Chemical properties describe how a substance interacts with other substances or transforms into new ones through chemical reactions. These characteristics are observed during a chemical process, where the substance’s identity changes. Lead, a metal known since antiquity, possesses distinct chemical properties that dictate its behavior, applications, and environmental interactions.
Lead’s Foundational Atomic Characteristics
Lead, represented by the chemical symbol Pb, has an atomic number of 82. It is positioned in Group 14, also known as the carbon group, and Period 6 of the periodic table. The electron configuration of a lead atom is [Xe]4f¹⁴5d¹⁰6s²6p². This configuration indicates that lead has four valence electrons in its outermost shell, specifically two in the 6s orbital and two in the 6p orbital.
Lead primarily exhibits two common oxidation states: +2 and +4. The +2 oxidation state is more prevalent and stable for lead due to a phenomenon called the inert pair effect. This effect describes the tendency of the two outermost s-orbital electrons to remain unshared and unreactive, making it easier for lead to lose only its two p-orbital electrons during chemical bonding. The +4 oxidation state, where all four valence electrons participate in bonding, is less common in inorganic lead compounds.
Reactivity with Common Substances
When freshly exposed to air, metallic lead rapidly tarnishes, developing a thin layer of lead(II) oxide (PbO) on its surface. This oxide layer serves as a protective barrier, preventing further oxidation, a process known as passivation. While lead generally resists pure water, it can slowly react in the presence of dissolved oxygen to form protective surface layers, such as lead carbonate or lead silicate.
Lead’s reaction with acids varies. With nitric acid, lead readily forms lead(II) nitrate, though concentrated nitric acid can form an insoluble lead oxide coating, stopping further reaction. With dilute sulfuric acid, lead reacts slowly, forming insoluble lead(II) sulfate (PbSO₄), which creates a protective layer inhibiting additional reaction. This passivation is why lead lines containers for concentrated sulfuric acid.
Similarly, lead reacts slowly with hydrochloric acid, producing insoluble lead(II) chloride (PbCl₂), which also forms a passivating layer. Lead also demonstrates amphoteric behavior, reacting with strong bases to form plumbites or plumbates.
The Nature of Lead Compounds
Lead forms various compounds. Among its oxides, lead(II) oxide (PbO) exists as red litharge and yellow massicot, both insoluble in water. This oxide dissolves in acids to yield Pb²⁺ salts and reacts with alkaline solutions to form plumbites. Lead(IV) oxide (PbO₂), a dark-brown or black crystalline powder, is also insoluble in water and is a powerful oxidizing agent. Red lead, or trilead tetroxide (Pb₃O₄), is a mixed-valence oxide, a combination of lead(II) and lead(IV) oxides.
Lead also forms various halides. Lead(II) chloride (PbCl₂) is a white solid with low water solubility, which increases when heated. Lead(II) iodide (PbI₂) also has limited water solubility. Lead(IV) halides are generally less stable; lead(IV) chloride (PbCl₄) is unstable, and lead(IV) bromide and iodide are not known.
Many common lead salts, such as lead sulfate (PbSO₄) and lead carbonate (PbCO₃), are largely insoluble in water. This low solubility influences lead’s environmental mobility, as they tend to precipitate from aqueous solutions. However, some lead salts like lead(II) nitrate and lead(II) acetate are water-soluble.