A breakthrough in superheavy element chemistry that confirms the predictive power of the periodic table
Imagine conducting a chemical experiment where your target material does not exist in nature, can only be produced one atom at a time, and vanishes in less time than it takes to blink.
Elements with atomic numbers greater than 103 that do not occur naturally and must be synthesized in laboratories.
The compound Sg(CO)6, where a seaborgium atom is bonded to six carbon monoxide molecules.
This is the extraordinary challenge facing chemists who study superheavy elements like seaborgium (element 106). In 2014, a team of scientists achieved a seemingly impossible feat: they created and identified a compound of seaborgium called seaborgium hexacarbonyl (Sg(CO)₆) 1 . This breakthrough opened a new window into the behavior of matter at the very limits of existence, testing whether the periodic table's patterns hold true for elements that are both incredibly massive and fleeting.
To appreciate the significance of this achievement, one must understand the theoretical stakes. Seaborgium resides directly below tungsten in group 6 of the periodic table, which suggests it should exhibit similar chemical properties. However, seaborgium atoms are so massive that their inner electrons are predicted to move at speeds approaching the speed of light. This causes relativistic effects – a phenomenon where:
Group 6 elements in the periodic table
For superheavy elements, electrons move at significant fractions of light speed, causing measurable changes in atomic properties that must be accounted for in quantum chemical calculations.
Advanced computational models predicted that seaborgium should form a stable hexacarbonyl complex with properties similar to its lighter homologs if relativistic effects were properly included 2 .
These effects could cause seaborgium to behave differently than its lighter relatives, molybdenum and tungsten. The synthesis of Sg(CO)₆ was designed as a crucial test. Metal carbonyl complexes are fundamental in chemistry, and their stability and volatility provide a sensitive probe of electronic structure. Theoretical predictions suggested that if relativistic effects were properly accounted for, seaborgium should form a stable hexacarbonyl complex with properties very similar to those of molybdenum and tungsten hexacarbonyls.
The 2014 experiment, led by an international team, was a masterpiece of precision and miniaturization, conducted at the RIKEN Nishina Center for Accelerator-Based Science in Japan. The entire process was tailored to work with single atoms on a timescale of seconds.
Seaborgium atoms were created by bombarding a curium-248 target with a beam of neon-22 nuclei from a particle accelerator. This nuclear reaction produced seaborgium-265, an isotope with a half-life of about 8 seconds 1 .
The newly born seaborgium atoms were immediately separated from other reaction products and transported in a stream of helium gas. This gas was mixed with carbon monoxide and passed through a device cooled to -170 °C.
Under these cold, high-pressure conditions, the individual seaborgium atoms reacted with six carbon monoxide molecules each to form the volatile Sg(CO)₆ complex.
The gas stream carried the carbonyl complexes through a chromatography column lined with silicon dioxide and equipped with sensitive radiation detectors. The key measurement was adsorption enthalpy—how strongly the molecules stuck to the silica surface. By comparing this "retention time" with that of its lighter cousins, the team could identify the compound 4 .
Working with individual atoms presents unique challenges in detection and analysis that require specialized equipment and techniques.
With a half-life of just 8 seconds, all chemical processes had to occur rapidly before the seaborgium atoms decayed.
The synthesis of Sg(CO)₆ required a specialized arsenal of instruments and reagents, each designed to handle the unique challenges of single-atom chemistry.
| Tool/Reagent | Function in the Experiment |
|---|---|
| Particle Accelerator | Produces seaborgium atoms by fusing lighter atomic nuclei. |
| Gas Chromatography | Separates volatile compounds based on their different interactions with a surface. |
| Silicon Dioxide (SiO₂) Surface | Acts as the stationary phase in chromatography; its interaction with carbonyl complexes helps identify them. |
| Carbon Monoxide (CO) Gas | The reagent that reacts with seaborgium atoms to form the volatile carbonyl complex. |
| Helium Gas Stream | Transports single atoms and molecules through the apparatus without chemical interaction. |
| Radiation Detectors | Detect the decay of radioactive atoms, allowing scientists to pinpoint the location and time of decay events. |
Essential for creating superheavy elements that don't exist naturally by accelerating atomic nuclei to high speeds and colliding them with target materials.
Enables separation and identification of volatile compounds based on their interaction with a stationary phase, crucial for detecting the carbonyl complex.
Highly sensitive detectors that can identify the characteristic decay patterns of individual superheavy atoms, allowing their presence to be confirmed.
The experiment yielded a decisive result: the seaborgium complex interacted with the silicon dioxide surface in a manner nearly identical to its tungsten and molybdenum analogs. This was compelling evidence that the team had indeed created Sg(CO)₆ and that its volatility was very similar to that of the other group 6 hexacarbonyls 1 4 .
| Property | Mo(CO)₆ | W(CO)₆ | Sg(CO)₆ |
|---|---|---|---|
| Metal Atomic Number | 42 | 74 | 106 |
| Molecular Mass (g/mol) | 264 | 352 | 437 1 |
| Volatility | High (volatile solid) | High (volatile solid) | High (similar volatility to Mo and W) 1 4 |
| Stability | Stable under standard conditions | Stable under standard conditions | Stable for a few seconds, allowing for gas-phase studies 1 |
| Parameter | Result for Sg(CO)₆ | Significance |
|---|---|---|
| Number of Molecules Detected | Approximately 18 7 | Provided enough data for statistical analysis despite extreme rarity. |
| Adsorption Behavior | Nearly identical to W(CO)₆ and Mo(CO)₆ 1 4 | Confirmed seaborgium behaves as a typical member of group 6. |
| Theoretical Agreement | Supported predictions that included relativistic effects 4 | Validated modern chemical models for superheavy elements. |
The detection of approximately 18 molecules of Sg(CO)₆ provided statistically significant evidence for the formation of this superheavy complex.
The similar adsorption behavior across group 6 elements demonstrates that periodic trends hold even for superheavy elements.
This successful synthesis confirmed that the periodic table's architecture holds strong, even for the superheavy elements. Relativistic effects, rather than breaking the group trends, were shown to be a predictable and integral part of the chemistry of the heaviest elements.
The study of seaborgium continues to advance. In June 2025, an international team at the GSI/FAIR facility in Germany announced the discovery of a new isotope, seaborgium-257, and found evidence for a special quantum state in seaborgium-259 known as a K-isomer 3 6 .
These discoveries are "mapping the coastline of the island of stability" and could provide a doorway to studying even more short-lived isotopes. This ongoing research promises to further illuminate the structure and stability of superheavy nuclei, complementing chemical studies like the carbonyl complex synthesis.
The creation of seaborgium hexacarbonyl is more than a technical triumph; it is a profound demonstration of scientific curiosity and ingenuity.
By coaxing a few atoms of a short-lived element into a known compound, scientists confirmed that the rules of chemistry, refined by relativity, extend to the farthest reaches of the periodic table. This work, conducted at the scale of single atoms and seconds, reassures us that the universe, even in its most exotic corners, operates on principles we can understand, predict, and continue to explore.