The heaviest element on the periodic table sits at the boundary of chemistry and nuclear physics, pushing the limits of what scientists can create. Elements are the fundamental building blocks of matter, defined by the number of protons in their atomic nucleus. The periodic table organizes these elements by increasing proton count, which measures an element’s identity and “heaviness.” The heaviest elements are highly unstable, fleeting atoms created artificially in laboratories, not found in nature. The current record holder, which completes the seventh row of the table, is Element 118, known as Oganesson.
How Scientists Define an Element’s Weight
The organization of the periodic table hinges on the atomic number (Z), which equals the number of protons in an atom’s nucleus. Changing this number changes the element’s identity, making the atomic number the definitive measure of its position. While neutrons contribute to the atom’s overall mass number, the proton count is the unchangeable characteristic used to classify a distinct element.
The actual weight of an atom is more accurately represented by its atomic mass, the sum of protons and neutrons in a specific isotope. For lighter, naturally occurring elements, the atomic weights listed are weighted averages accounting for various isotopes. For the heaviest elements, this distinction is less relevant because they exist only as single, short-lived isotopes created in a lab.
The vast majority of elements up to uranium (atomic number 92) occur naturally on Earth. Elements with an atomic number greater than 92 are classified as transuranic elements and must be synthesized by researchers. This group includes super-heavy elements, defined by their extraordinary instability.
As the number of protons increases, the repulsive force within the nucleus grows stronger, requiring more neutrons to provide the strong nuclear force necessary for stability. This results in an inverse relationship between an element’s mass and its lifetime; the heaviest elements are characterized by half-lives measured in fractions of a second.
Oganesson: The Heaviest Known Element
The element with the highest atomic number currently recognized is Oganesson (Og), Element 118. Its discovery was officially confirmed in 2016 and named to honor Russian nuclear physicist Yuri Oganessian, who was instrumental in the discovery of several super-heavy elements. Oganesson sits at the end of the seventh period in Group 18, the column of noble gases.
The synthesis of Oganesson-294 was achieved by bombarding a target of Californium-249 (\(\text{Cf}^{249}\)) with a beam of Calcium-48 (\(\text{Ca}^{48}\)) ions in a particle accelerator. This technique, known as cold fusion, involves the collision of two heavy nuclei that briefly fuse to form a single, highly unstable nucleus. The fusion reaction is extremely rare, requiring billions of collisions over months to produce only a handful of atoms.
The resulting isotope, Oganesson-294 (\(\text{Og}^{294}\)), is the single confirmed isotope. It is extremely short-lived, decaying within milliseconds, with a measured half-life of approximately \(0.7\) to \(0.89\) milliseconds. Because only a few atoms have ever been created, its chemical properties cannot be directly measured.
Theoretical models predict that Oganesson may not behave as a typical noble gas, despite its placement in Group 18. Relativistic effects, pronounced in atoms with such a high number of protons, are predicted to cause its outermost electrons to move sluggishly. This effect is thought to make Oganesson less inert, potentially enabling it to form chemical bonds and possibly exist as a solid or semi-conductor at room temperature, unlike its gaseous neighbors.
Searching Beyond the Periodic Table
The successful creation of Oganesson completes the seventh row of the periodic table, but the search for even heavier elements continues. Scientists are now focused on synthesizing Element 119 and Element 120, which would begin the eighth period. Creating these super-heavy elements requires high-energy particle accelerators to smash two smaller nuclei together.
The goal of this research is to explore a theoretical region known as the “Island of Stability.” This concept suggests that while the half-lives of super-heavy elements decrease with increasing atomic number, certain isotopes may possess a uniquely stable configuration of protons and neutrons. These special numbers of nucleons are called “magic numbers,” corresponding to filled nuclear shells, similar to the stability provided by filled electron shells in noble gases.
Elements located on this predicted island are theorized to have significantly longer half-lives than their immediate neighbors, potentially lasting minutes, days, or even years. This enhanced stability would allow researchers to study their chemical properties in detail for the first time, potentially revealing entirely new chemical behaviors dictated by the extreme atomic environment. Current experiments aim to use different combinations of target and projectile elements to reach the neutron-rich isotopes predicted to sit at the center of this island.
The quest to synthesize Elements 119 and 120 is challenging because the probability of the necessary nuclear fusion reaction decreases rapidly with each increase in atomic number. Every new element requires a more powerful particle accelerator, more sensitive detection equipment, and the precise combination of rare target materials. Reaching the Island of Stability remains a primary objective in nuclear physics, promising a fundamental expansion of our understanding of matter.