When does metallic bonding occur?
Metallic bonding occurs when you have a metal in the solid or liquid state.
The s and p valence electrons of metals are loosely held. They leave their “own” metal atoms. This forms a "sea" of electrons that surrounds the metal cations. The electrons are free to move throughout this electron sea.
In this model, the valence electrons are free, delocalized, and mobile.
Metallic bonding is the attractive force between the metal cations and the sea of electrons.
Note that it is not correct to speak of “a metallic bond”, because the electrons are not attracted to a single atom. We should speak of “metallic bonding”, because the electrons attract all the metal ions and not just one.