Back in high school I was taught a definition of metals:
Metals are a lattice of positive nuclei embedded within a sea of delocalised electrons
Metals are conductive because the electrons aren't bound to any particular atom, and hence can flow freely throughout the entire metal.
However, I was recently taught about electron orbitals, and I'm not sure how to reconcile the two pictures. If the metal atoms were sharing pi or sigma bonds, I'm not sure how any of the electrons could be truly delocalised, and how conductivity arises. In terms of the atomic orbital theory, how does conductivity arise in metals?