Post-transition metal
Lead ( ) is a chemical element with the symbol Pb (from the Latin plumbum) and atomic number 82. It is a heavy metal, denser than most common materials. Lead is soft, malleable, and has a relatively low melting point. When freshly cut or melted, it appears shiny silvery with a bluish tint, but tarnishes to dull gray on exposure to air. Lead has the highest atomic number of any stable element, and three of its isotopes are endpoints of major nuclear decay chains of heavier elements.
Lead is a relatively unreactive post-transition metal. Its weak metallic character is shown by its amphoteric behavior: lead and lead oxides react with both acids and bases, and it tends to form covalent bonds. Lead compounds usually occur in the +2 oxidation state rather than the +4 state common in lighter members of the carbon group, with exceptions mostly limited to organolead compounds. Like the lighter members of the group, lead can bond with itself, forming chains and polyhedral structures.
Since lead is easily extracted from its ores, prehistoric people in the Near East were aware of it. Galena is a principal ore of lead which often bears silver. Interest in silver helped initiate widespread extraction and use of lead in ancient Rome. Lead production declined after the fall of Rome and did not reach comparable levels until the Industrial Revolution. Lead played a crucial role in the development of the printing press, as movable type could be relatively easily cast from lead alloys. …
Lead was first smelted in the 7th millennium BC, lead was widely adopted because of its ease of working and its association with silver ores, and it became especially important in the ancient Mediterranean, where Roman production reached unprecedented levels. After the decline and fall of the Western Roman Empire, lead use expanded in Asia and later revived in Europe during the Middle Ages and the Industrial Revolution. Growing awareness of lead's toxicity in the modern era led to regulatory restrictions, although the metal remains significant in certain industrial applications.
A lead atom has 82 electrons, with the electron configuration [Xe]4f145d106s26p2. The combined first and second ionization energies—the total energy required to remove the two 6p electrons—are similar to those of tin, lead's immediate neighbor above in the carbon group. This is unusual, as ionization energies typically decrease down a group due to the outer electrons being farther from the nucleus and more shielded by inner orbitals. However, the sum of the first four ionization energies of lead is higher than that of tin, contrary to periodic trends. This anomaly is explained by relativistic effects, which become significant in heavier atoms. These effects contract the s and p orbitals, giving lead's 6s electrons greater binding energies than its 5s electrons. This leads to the inert-pair effect, where the 6s electrons are less likely to participate in bonding. The result is stabilization of the +2 oxidation state and unusually long distances between nearest atoms in crystalline lead.
Lighter carbon-group congeners of lead form stable or metastable allotropes with the tetrahedrally coordinated, covalently bonded diamond cubic structure. In these elements, the s- and p-orbital energy levels are close enough to allow mixing into four hybrid sp3 orbitals. In lead, however, the inert pair effect increases the separation between s- and p-orbitals so much that the energy gain from hybridization is insufficient to overcome this gap. …
Lead's per-particle abundance in the Solar System is 0.121 ppb (parts per billion). This figure is two and a half times higher than that of platinum, eight times more than mercury, and seventeen times more than gold. The amount of lead in the universe is slowly increasing as most heavier atoms (all of which are unstable) gradually decay to lead. The abundance of lead in the Solar System since its formation 4.5 billion years ago has increased by about 0.75%. The Solar System abundances table shows that lead, despite its relatively high atomic number, is more prevalent than most other elements with atomic numbers greater than 40.
Primordial lead—which comprises the isotopes lead-204, lead-206, lead-207, and lead-208—was mostly created as a result of repetitive neutron capture processes occurring in stars. The two main modes of capture are the s- and r-processes.
In the s-process (s is for "slow"), captures are separated by years or decades, allowing less stable nuclei to undergo beta decay. A stable thallium-203 nucleus can capture a neutron and become thallium-204; this undergoes beta decay to give stable lead-204; on capturing another neutron, it becomes lead-205, which has a half-life of around 17 million years. Further captures result in lead-206, lead-207, and lead-208. On capturing another neutron, lead-208 becomes lead-209, which quickly decays into bismuth-209. …
Contrary to popular belief, pencil leads in wooden pencils have never been made from lead. When the pencil originated as a wrapped graphite writing tool, the particular type of graphite used was named plumbago (literally, lead mockup).
Lead metal has several useful mechanical properties, including high density, low melting point, ductility, and relative inertness. Many metals are superior to lead in some of these aspects but are generally less common and more difficult to extract from parent ores. Lead's toxicity has led to its phasing out for some uses.
Lead was used to cover the ramparts protecting the ascent to the Alamut Castle in Persia, which could absorb attacks by siege engines.
Lead has been used for bullets since their invention in the Middle Ages. It is inexpensive; its low melting point means small arms ammunition and shotgun pellets can be cast with minimal technical equipment; and it is denser than other common metals, which allows for better retention of velocity. It remains the main material for bullets, alloyed with other metals as hardeners. Concerns have been raised that lead bullets used for hunting can damage the environment. Shotgun cartridges used for waterfowl hunting must today be lead-free in the United States, Canada, and in Europe.
Lead's high density and resistance to corrosion have been exploited in a number of related applications. …
Natural lead consists of four stable isotopes with mass numbers 204, 206, 207, and 208, along with traces of six short-lived radioisotopes with mass numbers 209–214. The relatively high number of isotopes is consistent with lead's even atomic number. Lead has a magic number of protons (82), making its nucleus especially stable according to the nuclear shell model. Lead-208 also has 126 neutrons, another magic number, which may account for its exceptional stability.
With its high atomic number, lead is the heaviest element whose natural isotopes are considered stable; lead-208 is the heaviest stable nucleus known. This distinction previously belonged to bismuth (atomic number 83) until its sole primordial isotope, bismuth-209, was found in 2003 to decay extremely slowly. Although the four stable isotopes of lead could theoretically undergo alpha decay to mercury isotopes with an energy release, no such decay has been observed; their predicted half-lives range from 1035 to 10189 years, at least 1025 times the current age of the universe.
Three of lead's stable isotopes—lead-206, lead-207, and lead-208—are the end products of the three major natural decay chains: the uranium chain (from uranium-238), the actinium chain (from uranium-235), and the thorium chain (from thorium-232), respectively. …
Lead has no confirmed biological role, and there is no confirmed safe level of lead exposure. A 2009 Canadian–American study concluded that even at levels that are considered to pose little to no risk, lead may cause "adverse mental health outcomes". Its prevalence in the human body—at an adult average of 120 mg—is nevertheless exceeded only by zinc (2500 mg) and iron (4000 mg) among the heavy metals. Lead salts are very efficiently absorbed by the body. A small amount of lead (1%) is stored in bones; the rest is excreted in urine and feces within a few weeks of exposure. Only about a third of lead is excreted by a child. Continual exposure may result in the bioaccumulation of lead.
Lead is a highly poisonous metal (whether inhaled or swallowed), affecting almost every organ and system in the human body. At airborne levels of 100 mg/m3, it is immediately dangerous to life and health. Most ingested lead is absorbed into the bloodstream. The primary cause of its toxicity is its predilection for interfering with the proper functioning of enzymes. It does so by binding to the sulfhydryl groups found on many enzymes, or mimicking and displacing other metals that act as cofactors in many enzymatic reactions. The essential metals that lead interacts with include calcium, iron, and zinc. High levels of calcium and iron tend to provide some protection from lead poisoning; low levels cause increased susceptibility. …