the metallic bond

Metal atoms hold their outer electrons so loosely that, when packed together, they simply let go of them into a common pool. The result is the famous picture of a sea of electrons: a regular array of positive ion cores bathed in a fluid of shared, mobile electrons that belong to no single atom but to the whole crystal. The negative sea glues the positive cores together, and that shared, roaming glue is the metallic bond.

Because the bonding electrons are not pinned in a particular direction or between a particular pair, the bond is nondirectional — an ion core is happily held by the electron sea no matter which way its neighbours sit. This has two large consequences. First, the free electrons can drift in response to a voltage or carry thermal energy, which is exactly why metals conduct electricity and heat so well. Second, and just as important, you can slide one layer of ion cores over another and the electron sea simply flows to keep gluing them — the bond does not care about the new arrangement.

That second point is the deep reason metals are ductile: they bend, draw into wire, and hammer into sheet instead of shattering, because their bonding tolerates atoms rearranging. Metallic bond energies are large (roughly 100 to 850 kJ/mol), so most metals are also strong and have high melting points, and their bonding is what lets them be shiny (the free electrons reflect light). This combination — conductive, formable, tough — is why metals became the backbone of engineering.