Unquadennium

Unquadennium is the temporary systematic name assigned to the hypothetical chemical element with atomic number 149. No element with this atomic number has been confirmed.

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Atomic number: The number of protons in an atom's nucleus, which defines its chemical element. Unquadennium's defining property is the proposed atomic number 149.
Superheavy element: A chemical element with an atomic number greater than 103. Atomic number 149 would place unquadennium among the superheavy elements.
IUPAC: The international organization that standardizes chemical nomenclature, terminology, and measurement conventions. IUPAC defines the systematic rules behind unquadennium's temporary name.
Extended periodic table: A proposed continuation of the periodic table beyond the currently confirmed elements. Models differ on how far recognizable periodic structure could extend toward element 149.
Periodic table: A tabular arrangement of chemical elements ordered primarily by increasing atomic number and recurring chemical properties. Element 149 would extend the periodic table beyond all confirmed elements.
Nuclear shell model: A model describing protons and neutrons as occupying quantized energy levels within an atomic nucleus. Nuclear shell structure helps estimate whether a nucleus as heavy as element 149 could persist.
Naming of chemical elements: The process by which newly recognized elements receive names and symbols under international standards. Unquadennium remains a systematic placeholder until element 149 is discovered and officially named.
Relativistic effect: A change in electron behavior caused by relativistic speeds and effects in heavy atoms. Such effects may make element 149's chemistry depart from simple periodic trends.
IUPAC systematic element name: A temporary element name formed from digit-derived roots and the suffix -ium under IUPAC's naming rules. Unquadennium follows the systematic naming scheme for elements without permanent names.
Island of stability: A predicted region of enhanced stability among superheavy nuclei with particular proton and neutron numbers. Predictions of unusual nuclear stability motivate searches for elements far beyond known nuclei.