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.
It tarnishes on contact with moist air, and takes on a dull appearance the hue of which depends on the prevailing conditions.
The sum of the first four ionization energies of lead exceeds that of tin, Lead's lighter carbon group congeners form stable or metastable allotropes with the tetrahedrally coordinated and covalently bonded diamond cubic structure.
The energy levels of their outer s- and p-orbitals are close enough to allow mixing into four hybrid sp orbitals.
Three of the stable isotopes are found in three of the four major decay chains: lead-206, lead-207, and lead-208 are the final decay products of uranium-238, uranium-235, and thorium-232, respectively.
When freshly cut, lead is bluish-white; it tarnishes to a dull gray color when exposed to air.Exceptions are mostly limited to organolead compounds.Like the lighter members of the group, lead tends to bond with itself; it can form chains, rings and polyhedral structures.In the late 19th century, lead's toxicity was recognized, and its use has since been phased out of many applications.Lead is a neurotoxin that accumulates in soft tissues and bones, damages the nervous system, and causes blood disorders.As uranium decays into lead, their relative amounts change; this is the basis for uranium–lead dating.The Holsinger meteorite, the largest piece of the Canyon Diablo meteorite.It is particularly problematic in children, in that permanent brain damage may result, even if blood levels are promptly normalized with treatment. The sum of lead's first and second 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 nucleus, and more shielded by smaller orbitals.The similarity of ionization energies is caused by the lanthanide contraction—the decrease in element radii from lanthanum (atomic number 57) to lutetium (71), and the relatively small radii of the elements from hafnium (72) onwards.This is due to poor shielding of the nucleus by the lanthanide 4f electrons.