Concept:Some reactive metals undergo "passivation," where initial contact with air forms a non-porous, chemically stable oxide layer that completely seals the metal from further atmospheric attack.
Solution:- Aluminum (Al) is a reactive Period 3 metal.
- When freshly exposed to air, it reacts instantly with oxygen to form a thin, transparent, and incredibly tough layer of Aluminum oxide (\( \text{Al}_2\text{O}_3 \)).
- Unlike iron rust, which flakes off, this oxide layer tightly adheres to the metal surface.
- This layer acts as an impenetrable shield, which is why Aluminum does not corrode readily in daily use despite being a reactive metal.
Why other options are incorrect:- Na: Its oxide does not protect it; it continues to react violently with moisture and air.
- Mg: Also forms a protective layer (MgO), but Aluminum is the classic, textbook example of this phenomenon used heavily in industry (anodization).
- Si: Is a metalloid and does not undergo rapid metallic oxidation in this manner at room temperature.
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