Editing Lead (section)

=== Atomic ===
A lead [[atom]] has 82 [[electron]]s, arranged in an [[electron configuration]] of &#91;[[xenon|Xe]]&#93;4f<sup>14</sup>5d<sup>10</sup>6s<sup>2</sup>6p<sup>2</sup>. The sum of lead's first and second [[ionization energy|ionization energies]]—the total energy required to remove the two 6p electrons—is close to that of [[tin]], lead's upper neighbor in the [[carbon group]]. This is unusual; ionization energies generally fall going down a group, as an element's outer electrons become more distant from the [[atomic nucleus|nucleus]], and more [[shielding effect|shielded]] by smaller orbitals. The sum of the first four ionization energies of lead exceeds that of tin,{{sfn|Lide|2005|pp=10-179}} contrary to what [[periodic trends]] would predict. This is explained by [[relativistic quantum chemistry|relativistic effects]], which become significant in heavier atoms,{{sfn|Pyykkö|1988|pp=563–594}} which contract s and p orbitals such that lead's 6s electrons have larger binding energies than its 5s electrons.{{sfn|Claudio|Godwin|Magyar|2002|pp=1–144}} A consequence is the so-called [[Inert-pair effect|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 [[Crystal|crystalline]] lead unusually long.{{sfn|Norman|1996|p=36}}

Lead's lighter carbon group [[congener (chemistry)|congeners]] form stable or metastable [[Allotropy|allotropes]] with the tetrahedrally coordinated and [[covalent bond|covalently bonded]] [[diamond cubic]] structure. The energy levels of their outer [[Atomic orbital|s-]] and [[Atomic orbital|p-orbitals]] are close enough to allow mixing into four [[orbital hybridisation|hybrid]] sp<sup>3</sup> orbitals. In lead, the inert pair effect increases the separation between its s- and p-orbitals, and the gap cannot be overcome by the energy that would be released by extra bonds following hybridization.{{sfn|Greenwood|Earnshaw|1998|pp=226–227, 374}} Rather than having a diamond cubic structure, lead forms [[metallic bonding|metallic bonds]] in which only the p-electrons are delocalized and shared between the Pb<sup>2+</sup> ions. Lead consequently has a [[cubic crystal system|face-centered cubic]] structure{{sfn|Christensen|2002|p=867}} like the similarly sized{{sfn|Slater|1964}} [[Valence (chemistry)|divalent]] metals [[calcium]] and [[strontium]].{{sfn|Considine|Considine|2013|pp=501, 2970}}{{efn|The tetrahedral allotrope of tin is called α- or gray tin and is stable only at or below 13.2&nbsp;°C (55.8&nbsp;°F). The stable form of tin above this temperature is called β- or white tin and has a distorted face centered cubic (tetragonal) structure which can be derived by compressing the tetrahedra of gray tin along their cubic axes. White tin effectively has a structure intermediate between the regular tetrahedral structure of gray tin, and the regular face centered cubic structure of lead, consistent with the general trend of increasing metallic character going down any representative group.{{sfn|Parthé|1964|p=13}}}}{{efn|A [[quasicrystal]]line [[thin-film]] allotrope of lead, with pentagonal symmetry, was reported in 2013. The allotrope was obtained by depositing lead atoms on the surface of an [[icosahedron|icosahedral]] silver-[[indium]]-[[ytterbium]] quasicrystal. Its conductivity was not recorded.{{sfn|Sharma|Nozawa|Smerdon|Nugent|2013}}{{sfn|Sharma|Smerdon|Nugent|Ribeiro|2014|p=174710}}}}{{efn|Diamond cubic structures with lattice parameters around the lattice parameter of silicon exists both in thin lead and tin films, and in massive lead and tin, freshly solidified in vacuum of ~5 x 10<sup>−6</sup> Torr. Experimental evidence for almost identical structures of at least three oxide types is presented, demonstrating that lead and tin behave like silicon not only in the initial stages of crystallization, but also in the initial stages of oxidation.{{sfn|Peneva|Djuneva|Tsukeva|1981}}}}