Answer:
looseness of outer electrons in metal atoms
Step-by-step explanation:
Unlike in insulators the valance electrons in metals are free to roam inside the boundaries of the metal slab. These electrons are called free electrons.
These electrons are free to move because there is no gab between their valance band and conduction band.
Since they are free to move they can transfer charges easily and be a good conductor.
Also heat exchange takes place when one electron collides with another moving electron or a stationary atom.
This is why metals are both good heat conductors and good electrical conductors.