Inert pair effect

The effect of the inert electron pair or Inert Pair Effect is a relativistic effect, which describes the expected, the preferential formation of ions with two oxidation states below. This is the case of elements with p- electrons of group 13 ( main group 3 ).

From the 4th period starting, but especially from the sixth period, the electrons move in the inner s orbitals nearly the speed of light. The result is that the mass of the s- electrons and thus increases their average distance decreases to the core. The orbital " shrinks " as it were. Due to the orthogonality of the s orbitals is a contraction of the 1s orbital has a direct effect on all higher s orbitals, which thus also contract. One consequence of this is that the ionization energies ( normally within a main group down decrease ) increase. For example, the ionization energy of the electrons in the S- thallium greater than aluminum. The s- electrons thus behave "inert" against ionization. Thereby forming, for example, Tl- (I) preferably with Tl (III), and Pb (II) more readily than Pb (IV).