Effect of the inert electron pair
The effect of the inert electron pair or inert pair effect is a relativistic effect that describes the preferred formation of ions with two oxidation states below those to be expected. This happens with elements with p- electrons from the 13th group (3rd main group).
Starting from the 4th period, but especially from the 6th period, the electrons in the inner s orbitals move almost at the speed of light . This leads to an increase in the mass of the s electrons and thus to a decrease in their mean distance from the nucleus . The orbital "shrinks" so to speak. Due to the orthogonality of the s orbitals, a shrinking of the 1s orbitals has a direct effect on all higher s orbitals, which thus also contract. One consequence of this is that the ionization energies (which normally decrease downwards within a main group ) increase. For example, the ionization energies of the s electrons in thallium are greater than in aluminum . The s-electrons behave "inertly" towards ionization. For example, Tl (I) is formed more easily than Pb (IV) in preference to Tl (III) and Pb (II).
The name comes from Nevil Vincent Sidgwick . This originally explained it by the fact that in the p-blocks (p-electrons under the valence electrons) the s-electrons under the valence electrons are more strongly bound when one goes to higher periods and are therefore no longer available for covalent bonds. For example, in the 4th main group (carbon group), carbon and silicon form tetravalent compounds with chlorine, but the elements tin and lead only form divalent compounds in the higher periods. The explanation was later felt to be inadequate, for example the ionization energies of the s electrons in the 3rd main group (boron group) did not behave as expected according to this explanation. Instead, relativistic effects were used to explain.
literature
- AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 305.
Individual evidence
- ↑ Scerri, Who was John David Main Smith? , Oxford University Blog, March 12, 2016