The question of the heaviest element on the periodic table is not a simple one-word answer because the term “heavy” can be defined in two distinct scientific ways. The most common understanding of an element’s weight relates to its atomic mass, which is a measure of the total matter contained within a single atom. To provide the most complete answer, we must consider both the elements found naturally on Earth and the synthetic elements created in specialized laboratories.
Defining Heaviness: Atomic Mass Versus Density
The standard measure of an element’s weight on the periodic table is its atomic mass, also called atomic weight. This value is derived from the number of protons and neutrons packed into the atom’s nucleus. For example, an atom of hydrogen is significantly lighter than an atom of carbon, which typically contains six protons and six neutrons.
Atomic mass is distinct from an element’s density, which is a measure of mass per unit volume. The element with the highest known density is osmium, which can have a density of approximately 22.59 grams per cubic centimeter at room temperature.
Osmium has an atomic number of 76, meaning its individual atoms are much lighter than those at the end of the periodic table. Its high density is due to its atomic structure, where the atoms nest closely together. For finding the heaviest element, the determining factor is the atomic mass, which increases consistently with the element’s atomic number.
The Heaviest Naturally Occurring Element
When limiting the search to elements found in significant quantities in the Earth’s crust, the answer is uranium, which has an atomic number of 92. The most common isotope, uranium-238, has an atomic mass of approximately 238 atomic mass units. Uranium is the heaviest element that is found naturally and has an atomic structure stable enough to have survived since the Earth was formed.
Uranium is found in various minerals, and its presence is a consequence of its relatively long radioactive half-life. While elements with atomic numbers greater than 92, such as neptunium and plutonium, can sometimes be found in trace amounts, they are typically produced as intermediate decay products of uranium. These trace amounts are not considered naturally occurring in the same way, making element 92 the established upper limit for naturally abundant elements.
The Heaviest Known Element
The absolute heaviest element discovered is oganesson, which holds the current record with an atomic number of 118. It has an approximate atomic mass of 294 atomic mass units. This element is entirely synthetic, meaning it does not exist in nature and was created by scientists in a laboratory setting.
Oganesson was first produced through a collaboration between the Joint Institute for Nuclear Research in Russia and the Lawrence Livermore National Laboratory in the United States. Its creation involved bombarding a target of californium-249 atoms with a beam of calcium-48 ions inside a particle accelerator. This fusion process resulted in the formation of a few atoms of the new, superheavy element.
Because it is so short-lived, oganesson’s properties are mostly theoretical. Its placement in Group 18 suggests it should be a noble gas, but relativistic effects caused by its high atomic number may cause it to behave like a solid or liquid. Its synthesis represents the current boundary of human capability in extending the periodic table.
Limits to Creating Heavier Elements
The creation of superheavy elements like oganesson faces severe limitations due to nuclear instability. As the number of protons in an atomic nucleus increases, the repulsive force between them grows much faster than the strong nuclear force that holds the nucleus together. This imbalance results in a highly unstable nucleus that breaks apart almost instantly.
The most stable isotope of oganesson, oganesson-294, has an incredibly short half-life, measured in less than a millisecond. This rapid decay is a direct consequence of the overwhelming repulsive forces within the massive nucleus.
Research continues with the hope of reaching a theoretical region known as the “Island of Stability.” This concept predicts that certain combinations of protons and neutrons in superheavy elements might create a more stable, spherical nuclear configuration. Elements located on this predicted island would still be radioactive but could possess half-lives of minutes or even years, allowing for more detailed study.