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In my lecture notes for inorganic chemistry, it is stated that magnesium has high affinity for oxygen but is rather stable at room temperature in air.

Can anyone explain why this is the case?

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In fact we could have the reaction O2 + 2Mg --> 2MgO , so can you have some point missed. Because they can have ionic repulsion, but if I am wrong correct me. –  ordinary chemistry student Nov 23 '12 at 15:27
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2 Answers

I believe that magnesium exposed to air quickly ends up with a coating of magnesium oxide on its surface. This oxide coating is quite uniform and does not allow any further oxygen to reach the surface.

Several metals, but NOT iron, have this property of a protective oxide coating. In iron the coating forms a spongey surface easily penetrated by further oxygen.

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Magnesium's oxide layer is not protective- its not tenacious like aluminium oxide on aluminium. Its simply that virtually all metals form an oxide layer almost instantaneous with cold dry air- the activation potential is small because the surface of metals has high energy due to deformed metal bonds to form the surface. After the initial oxide is formed, the reaction always slows down as a greater activation energy is needed for it to continue- both iron and magnesium are still corroding but just its not obvious. Also don't confuse iron oxide (which would initially FeO) with rust Fe2O3.xH20- with the normal thickness of FeO on iron its just about clear.

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