The element with the largest atomic mass depends on whether it was found in nature or created in a laboratory. All elements are defined by the number of protons (the atomic number), but the total mass of an atom is determined by the combined count of protons and neutrons. Scientists have successfully synthesized elements with atomic masses far exceeding those found naturally on Earth. This ongoing research into the heaviest elements pushes the boundaries of the periodic table and deepens the understanding of nuclear physics.
Defining Atomic Mass and Atomic Weight
The concept of atomic mass is fundamental to chemistry and physics, quantifying the matter contained within a single atom. More precisely, the mass number represents the total count of protons and neutrons, collectively called nucleons, in the nucleus of a specific isotope. The actual atomic mass is measured in atomic mass units (amu). The slight deviation from a whole number is due to the small mass contribution of electrons and the nuclear binding energy.
It is important to distinguish this from atomic weight, which is the value typically listed on the periodic table. Atomic weight, also known as relative atomic mass, is a weighted average of the atomic masses of all naturally occurring isotopes of an element. This average accounts for the relative abundance of each isotope found in a typical sample of the element on Earth.
For a synthetic element that does not exist in nature, a true atomic weight cannot be calculated because the relative abundance is zero. In these cases, the mass is often cited as the mass number of the most stable or longest-lived isotope that has been produced. Therefore, when discussing the “largest atomic mass,” it usually refers to the mass number of the heaviest known isotope, regardless of its natural abundance.
The Heaviest Synthetically Created Element
The element with the current highest known atomic mass is Oganesson (Og, atomic number 118). Oganesson is a synthetic element, meaning it must be produced artificially in a laboratory setting. The heaviest confirmed isotope, Oganesson-294, has an atomic mass of 294 atomic mass units, derived from its 118 protons and 176 neutrons.
Oganesson-294 only exists for fractions of a second, possessing an extremely short half-life of less than one millisecond. Its fleeting existence is why its physical and chemical properties remain largely theoretical. Its placement at the end of the seventh period makes it the heaviest element discovered to date.
The Heaviest Naturally Occurring Elements
In contrast to the transient superheavy synthetic elements, the heaviest element found in significant quantities in nature is Uranium (element 92). Uranium is abundant enough in the Earth’s crust to be mined commercially. Its most common isotope, Uranium-238, has an atomic mass of approximately 238 amu, and its long half-life allows it to persist since the Earth’s formation.
Trace amounts of elements with higher atomic numbers can also be found. Neptunium (element 93) and Plutonium (element 94) occur naturally, generated continuously as part of the natural decay and neutron capture processes within uranium ore deposits.
Plutonium-244, with an atomic mass of 244, is technically the heaviest nuclide ever confirmed to exist in the Earth’s crust, though its abundance is minuscule. This distinction is made because these heavier elements are constantly being created and destroyed in a geological cycle, keeping their natural quantities extremely low.
Creating and Characterizing Superheavy Elements
The synthesis of superheavy elements, those with atomic numbers 104 and greater, requires specialized equipment like powerful particle accelerators. Scientists create these atoms by accelerating a beam of lighter ions, such as Calcium-48, and smashing them into a target made of a heavy element. This technique, known as nuclear fusion-evaporation, aims to combine the two nuclei into a single, heavier nucleus. The process is incredibly inefficient; often, only a few atoms of the new element are produced after weeks or months of continuous bombardment.
Once a superheavy atom is formed, it immediately begins to decay, making its characterization exceptionally difficult. The half-lives are so short that scientists must use sophisticated physical separators to isolate the newly formed nucleus from the beam and target material before it decays. By observing the distinct decay chain—the sequence of alpha particles and other emissions—physicists can confirm the existence and mass of the original superheavy atom.
The scientific effort to create increasingly heavy elements is driven by the theoretical concept of the “Island of Stability.” This is a predicted region on the nuclear chart where certain combinations of protons and neutrons, often referred to as “magic numbers,” are theorized to result in a dramatic increase in nuclear stability. Researchers are working to synthesize elements with atomic numbers and neutron counts that fall within this proposed island, hoping to find superheavy isotopes that could have half-lives of minutes, hours, or potentially even longer.