Nonanions

Nonanions are a class of hypothetical chemical species that carry a net negative charge of nine. The term is derived from the prefix "non-" meaning nine and "-anion" referring to a negatively charged ion. In standard chemical nomenclature, ions are typically named based on their charge, with univalent anions (charge -1) often ending in "-ide," bivalent anions (charge -2) also sometimes using "-ide" or other specific nomenclature, and higher charged anions being less common and often having specific names.

The existence of a stable nonanion in isolation or in common chemical compounds is considered highly improbable. The strong electrostatic repulsion between nine excess electrons would be extremely difficult to overcome by any stabilizing forces, such as the positive charge of a cation or solvation effects. Theoretical calculations and models of atomic and molecular electronic structure suggest that forming such a highly charged negative species would require an enormous amount of energy and would likely be unstable against decomposition or electron ejection. While extremely high charge states can be observed in highly energetic plasma environments or in the gas phase under extreme conditions, these are not typically what is meant by a chemical nonanion in the context of stable matter. Therefore, nonanions are primarily a theoretical concept rather than a commonly encountered or observed chemical entity.