The periodic table contains 118 elements, defined by the number of protons in its nucleus. While many elements like oxygen and silicon are abundant, others are incredibly scarce. The term “rarity” in elemental science is not a single answer, but a concept defined by the specific metric used—whether it is nuclear stability, geological concentration, or origin. The true rarity of any element is a function of its physical properties and the mechanisms that create or destroy it in nature.
Defining Rarity in Elemental Science
Elemental rarity can be categorized into three distinct scientific metrics that determine an element’s availability.
Nuclear Instability
This occurs when an element’s nucleus is so unstable that it rapidly undergoes radioactive decay, severely limiting its existence. These elements possess extremely short half-lives, meaning they transform into other elements almost as quickly as they are created.
Geological Concentration
This relates to a stable element’s low abundance in the Earth’s crust. Elements that are widely dispersed or concentrated deep within the planet’s mantle and core are considered rare because they are inaccessible or exist only at parts-per-billion levels in the crust. This scarcity is defined by distribution and concentration.
Synthetic Origin
This applies to elements that are not naturally found on Earth, or only in trace amounts following nuclear events. These elements must be manufactured in specialized laboratories, reactors, or particle accelerators through nuclear bombardment. Their rarity is absolute, defined by their non-existence in the natural environment.
The Rarest Naturally Occurring Element on Earth
When considering elements naturally present in the Earth’s crust, the rarest non-transuranic element is generally Astatine (At), element number 85. Astatine’s extreme scarcity is linked to its profound nuclear instability; its name is derived from the Greek word for “unstable.” Less than 25 grams of Astatine are estimated to exist in the entire Earth’s crust at any single moment. It exists only as a transient intermediate product in the natural decay chains of heavier elements like Uranium and Thorium. All of Astatine’s isotopes are short-lived; the most stable isotope, Astatine-210, has a half-life of only 8.1 hours. This rapid decay prevents any measurable quantity from accumulating. Francium and Promethium are close contenders for this title, as they also have extremely short half-lives.
Elements Scarce in the Earth’s Crust
A different perspective on rarity focuses on stable elements that are scarce due to low geological concentration or extreme dispersion. The Platinum Group Metals (PGMs)—Ruthenium, Rhodium, Palladium, Osmium, Iridium, and Platinum—exemplify this type of rarity. These metals are highly valued for their unique chemical and physical properties, such as excellent thermal and electrical conductivity and resistance to corrosion. The PGMs are siderophilic, meaning they are “iron-loving,” and their high density caused them to sink and concentrate primarily in the Earth’s core during the planet’s formation. Consequently, their natural abundance in the continental crust is extremely low, often measured in parts per billion (ppb). For instance, Iridium is found at a concentration of about 0.02 nanograms per gram in the crust, making it difficult and costly to extract. This geological scarcity, rather than nuclear decay, defines the rarity and high cost of these stable, economically important elements.
Elements Created Only in Laboratories
The ultimate form of elemental rarity is represented by transuranic elements, which are those with an atomic number greater than 92 (Uranium). These elements are so unstable that, with the exception of trace amounts of Neptunium and Plutonium found in some uranium ores, they do not occur naturally on Earth. All transuranic elements must be synthesized by humans, typically in nuclear reactors or particle accelerators. The synthesis process involves bombarding a target atom with high-energy particles, such as neutrons or lighter nuclei, to force the nuclei to fuse and create a heavier element. Elements beyond Plutonium, such as Californium and Livermorium, are entirely synthetic on Earth and are created atom by atom. Their half-lives are often extremely short, sometimes mere fractions of a second. This combination of being man-made and having fleeting existence establishes these transuranic elements as the rarest of all, existing only through scientific intervention.