One of the conceptually simpler molecular changes into which an @E02035@ can be notionally dissected. Such changes include bond rupture, bond formation, internal rotation, change of @BT07003@ or bond @A00346@, bond @M03920@, redistribution of charge, etc. The concept of primitive changes is helpful in the detailed verbal description of elementary reactions, but a primitive change does not represent a process that is by itself necessarily observable as a component of an @E02035@.