As elements increase in atomic size, their stability decreases significantly. The stability of an element is determined not by its chemical reactivity, but by the resilience of its nucleus to spontaneous breakdown, a process known as radioactivity. The search for the “most unstable element” is a quest to find the one whose atomic nucleus decays the fastest. This instability is most dramatic at the extreme upper end of the periodic table, where elements exist only for fractions of a second.
Defining Instability: The Role of Half-Life
Instability in an element is measured by its radioactivity, the process where an atomic nucleus spontaneously emits energy and particles to transform into a different, more stable nucleus. This decay occurs because the forces holding the nucleus together are outweighed by the forces pushing it apart. Scientists quantify this rate of decay using a metric called the half-life.
Half-life (\(T_{1/2}\)) is the time required for half of the atoms in a given sample of a radioactive substance to undergo decay. This concept means the probability of any single atom decaying is constant over time, resulting in an exponential rate of breakdown. A shorter half-life indicates a proportionally higher rate of nuclear instability. For instance, an isotope with a half-life of one minute will have half its atoms remaining after sixty seconds.
The half-life serves as the standard for comparing the inherent instability of different atomic nuclei. Elements whose half-lives are measured in years are considered relatively stable, while those measured in milliseconds or microseconds are profoundly unstable. These short half-lives are characteristic of the heaviest elements, which are only created in specialized laboratories.
Identifying the Element with the Shortest Existence
The element currently holding the record for the shortest measured half-life is Oganesson (element 118), the heaviest element known. The specific isotope, Oganesson-294, has been observed to exist in its ground state for a brief period. Its measured half-life is approximately \(0.7\) to \(0.89\) milliseconds, meaning half of any synthesized sample decays in less than a thousandth of a second.
This millisecond existence places Oganesson-294 at the edge of nuclear stability for chemically defined elements. Its short lifetime makes it nearly impossible to study its chemical properties before it transforms into a different element. For comparison, Tennessine-294 (element 117) has a half-life of around \(51\) to \(80\) milliseconds, which is significantly longer.
While Oganesson-294 is the least stable element in its ground state, certain extremely neutron-deficient nuclides of lighter elements decay even faster. These include isotopes like Hydrogen-7, whose half-life is measured in yoctoseconds. However, these are not considered elements in the same context as the superheavy atoms. Oganesson represents the practical limit of the periodic table regarding nuclear stability.
Creating the Heaviest Elements
Oganesson and other superheavy elements are fleeting because they do not exist naturally on Earth and must be created synthetically. This process requires a specialized laboratory environment, typically involving a particle accelerator and significant energy. The synthesis of new elements involves a high-speed collision known as a fusion-evaporation reaction.
To create Oganesson, scientists fire a beam of lighter atomic nuclei (the projectile) at a target made of a heavy element. For the discovery of Oganesson-294, scientists at the Joint Institute for Nuclear Research used a beam of Calcium-48 ions and aimed it at a target of Californium-249. The calcium ions were accelerated to approximately \(10\%\) of the speed of light before impact.
The goal is for the two nuclei to fuse into a single, highly energetic “compound nucleus” that immediately sheds its excess energy, typically by evaporating a few neutrons. This fusion event is extremely rare; only a few atoms of the new element are produced, even after weeks or months of bombardment. The resulting atom of Oganesson-294 is then quickly separated and directed to a detector to measure its decay chain and fleeting half-life.
The Physics of Nuclear Limits and Stability
The instability of Oganesson and other superheavy elements is rooted in the fundamental forces governing the atomic nucleus. The stability of a nucleus is a continuous battle between two opposing forces: the attractive strong nuclear force and the repulsive electrostatic force (Coulomb repulsion). The strong nuclear force binds protons and neutrons together but only acts over extremely short distances, approximately the diameter of a few nucleons.
Conversely, the electrostatic force is a long-range force that causes all positively charged protons to push away from each other. As the number of protons increases, the total repulsive force grows much faster than the strong nuclear force, which only benefits from the nearest neighbors. Beyond a certain size, reached with the superheavy elements, the cumulative proton repulsion overwhelms the binding power of the strong force, leading to immediate nuclear decay.
Despite this overwhelming trend toward instability, physicists theorize the existence of an “Island of Stability,” a region where certain superheavy isotopes might have much longer half-lives. This hypothesized stability is predicted to occur when a nucleus contains specific “magic numbers” of protons and neutrons that correspond to completely filled nuclear shells. Finding an element within this theoretical island, perhaps with a half-life of minutes or even days, remains a primary goal of superheavy element research.