metals

Metals are the shiny, dense, workable materials most people picture as 'solid stuff': iron and steel, aluminum, copper, gold, titanium. What makes a metal a metal is its bonding - the outer electrons are not tied to any one atom but form a shared 'sea' of electrons drifting through a lattice of positive ion cores. That electron sea explains almost everything metals do.

Because the electrons flow freely, metals conduct electricity and heat extremely well and reflect light (their shine). Because the bonding has no direction, layers of atoms can slide over one another, so metals are ductile - you can hammer, roll, draw, and bend them into shape without shattering (this sliding happens by dislocation motion). They are generally strong and stiff: steel's Young's modulus is about 200 GPa. Pure metals are usually soft, so we mostly use alloys - mixtures such as steel (iron plus carbon) or brass (copper plus zinc) - where the added atoms make dislocations harder to move and so raise the strength.

Metals dominate structures, machines, wiring, and tools because they combine strength with the ability to deform rather than crack. Their big weaknesses are that many corrode (iron rusts) and they are heavy (steel's density is about 7.9 g/cm^3, over seven times water). And making a metal 'stronger' almost always means making it harder for dislocations to move, which usually costs some ductility - a real trade-off.