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 @[email protected], e.g. \(\frac{k_{\text{Br}}}{k_{\text{Cl}}}\). As for @[email protected], 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]