Answer
See explanation.
Work Step by Step
The free electron model is discussed on page 1420.
The speed of electrons is so high that we may assume they see a uniform, spatially-averaged potential. It is also reasonable to assume that the electron is free to move anywhere within the metal because it is so mobile. Finally, the assumption that electrons don’t interact with ions may be defended by noting that the ions’ positive charges are shielded by other electrons’ negative charges.
