Final answer:
Metals tarnish when exposed to air due to the formation of an oxide layer, with the correct answer being A) Oxide. Corrosion often results in protective or destructive oxide layers, depending on the metal and conditions.
Step-by-step explanation:
Metals get tarnished when exposed to air due to the formation of an oxide layer on their surface. The correct answer is A) Oxide. Corrosion, a galvanic process involving the oxidation of metals, commonly results in the production of metal oxides. For instance, silver tarnishing and the protective films on aluminum and chromium are results of oxide formation. This process, also known as a REDOX (reduction-oxidation) reaction, leads to metals losing electrons and forming a layer of metal oxide that can either be protective, as in the case of aluminum, or destructive, such as the rusting of iron.
Some metals like aluminum form a thin, protective oxide layer that acts as a barrier and prevents further corrosion, whereas others like iron form a flaky oxide that does not protect the underlying metal. The formation of aluminum oxide on the surface of aluminum and the development of patina, a copper (II) carbonate layer on copper, are examples of protective corrosion. The significant economical impact of corrosion, especially on iron-containing objects, emphasizes the importance of understanding and preventing this process.