Post-transition metals sit in an interesting no-man's-land on the periodic table — to the right of the transition metals and to the left of the metalloids. They include aluminum, gallium, indium, tin, thallium, lead, and bismuth. While they are still metals — they conduct electricity, have a metallic sheen, and form positive ions — their metallic character is noticeably weaker than that of transition metals. They tend to be softer, melt at lower temperatures, and are poorer conductors. Gallium famously melts at just 29.76°C, meaning it will literally liquefy in your hand on a warm day.
Aluminum is the star of this group and the most abundant metal in the Earth's crust, making up about 8% by mass. Despite this abundance, it was once more expensive than gold — Napoleon III reportedly served honored guests on aluminum plates while lesser guests ate from gold. The invention of the Hall-Héroult electrolytic process in 1886 made aluminum cheap and ubiquitous: today, about 65 million tonnes are produced annually for aircraft, packaging, construction, and cookware. Tin has been vital since the Bronze Age (bronze is a copper-tin alloy), and tin cans revolutionized food preservation. Lead, despite its toxicity, shaped history through Roman plumbing (the Latin word "plumbum" gives us both Pb and "plumber") and is still used in car batteries.
A distinguishing chemical feature of post-transition metals is the "inert pair effect" — a tendency for the two s-electrons in their outer shell to resist participating in bonding as you go down the group. This is why thallium prefers the +1 state over +3, and lead favors +2 over +4. Bismuth is noteworthy as the heaviest element with a stable (or nearly stable) isotope — bismuth-209 has a half-life of about 1.9 × 10¹⁹ years, far longer than the age of the universe. Bismuth crystals, with their iridescent, staircase-like geometry, are among the most beautiful mineral specimens you can grow at home.