In chemistry, post-transition metals are the metallic elements in the periodic table located between the transition metals to their left and the metalloids to their right. Usually included in this category are Gallium, Indium,Thallium, Tin, Lead and Bismuth. Which elements are counted as post-transition metals depends, in periodic table terms, on where the transition metals are taken to end and where the metalloids or Nonmetals are taken to start.
Physically, post-transition metals are soft (or brittle), have poor mechanical strength, and melting points lower than those of the transition metals; most also have boiling points lower than those of the transition metals. Being close to the metal-nonmetal border, their crystalline structures tend to show covalent or directional bonding effects, having generally greater complexity or fewer nearest neighbours than other metallic elements.
Chemically, they are characterised—to varying degrees—by covalent bonding tendencies, acid-base amphoterism and the formation of anionic species such as aluminates, stannates, and bismuthates (in the case of aluminium, tin, and bismuth, respectively). They can also form Zintl phases (half-metallic compounds formed between highly electropositive metals and moderately electronegative metals or metalloids).