element effect

https://doi.org/10.1351/goldbook.E02036
The ratio of the rate constants of two reactions that differ only in the identity of the element of the atom in the
leaving group
, e.g. kBrkCl. As for
isotope effects
, a ratio of unity is regarded as a 'null effect'.
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1111 [Terms] [Paper]