Elemental rarity is complex because scarcity stems from different physical and chemical realities. Abundance is often measured by how much of an element exists in the Earth’s crust. However, this metric fails to account for elements that are fleeting or only exist because of human intervention. The rarest elements either have a temporal existence measured in seconds or have an abundance of zero outside of a laboratory setting. Understanding rarity requires distinguishing between naturally trace elements and purely artificial creations.
Establishing Rarity Metrics
Rarity is defined by two distinct metrics: crustal abundance and temporal existence or synthetic origin. Crustal abundance refers to the concentration of an element within the Earth’s outer layer, typically measured in parts per billion (ppb). Elements like gold and platinum are scarce because their low crustal concentration makes them difficult to extract in commercially viable deposits.
A greater rarity is defined by an element’s half-life, the time it takes for half of a radioactive sample to decay. Elements that decay as fast as they are formed are rare because their existence is momentary and cannot be sustained over geological time scales. This contrasts with purely synthetic elements, which are inherently the rarest because they do not occur on Earth and must be created by scientists in specialized facilities.
The Most Transient Elements
The rarest elements that occur naturally on Earth are transient elements, existing primarily as intermediate products in the decay chains of uranium and thorium. Their scarcity is a function of time, as their extremely short half-lives mean they constantly disappear almost immediately after formation. They exist only in infinitesimal quantities across the entire globe.
Astatine (Atomic Number 85) is considered the rarest naturally occurring element in the Earth’s crust. Its total estimated global quantity at any moment is less than 30 grams. The longest-lived naturally occurring isotope, Astatine-219, has a half-life of only 56 seconds. The element is so intensely radioactive that any visible sample would instantly vaporize itself due to the heat generated by its own decay.
Francium (Atomic Number 87) is the second rarest, with a total crustal quantity estimated to be in the range of 20 to 30 grams. It is formed by the alpha decay of actinium. Its longest-lived isotope, Francium-223, has a half-life of only 22 minutes. Since its half-life is measured in minutes, Francium exists only as a momentary step in the decay process, making it virtually impossible to study in bulk.
Promethium (Atomic Number 61) is the only entirely radioactive and unstable lanthanide series element. It is found in trace amounts within uranium ore, resulting from the spontaneous fission of Uranium-238. Its longest-lived isotope, Promethium-145, has a half-life of 17.7 years. Despite this relatively longer half-life, the estimated total quantity in the crust is still only about 572 grams.
Elements That Are Purely Man-Made
The rarest elements are those with zero natural abundance, existing only when scientists actively create them. These synthetic creations are typically classified as transuranic elements, possessing an Atomic Number of 93 or higher. Superheavy elements in this category are produced atom-by-atom in particle accelerators.
The creation process involves bombarding a target element with a beam of lighter nuclei, using immense energy to force the two nuclei to fuse. These fusion reactions are highly inefficient, often creating only a few atoms over weeks or months of operation. The resulting superheavy atoms are extremely unstable, decaying almost instantly.
Oganesson (Atomic Number 118) is the heaviest element discovered to date. Its most stable known isotope, Oganesson-294, has a half-life of only 0.7 milliseconds. This extremely short existence means the element cannot be chemically studied in a traditional sense. Its properties are known only through theoretical prediction and the detection of its decay products.
The “Island of Stability” is a theoretical concept predicting a region of the periodic table where certain superheavy isotopes might possess much longer half-lives. While current superheavy elements decay in milliseconds, isotopes on this predicted island could potentially exist for seconds or longer. The rarity of these purely synthetic elements is a function of the astronomical energy and specialized equipment required to momentarily bring them into existence.
Clarifying Common Misconceptions
A common misunderstanding involves Rare Earth Elements (REEs), a set of 17 metallic elements including the lanthanides, Scandium, and Yttrium. Although the name suggests extreme scarcity, these elements are not rare in terms of low crustal abundance. Cerium, the most common REE, is the 25th most abundant element in the crust, comparable to the industrial metal copper.
The “rare” misnomer originates from the difficulty of extraction, not overall volume. Rare Earth Elements are chemically very similar, making them difficult and expensive to separate from their host minerals. They are rarely found concentrated in economically viable deposits, instead usually being dispersed throughout the crust. Therefore, the term “rare” refers to the rarity of a profitable mining deposit, not the scarcity of the element itself.