Ais a group of atoms that behave like they are one ion

Definition of atomic group. Certain atoms form especially tightly bonded atomic groups. In minerals, these usually consist of an atom covalently bonded to three or four surrounding oxygen atoms. These groups usually have a net negative charge, in which case they are called anionic groups.

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a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.