molecular entity quality atomic quality reactivity As applied to a chemical species, the term expresses a kinetic property. A species is said to be more reactive or to have a higher reactivity in some given context than some other (reference) species if it has a larger rate constant for a specified elementary reaction [source: IUPAC Gold Book - https://doi.org/10.1351/goldbook.R05180] https://doi.org/10.1351/goldbook.R05180