The sum of the first four ionization energies of lead exceeds that of tin, contrary to what periodic trends would predict. This is explained by relativistic effects, which become significant in heavier atoms, which contract s and p orbitals such that lead's 6s electrons have larger binding energies than its 5s electrons. A consequence is the so-called inert pair effect: the 6s electrons of lead become reluctant to participate in bonding, stabilising the +2 oxidation state and making the distance between nearest atoms in crystalline lead unusually long.
The sum of the first four ionization energies of lead exceeds that of tin contrary to what periodic trends would predict This is explained by relativistic effects which become significant in heavier atoms which contract s and p orbitals such that lead's 6s electrons have larger binding energies than its 5s electrons A consequence is the so-called inert pair effect the 6s electrons of lead become reluctant to participate in bonding stabilising the +2 oxidation state and making the distance between nearest atoms in crystalline lead unusually long