80 million years |
| Curium | 96 | Cm-247 | 16 million years |
| Californium | 98 | Cf-251 | 900 years |
| Fermium | 100 | Fm-257 | 100 days |
| Nobelium | 102 | No-259 | 58 minutes |
| Seaborgium | 106 | Sg-269 | 14 seconds |
| Copernicium | 112 | Cn-285 | 30 seconds |
| Oganesson | 118 | Og-294 | 0.7 milliseconds |
A theoretical region where superheavy elements with specific "magic numbers" of protons and neutrons may exhibit significantly longer half-lives than neighboring elements.
The journey beyond uranium is a testament to human ingenuity and our relentless drive to explore the unknown. From the first milligram of plutonium to the fleeting atoms of oganesson, the creation of the transuranium elements has reshaped the periodic table from a static map of nature into a dynamic canvas of human achievement.
This half-century quest has not only filled in the blanks at the bottom of the chart but has also provided profound insights into the core of what makes up our universe, proving that the limits of matter are defined only by the limits of our imagination and perseverance. The voyage to the Island of Stability continues, and the next chapter in this incredible story is yet to be written.
From Neptunium in 1940 to Oganesson in 2006, the quest continues.
Scientists worldwide continue the hunt for elements beyond oganesson.