Inert pair effect

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Inert pair effect refers to the fact that "valence electrons in an s orbital penetrate to the nucleus better than electrons in p orbitals, and as a result they're more tightly bound to the nucleus and less able to participate in bond formation. A pair of such electrons is called an "inert pair". The inert pair effect explains why common ions of Pb are Pb⁴⁺ and Pb²⁺, and not just Pb⁴⁺ as we might expect from the octet rule." [1]

The heuristic device of pretending an electron orbits the nucleus like a planet orbits a star is sometimes used to "explain" this relativistic effect on the shape of a molecule's electron which is the three dimensional shape of its potential for interacting (due to its energy level) called a molecular orbital. This is at the expense of correctly understanding both the quantized and wave-particle duality aspects of fundamental particles.

The inert pair effect is apparent from the chemistry of the Group III and Group IV elements and beyond. The lighter elements in Group IV tend to have a valency of +4, whereas the heavier elements form 2+ ions that are more stable than 4+ ions. For instance, Pb O is much more stable than Pb O₂ which decomposes readily to Pb O.

The inert pair effect can explain why Thallium forms Tl₂O while aluminium in the same group forms Al₂O₃.In aluminium, the electronic configuration is [Ne]3s² 3p_x¹. Because of the screening effect of the 2p electrons, one of the 3s electron can be excited easily to the 3p_y orbital. So Al mostly exhibits a valency of 3. However in case of Tl, the electronic configuration is [Xe]4f¹⁴ 5d¹⁰ 6s² 6p_x¹. Due to the poor screening effect of the 5d orbital, the effective nuclear charge on the 6s electron is high. Hence it cannot be excited to the 6p_y orbital. So the only unpaired electron remains in the 6p_x orbital. Hence Thallium exhibits a valency of 1 and forms Tl₂O. Similar effect is observed in tin and lead.

The inert pair effect can be used to make predictions concerning the properties of elements; for instance, although roentgenium has never been seen by the naked eye, the inert pair effect suggests that it should be colored like gold. ^{[citation needed]}