Untrioctium

Untrioctium is the temporary systematic name for the hypothetical chemical element with atomic number 138. No element with this atomic number has been confirmed.

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Atomic number: The number of protons in an atomic nucleus, which defines a chemical element. Untrioctium’s proposed identity is fixed by its 138 protons.
Nuclear stability: The tendency of an atomic nucleus to resist radioactive decay or fission. A sufficiently long-lived isotope would be needed to identify untrioctium experimentally.
Heavy-ion fusion: A nuclear reaction in which two atomic nuclei collide and fuse to form a heavier nucleus. Fusion is a principal proposed route to creating nuclei beyond known elements.
IUPAC: The international organization that standardizes chemical nomenclature, terminology, and measurement. IUPAC assigns temporary systematic names and approves permanent names for confirmed elements.
Chemical element: A pure substance whose atoms all have the same number of protons. Untrioctium would count as an element if nuclei with 138 protons were established.
Radioactive decay: The spontaneous transformation of an unstable atomic nucleus into another nucleus or particles. Any produced untrioctium isotope would be expected to decay on a measurable timescale.
Nuclear fusion reaction: A reaction in which atomic nuclei combine to form a heavier nucleus. Element-synthesis proposals rely on fusion to assemble its 138 protons.
Oganesson: The confirmed chemical element with atomic number 118, the highest atomic number currently recognized. Unlike untrioctium, oganesson has been experimentally produced and officially named.
Periodic table: A tabular arrangement of chemical elements ordered chiefly by atomic number. Element 138 would extend the table beyond all currently confirmed elements.
Nuclear fission: The splitting of an atomic nucleus into lighter nuclei, often accompanied by particle emission and energy release. Fission could rapidly destroy nuclei formed at atomic number 138.