Chemical elements form the fundamental building blocks of our universe. While some are abundant, others exist in vanishingly small quantities, making them incredibly rare. Defining “the most rare” element is complex, as rarity can refer to elements found in minuscule natural amounts, those that are unstable and rapidly decay, or those created only in laboratories.
Defining Elemental Rarity
Elemental rarity is assessed through several lenses. Natural abundance quantifies how much of an element is present in Earth’s crust, oceans, or atmosphere. Elements found in extremely low concentrations globally are considered rare by this criterion.
Rarity also relates to an element’s stability and its half-life. A half-life is the time it takes for half of a radioactive sample to decay into another element. Elements with very short half-lives are inherently rare because they quickly transform, meaning only fleeting amounts can exist.
Finally, some elements are rare because they do not occur naturally on Earth and must be synthesized in laboratories. These synthetic elements are produced through complex and energy-intensive processes, making them inherently scarce due to the difficulty and cost involved in their creation.
The Rarest Naturally Occurring Elements
Among naturally occurring elements, astatine (At) is the rarest in Earth’s crust, with less than 1 gram present at any time. All astatine isotopes are short-lived; its most stable isotope, astatine-210, has a half-life of approximately 8.1 hours. This instability means astatine continuously forms as a decay product of heavier elements, only to quickly disappear.
Francium (Fr) is another incredibly rare naturally occurring element, often considered the second rarest. Its most stable isotope, francium-223, has a remarkably short half-life of just 22 minutes. Like astatine, francium is primarily found as a transient product of radioactive decay chains, with less than 30 grams estimated to exist in the Earth’s crust.
Promethium (Pm) is a rare earth element that also has no stable isotopes, making it exceptionally rare in nature. Its longest-lived isotope, promethium-145, has a half-life of 17.7 years, while promethium-147 has a half-life of 2.62 years. Naturally occurring promethium is found only in trace quantities in uranium ores, resulting from the spontaneous fission of uranium, with the total natural amount estimated at less than one kilogram globally.
The Rarest Synthetic Elements
Synthetic elements are exclusively human-made, often existing for mere fractions of a second. Transuranic elements, those with atomic numbers greater than uranium (Z=92), fall into this category.
Oganesson (Og), atomic number 118, is the element with the highest known atomic number and mass. Only a handful of oganesson atoms have ever been produced. Its only known isotope, oganesson-294, is highly radioactive with a half-life of approximately 0.7 to 0.89 milliseconds.
Tennessine (Ts), element 117, is the second-heaviest element created. Its known isotopes, such as tennessine-293 and tennessine-294, have extremely short half-lives, ranging from 14 to 80 milliseconds. The rarity of these synthetic elements stems from the immense energy and specialized equipment required for their creation, often yielding only a few atoms.
The Science Behind Extreme Rarity
The extreme rarity of certain elements is fundamentally rooted in their nuclear stability. Atomic nuclei can be unstable, meaning they undergo radioactive decay, transforming into other elements over time. The half-life is a measure of this instability; elements with very short half-lives exist for only a fleeting moment before decaying, making them inherently difficult to observe or accumulate in significant quantities. For superheavy elements, theoretical models predict an “island of stability” where certain combinations of protons and neutrons might lead to isotopes with considerably longer, though still short, half-lives than their immediate neighbors.
The formation of elements across the universe is explained by cosmic nucleosynthesis. The lightest elements, hydrogen and helium, were formed during the Big Bang. Heavier elements, up to iron, are primarily forged in the cores of stars through nuclear fusion. Elements heavier than iron are typically created in more energetic events like supernovae explosions or neutron star mergers, through processes like rapid neutron capture. The natural scarcity of very heavy, unstable elements on Earth is due to these cosmic formation processes and their subsequent rapid decay over cosmic timescales.
Synthetic elements are produced in laboratories using powerful particle accelerators. These machines accelerate beams of lighter atomic nuclei to extremely high speeds. These accelerated “projectiles” are then directed at target nuclei of heavy elements, with the goal of fusing them together. The immense energy of these collisions can briefly overcome the repulsive forces between the positively charged nuclei, allowing them to merge and form a new, heavier element. The challenges in creating these elements include the low probability of successful fusion events and the extremely short lifetimes of the newly formed atoms.