Protective Oxide Layer on Aluminum

Steps:

• Metals oxidize in air to form surface oxide layers.
• Aluminum's oxide (Al₂O₃) is thin, dense, and adheres tightly to the metal.
• This layer acts as a barrier, blocking oxygen access and halting further reaction.
• Result: Aluminum appears less reactive despite high inherent reactivity.

Why A is correct:

• Aluminum forms a passive Al₂O₃ layer per the passivation principle, which self-limits corrosion by impermeability.

Why the others are wrong:

• B. Copper's CuO layer is porous, allowing ongoing oxidation to verdigris.
• C. Iron's Fe₂O₃ rust layer flakes off, exposing metal for continued reaction.
• D. Silver's Ag₂O layer is unstable and thin, not preventing tarnishing or further exposure.

Final answer: A