Lead is a metallic element with the chemical symbol Pb. Its principal ore is galena, and it has important applications in lead-acid batteries, radiation shielding, and roofing. Lead is the heaviest stable element and has the ability to absorb high frequency electromagnetic radiation. It is chemically most similar to tin and is resistant to corrosion by most acids. However, it is highly toxic and chronic exposure to low levels of lead can lead to serious symptoms. Its use has declined due to its toxicity, and leaded petrol has been phased out in many countries.
Lead, atomic number 82 in the periodic table, is a metallic element with the chemical symbol Pb, which stands for plumbum, the Latin name for the element. It is a soft and ductile metal, silvery-white when cut, but when exposed to air it quickly acquires a dull gray appearance due to the formation of an oxide layer. Although occasionally found in the elemental state, the principal ore of lead is galena, or lead sulfide (PbS); other lead minerals include cerussite – lead carbonate (PbCO3) – and anglesite – lead sulphate (PbSO4). Historically, the chemical and physical properties of lead made it a very useful element, but since the late 20th century its use has declined due to its toxicity. However, lead still has a number of important applications, for example in lead-acid batteries, for radiation shielding and as a flexible and resilient roofing material.
The metal melts at 622.4°F (328°C) and boils at 3,164°F (1,740°C). The four stable isotopes of lead are the end products of the decay of various naturally occurring radioactive elements, such as uranium and thorium, through a series of steps. Lead is the heaviest stable element, a distinction that belonged to bismuth – element number 83 – until it was discovered to be mildly radioactive. One of the most important physical properties of lead is its ability to absorb high frequency electromagnetic radiation, such as X-rays and gamma rays. This is due to its high density and large number of electrons in the lead atom.
Lead belongs to the same group as carbon, silicon, germanium and tin. These elements become more metallic in character with increasing atomic weight, and while lead’s chemical properties bear some resemblance to those of the other members of the group, it is chemically most similar to the metal, tin. In its compounds, lead usually has an oxidation state of +2, which means it donates two electrons to other atoms or molecules. Less commonly, it can have an oxidation state of +4.
The metal combines with oxygen to form different oxides. “Red lead,” formed by heating lead in air, has the formula Pb3O4, but is thought to be a compound of lead oxide (PbO) and lead dioxide (PbO2). Lead oxide, also known as litharge, forms when the metal is heated strongly in air and can take the form of a yellow powder or red crystalline material.
The “white lead” is basic lead carbonate (2PbCO3 Pb(OH)2). It was formerly used extensively in paints due to its strong white color before being largely replaced by the non-toxic titanium dioxide. Aside from its toxicity, one problem with white lead was that it tended to react slowly with trace amounts of hydrogen sulfide (H2S) in the air to form black lead sulfide. This is a good test for H2S, but it meant that old paintings tended to darken over time.
Lead is resistant to corrosion by most acids, as most lead salts have little or no solubility in water and form a layer that protects the lead from further action. However, it will react with acetic and nitric acids, since the salts formed by these reactions – lead acetate and lead nitrate, respectively – are very soluble. Lead reacts with “hard” water to form insoluble basic lead carbonate, but forms soluble compounds with soft water, meaning lead water pipes pose a higher risk of lead poisoning in soft water areas .
Probably the best known of lead’s properties is its toxicity. Cases of acute lead poisoning are rare, but it is a cumulative poison and chronic exposure to low levels of lead can lead to a number of serious symptoms. It deactivates the enzymes that produce hemoglobin, leading to a buildup of the precursor chemical – this can paralyze the intestines, resulting in constipation and abdominal pain, and cause fluid to build up in the brain, causing headaches. Over a longer period, it causes anemia and neurological problems.
Chronic lead poisoning has been a significant problem due to the widespread use of lead in applications that allow it to enter the environment. For example, metallic lead was formerly used in water pipes and lead compounds were used in paints. These uses have been discontinued in most countries and lead pipes have been replaced by non-toxic alternatives. The largest source of lead in the environment has been the compound tetraethyl lead, which has been added to gasoline for smoother combustion. Due to concerns about the health effects of lead in the environment, particularly on children in urban areas, leaded petrol has also been phased out in many countries.
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