207Chemical Reactivity
Lead is classified as a post-transition metal and is also a member of the carbon group. Lead only
forms a protective oxide layer although finely powdered highly purified lead can ignite in air. Melted
lead is oxidized in air to lead monoxide. All chalcogens oxidize lead upon heating.
Fluorine does not oxidize cold lead. Hot lead can be oxidized, but the formation of a protective
halide layer lowers the intensity of the reaction above 100 °C (210 °F). The reaction with chlorine
is similar: thanks to the chloride layer, lead persistence against chlorine surpasses those of copper
or steel up to 300 °C (570 °F).
Water in the presence of oxygen attacks lead to start an accelerating reaction. The presence of
carbonates or sulfates results in the formation of insoluble lead salts, which protect the metal from
corrosion. So does carbon dioxide, as the insoluble lead carbonate is formed; however, an excess
of the gas leads to the formation of the soluble bicarbonate; this makes the use of lead pipes
dangerous. Lead dissolves in organic acids (in the presence of oxygen) and concentrated (≥80%)
sulfuric acid thanks to complexation; however, it is only weakly affected by hydrochloric acid and is
stable against hydrofluoric acid, as the corresponding halides are weakly soluble. Lead also
dissolves in quite concentrated alkalis (≥10%) because of the amphoteric character and solubility
of plumbites.
Compounds
Lead compounds exist mainly in two main oxidation states, +2 and +4. The former is more common.
Inorganic lead(IV) compounds are typically strong oxidants or exist only in highly acidic solutions.
Oxides and Sulfides
Three oxides are known: lead(II) oxide or lead monoxide (PbO), lead tetroxide (Pb 3 O 4 ) (sometimes
called "minimum"), and lead dioxide (PbO 2 ). The monoxide exists as two allotropes: α-PbO and β-
PbO, both with layer structure and tetracoordinated lead.
The alpha polymorph is red-colored and has the Pb–O distance of 230 pm; the beta polymorph is
yellow-colored and has the Pb–O distance of 221 and 249 pm (due to asymmetry). Both
polymorphs can exist under standard conditions (beta with small (10−5 relative) impurities, such as
Si, Ge, Mo, etc.). PbO reacts with acids to form salts, and with alkalis to give plumbites, [Pb(OH) 3 ]−
or [Pb(OH) 4 ]2−.The monoxide oxidizes in air to trilead tetroxide, which at 550 °C (1020 °F) degrades
back into PbO.
The dioxide may be prepared by, for example, halogenization of lead(II) salts. Regardless the
polymorph, it has a black-brown color. The alpha allotrope is rhombohedral, and the beta allotrope
is tetragonal. Both allotropes are black-brown in color and always contain some water, which
cannot be removed, as heating also causes decomposition (to PbO and Pb 3 O 4 ).
The dioxide is a powerful oxidizer: it can oxidize hydrochloric and sulfuric acids. It does not react
with alkaline solution, but reacts with solid alkalis to give hydroxyplumbates, or with basic oxides to
give plumbates.
Reaction of lead salts with hydrogen sulfide yields lead monosulfide. The solid has the rocksalt-like
simple cubic structure, which it keeps up to the melting point, 1114 °C (2037 °F). When heated in
air, it oxidizes to the sulfate and then the monoxide. Lead monosulfide is almost insoluble in water,
weak acids, and (NH 4 ) 2 S/(NH 4 ) 2 S 2 solution is the key for separation of lead from analytical groups
I to III ions, tin, arsenic, and antimony.