Lead has been known since old ages and its use has been largely limited due to its high toxicity. It is the heaviest stable element and is resistant to corrosion.
History and Discovery
Lead is a prehistoric metal and was used by people of ancient Rome and Western Asia around 7000 BCE. It was widely used to make water pipes by the Romans and named it as plumbum nigrum (black lead). The use of lead decreased with the fall of the Roman Umpire. Lead was used as a currency in Ancient Chinese courts. Civilizations of Indus valley and Ancient Greece used lead to make amulets, glasses, ornaments, glazes and sinkers in fishing net. The name plumbum, was given to the metal and the English word plumbing was originated from this Latin word. The origin of name of lead is intertwined with tin, as olovo translates to lead in Czech, while it translate to tin in Russian language. The word lead has been derived from Old English word lead.
Lead
| Periodic Table Classification | Group 14 Period 6 |
|---|---|
| State at 20C | Solid |
| Color | Metallic gray |
| Electron Configuration | [Xe] 4f14 5d10 6s2 6p2 |
| Electron Number | 82 |
| Proton Number | 82 |
| Electron Shell | 2, 8, 18, 32, 18, 4 |
| Density | 11.35 g.cm-3 at 20°C |
| Atomic number | 82 |
| Atomic Mass | 207.20 g.mol -1 |
| Electronegativity according to Pauling | 1.87 (+2) |
Occurrence
Lead is an abundant metal and is present around 14 ppm in the Earth’s crust. It is considered as the 38th most abundant element on Earth [1]. It is primarily present in combined form with sulfur and is rarely found in elemental or metallic form. The most common mineral of lead is galena (PbS). It is found in zinc ores. There are various impurities in lead minerals and ores, including tin, arsenic, gold, silver and copper. The largest deposits of lead are found in Australia, Russia, China, US, and Ireland [2].
Physical Characteristics
Lead is a silver colored metal with a tint of blue. It tarnishes to dark gray color when exposed to air. Lead is soft but is significantly denser than other metals. it is malleable and has a low melting point. Lead has the atomic number of 82, which is considered as the highest among all stable naturally occurring elements.
Chemical Characteristics
Lead is not very reactive. It belongs to the post-transition metal group in the periodic table. Lead frequently form stable covalent bonds as compared ionic bonding. Lead reacts with acids and bases. The common oxidation state of lead in compounds is +2. Lead can also form bonds with other lead molecule and can acquire unique arrangement, such as rings, chain and polyhedral structures.
Significance and Uses
- Lead toxicity has led to major decline in its uses since mid-1980s. It is still used in various industries where its toxic effects are minimal for the environment and humans.
- Lead has been widely used for bullets.
- It is used in weight belts used by scuba divers to counteract their buoyancy.
- Lead is used to cover underwater cables as it is resistant to corrosion.
- It is widely used in construction industry. And is part of gutters, roofing material, cladding etc., to reinforce the structure.
- Lead is used to make sculptures and statues.
- Lead alloys with copper, including bronze and brass are used to make components of machinery.
- Lead is used in lead batteries and various supercapacitors are being used to in US, Australia and Japan in various megawatt scale applications.
- Lead is used to detect aldehydes, and organic acids (Oddy test).
Health Hazards
Lead is highly toxic element. Its toxicity was recognized in the late 19th century. It can accumulate in bones and soft tissues and damages the neurons and nervous system. Lead has the ability to cross the blood-brain barrier and cause damage to brain and other organs. Lead toxicity is specially alarming for children as it can lead to lifelong neurological disorders, and behavioral challenges. Lead exposure during pregnancy can cause miscarriage and can lead to delay in puberty in girls. Lead is known to interfere with many biologically significant enzymes, including enzyme of heme synthesis pathway [3]. Lead toxicity is primarily caused by the ingestion of lead-based paints, in toys, or in home. Lead dust from furniture or window sill painted with lead can enter the body via hand to mouth contact. Inhalation of lead is the second most common pathway of lead toxicity, and lead from automobiles and cigarette smoking can cause severe toxic effects on the human body. About 100 mg/m3 of airborne lead concentration is considered dangerous to health and life [4]. Lead can persist in our environment, especially organic soil and can remain there for hundreds of years. From contaminate soil and water, lead can enter the food chain and effect animals. It also enters the human body through consumption of contaminated fruits, and vegetables. Lead can accumulate in bodies of marine animals, especially fishes and cause bioaccumulation.
Isotopes of Lead
Lead consist of four stable isotopes, lead-204, lead-206, lead-207 and lead-208 [5]. There are five unstable isotopes of lead. The atomic number of lead is 83, which makes lead the heaviest stable element and isotope-208 is the heaviest stable nucleus. The theoretical half-life of isotopes of lead is around 1035 to 10189. There are around 43 artificial isotopes of lead, and their atomic mass range from 178-220.
REFERENCES
[1]. Elemental abundance figures are estimates and their details may vary from source to source
[2]. United States Geological Survey 2017, p. 97.
[3]. Cohen, Trotzky & Pincus 1981, pp. 904–06.
[4]. National Institute for Occupational Safety and Health.
[5]. IAEA – Nuclear Data Section 2017