the metallic bond
Metals bond in a third, distinctive way: instead of trading electrons pairwise, the atoms each donate their loose outer electrons into a common pool that flows freely through the whole solid. Picture positive ion cores sitting in a shared sea of mobile electrons, glued together by the attraction between the positive cores and the negative sea. That collective arrangement is the metallic bond.
It forms among atoms of low electronegativity, metals, which hold their few valence electrons weakly and are content to let them go communal. Like the ionic bond, the metallic bond is non-directional: the electron sea surrounds every ion core evenly, pulling equally in all directions, with no preferred angle. So metal atoms behave like equal spheres free to pack as tightly as geometry allows, and they overwhelmingly choose dense, high-coordination arrangements: face-centred cubic and hexagonal close packing (coordination 12, 74 percent filled) or body-centred cubic (coordination 8).
The free, mobile electron sea explains a metal's signature properties in one stroke. The electrons carry charge and heat easily, so metals conduct electricity and warmth well. The non-directional bonding lets planes of atoms slide over one another without breaking the bond (the sea simply flows along), which is why metals are ductile and malleable rather than brittle; you can hammer and draw them. And because the bond does not point anywhere, dislocations can glide easily, giving metals their formability. Non-directional plus dense-packed plus a shared electron sea is the whole personality of a metal.
Copper atoms give up their outer electrons to a shared sea and pack face-centred cubic (coordination 12, packing factor 0.74). Those roaming electrons make copper an excellent conductor, and the non-directional bonding lets its atomic planes slip, so copper can be drawn into fine wire without shattering.
A shared, non-directional electron sea favours dense close packing and gives metals conduction and ductility.
The electron sea is a deliberately simple cartoon; the full quantum picture (energy bands) is more accurate and explains why some metals are poor conductors and why band structure matters. But for understanding why metals close-pack and deform, the non-directional electron-sea idea is exactly the right level.