Short answers, pulled from the story.
A metal is a material that shows a lustrous appearance when polished or fractured and conducts electricity and heat relatively well. These properties come from having electrons available at the Fermi level, unlike nonmetallic materials. A metal can be a chemical element such as iron, an alloy such as stainless steel, or a molecular compound such as polymeric sulfur nitride.
Most metals are malleable and ductile because metallic bonding is non-directional, which keeps the Peierls stress low and lets dislocations move easily. Close-packed atoms give dislocations small Burgers vectors, so little energy is needed to deform the metal. By contrast, ionic compounds like table salt have much larger Burgers vectors and cleave instead of bending.
The history of refined metals is thought to begin with the use of copper about 11,000 years ago. Gold, silver, iron as meteoric iron, lead, and brass were in use before bronze first appeared in the fifth millennium BCE. Sodium, the first light metal, was discovered in 1809.
Base metals oxidize or corrode easily, such as iron, nickel, lead, and zinc, and many react with dilute hydrochloric acid to form a chloride and hydrogen. Noble metals like gold, platinum, silver, rhodium, iridium, and palladium resist corrosion and oxidation. Noble metals tend to be precious metals, often because of perceived rarity.
Metallic elements up to around iron are made by stellar nucleosynthesis, where light elements from hydrogen to silicon fuse and release energy. Heavier elements form by neutron capture in the slow s-process and the rapid r-process. The r-process can skip the zone of instability at bismuth to build heavier elements such as thorium and uranium.
Metals are inherently recyclable and can be used repeatedly, which saves energy and reduces environmental impact. Recycling aluminium from old material saves 95 percent of the energy needed to make it from bauxite ore. In 2010 the International Resource Panel warned that low recycling rates for rare metals in mobile phones, hybrid car batteries, and fuel cells could make these critical metals unavailable.
After World War II metallurgists developed superalloys, bulk metallic glasses, shape-memory alloys, quasicrystalline alloys, complex metallic alloys, high-entropy alloys, and MAX phases. The first metallic glass, a gold-silicon alloy, was produced at Caltech in 1960. Dan Shechtman discovered an aluminium-manganese quasicrystal in 1984 and won the Nobel Prize in Chemistry in 2011.