posttransitionmetal

Posttransition metals are metallic elements that lie after the transition metal series on the periodic table. They are located in the p-block, generally in groups 13–16, and are commonly considered to include aluminum (Al), gallium (Ga), indium (In), thallium (Tl), tin (Sn), lead (Pb), and bismuth (Bi); polonium (Po) is sometimes included depending on the source.

These elements share metallic bonding and tend to be softer and have lower densities and melting points than many transition metals. They are typically less conductive and more prone to covalent bonding than d-block metals. Their chemistry is marked by a range of oxidation states: +3 is common for the group 13 elements (Al, Ga, In, Tl), while tin and lead exhibit +2 and +4, bismuth typically +3, and polonium shows multiple states but is highly radioactive.

The oxides of posttransition metals are often amphoteric (as with aluminum oxide) or basic, reflecting their hybrid character between metals and nonmetals. They form a variety of compounds and alloys; several are economically important. Aluminum is widely used for its light weight and corrosion resistance; gallium and indium are essential in electronics and optoelectronics; tin is used in solder and tinplate; lead is used in batteries and shielding; bismuth is employed in alloys and cosmetics. Thallium and polonium have niche applications but are toxic or highly radioactive.

Classification schemes vary, and some sources include or exclude polonium. Overall, posttransition metals occupy a middle ground between true transition metals and the heavier main-group metals, with properties that are intermediate between those two groups.