What is lead?
Lead is a chemical element or group-14 metal of the periodic table with the symbol Pb and atomic number 82. It has been used in plumbing in Roman civilization. Now, it has been stopped due to lead poisoning. Now, it is used for making storage batteries and antiknock compound (lead tetraethyl) to control air pollution. It exists in a cubic closed-packed crystal lattice with a density of 11.34g/cm3. Lead is soft, malleable post transition metal. The low melting point of lead suggests the fact that all four valence electrons in Pb do not participate in metallic bonding. +2 oxidation number is common for lead rather than +4.
Properties of lead
The chemistry and properties of group-14 elements follow from their electronic configuration. All the elements have an outermost quantum shell with an s2p2 configuration. Lead exists in a face-centered cubic closed packed crystal lattice with a density of 11.34 g/cm3. It is a low melting metal because it cannot use all four valence electrons for metallic bonding. Some important properties of lead are given below the table,
|Properties of lead
|Electronic configuration||[Xe] 4f14 5d10 6s2 6p2|
|Melting point||327.46 °C|
|Boiling point||1749 °C|
|Molar heat capacity||26.650 J mol-1K-1|
|Electrical resistivity||208 nΩ·m|
|Crystal structure||face centered cubic crystal|
|Common oxidation number||+2, +4|
|Electronegativity||Pauling scale: 2.33|
Facts about lead
- The chemical reactivity of group 14 elements like carbon, silicon, germanium, tin, and lead is decreasing down the group. Chemical potential values of Pb(II) to Pb is less than that of hydrogen but it cannot liberate hydrogen from acids.
- The finely divided lead powder is pyrophoric in nature. But the surface of the metal is inactive due to the formation of the protective oxide layer.
- It dissolves in organic acid like acetic acid or formic acid in presence of air.
- It does not dissolve in concentrated nitric acid or sulfuric acid due to insoluble coating.
Isotopes of metal
Naturally, lead has four stable isotopes with mass numbers of 204, 206, 207, and 208. Three of these four isotopes are found in the radioactive decay series. The relative abundance of these isotopes is 204Pb (1.4%), 206Pb (24.1%), 207Pb (22.1) and 208Pb (52.4%). It is the first heaviest element whose natural isotopes are stable.
Where is lead found
It is found 13 to 16 ppm in the earth’s crust. Galena (PbS) is the main source of lead metal. The other minerals which contain Pb(II) are anglesite (PbSO4) and cerussite (PbCO3). United States, Russia, Australia, Canada are the major producer of metal. In India, galena containing about 3 percent of Pb and a little amount of Ag. It is obtained from the Zawar mines at Udaipur in Rajasthan.
Extraction from galena
Galena is concentrated by the froth-flotation process. The concentrated ore is roasted by the limited supply of air to partially convert it to PbO. Self-reduction between unchanged PbS and PbO produces lead metal. (2PbO + PbS → 3Pb (l) + SO2 (g).
The crude metal contains many impurities like Cu, Ag, Au, Zn, As, Sb, and Sn. It is purified by the electrolysis process. Impure lead is crust to rods to serve as an anode in an electrolytic cell. Sheet of pure metal is used as cathode in PbSiF6 electrolyte. The electrolyte is prepared by the action of H2SiF6 on Pb. Impurities are thrown down in anode mud while more electropositive metal like iron retains in the solution.
In spite of their ns2np2 valence shell electronic configuration, the lighter elements like C or Si of group-14 do not have any bivalent compound under ordinary conditions. The bond energy values decrease significantly in heavier elements like Pb. Therefore, bivalent compounds are comparatively more stable at room temperature. The stability of group-14 hydrides (MH4) decreases from Ge to Pb due to decreases in M-H bond energy.
Yellow solid PbF4 is the only stable tetrahalide of Pb. It is decomposed to PbF2 and F2 on heating. It can be prepared by the action of fluorine or HF on lead compounds. Other tetrahalides are less stable and PbI4 does not form.
The dihalides of lead are stable crystalline solids slightly soluble in cold water but much more soluble in hot water. PbCl2 and PbBr2 are photosensitive which depositing electromagnetic spectrum radiation.
Pb(II) oxide or PbO2 is prepared by dissolving Pb3O4 in dilute nitric acid solution. It is a strong oxidizing agent and decomposes to PbO at a temperature above 300 °C.
PbO is another oxide of metal that exists in two forms like litharge and massicot. Litharge form is obtained by oxidizing molten Pb in the air above 600 °C in a reverberatory furnace. Massicot is obtained by heating Pb in air. PbO is basic in nature and dissolves in acids to form Pb(II) salts. It has an unusual layer structure in which four oxygen atoms form a base of the square pyramid with Pb metal at the vertex.
Lead(II) carbonate occurs in nature as cerussite with the chemical formula PbCO3. It is precipitated from Pb(NO3)2 solution by NaHCO3 at a low temperature. It has been used as a white pigment under the name white-Pb. But it is now getting substituted very fast by TiO2 due to the non-toxic nature of TiO2.
Lead acetate or Pb(CH3COO)2 is a white crystalline chemical compound with sweetness in taste. It can be made by boiling Pb with acetic acid and hydrogen peroxide. It is used in medicine for washing the eye. It is also used for making other lead compounds and mordant in the dyeing process.
Uses of lead
- It is used in plumbing in Roman civilization. Now it has been stopped due to lead poisoning.
- Worldwide production of increases due to the use of Pb-acid batteries.
- It has been used to make alloys like solder (50% Pb, 50% Sn) and type metal (70 to 80% Pb, 30 to 15%Sb). Because of their low melting point and expansion on solidification of Pb-Sb alloy, it is called type metal.
- Lead is used for cable coverings and making pigments. It is also used to store corrosive liquids.
- It is used for making antiknock compound like lead tetraethyl. But the manufacture of this compound is highly discouraged due to air pollution.