element effect

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. $k Br k Cl$ . 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