In metals, the valence electrons are loosely bound to their respective atoms. These valence electrons are delocalized, meaning that they are not confined to any particular atom but can move freely throughout the metal lattice. These delocalized valence electrons are responsible for the characteristic properties of metals, such as high electrical and thermal conductivity, as well as their malleability and ductility.

Therefore, the particle that is free to drift in metals is the valence electron.