The Drude model considers the metal to be formed of a
collection of positively charged ions from which a number of "free
electrons" were detached. These may be thought to be the valence
electrons of the atoms that have become delocalized due to the
electric field of the other atoms.
The Drude model neglects long-range interaction between the
electron and the ions or between the electrons; this is called the
independent electron approximation.
The electrons move in straight lines between one collision and
another; this is called free electron approximation.
The only interaction of a free electron with its environment
was treated as being collisions with the impenetrable ions
core.