Post-transition metals

Where they sit on the periodic table

They occupy groups 13 to 16: aluminum, gallium, indium and thallium in group 13, tin and lead in group 14, bismuth in group 15 and polonium in group 16. Their outer electrons are in s and p orbitals, as in aluminum ([Ne] 3s² 3p¹), and from gallium onward a full d subshell lies underneath.

Classifications vary, and polonium is sometimes called a metalloid. Nihonium, flerovium, moscovium and livermorium (113–116) sit below this family in period 7 and are expected to be post-transition metals, but so few atoms have been made that this site lists them as unknown.

Properties

These are true metals, shiny and good conductors, but softer than the transition metals and with low melting points. Except for aluminum (660 °C), all of them melt below about 330 °C. Gallium melts just under 30 °C, so it turns liquid in your hand, yet it does not boil until about 2400 °C. Many of their oxides are amphoteric, reacting with both acids and bases. Thallium and lead are toxic, and polonium is intensely radioactive.

Reactions and trends

Aluminum is reactive, but a thin, tough oxide layer forms as soon as it meets air and protects it: 4Al + 3O₂ → 2Al₂O₃. Its reactivity drives the thermite reaction, 2Al + Fe₂O₃ → Al₂O₃ + 2Fe, which produces molten iron for welding rail tracks.

Down groups 13 to 15 the heavier elements increasingly prefer an oxidation state two below the group number, because their outer pair of s electrons resists bonding. This inert pair effect makes thallium most stable as +1, lead as +2 and bismuth as +3, while aluminum is almost always +3.

Everyday uses

Aluminum is the most abundant metal in Earth’s crust and, after iron, the most widely used, in cans, foil, aircraft and power lines. Tin coats steel cans and is mixed with copper to make bronze. Lead is used in car batteries and X-ray shielding but has been removed from gasoline and paint because it is toxic. Gallium nitride and gallium arsenide are used in LEDs, indium tin oxide makes touchscreens conductive, and bismuth, unusually nontoxic for a heavy metal, is used in stomach remedies.