The term "relativistic effects" was developed in light of the history of quantum mechanics. Initially quantum mechanics was developed without considering the theory of relativity.Kaldor, U. "Theoretical Chemistry and Physics of Heavy and Superheavy Elements", Kluser Academic Publishers 2003. "Heavy elements" in this context refers to high atomic number elements in the later part of the periodic table where relativistic effects are important. Examples are elements found in the lanthanide and actinide series.
Relativistic effects in chemistry can be considered to be perturbations, or small corrections, to the non-relativistic theory of chemistry, which is developed from the solutions of the Schrödinger equation. These corrections have differential effects on the electrons in various atomic orbitals within the atom, according to the speed of... Read More