Does sanding the oxide layer of a magnesium sample break bonds? We all know that if we keep a piece of magnesium metal exposed in open air, an oxide layer forms on it. This layer prevents further oxidation of the bulk metal. If we then rub it with sand paper, the magnesium metal can react again because the oxide layer is removed. So my question is:

Does that mean abrasion with sandpaper can break the oxygen bonds with magnesium?
I upvoted the question because it is actually very insightful. We may think of simply removing the MgO layer, and voila, there is bare magnesium. I think not.

If you took bare Mg (no oxide at all) in a vacuum, and dropped powdered MgO onto the Mg, then exposed the metal to an argon atmosphere, I think you could blow off the oxide powder because there would be no significant adhesion.

In other words, the unreactivity of the oxide-coated magnesium arises because the oxide coat is strongly bonded to the metal. This transition from metal to metal oxide occurs over a few atoms and is probably very irregular. After all, even the surface of the metal won't even be flat to within 1 or 2 atoms. 

Visible scratches on the metal after sandpapering prove that some mechanical effect has removed metal. If you remove metal, you must have broken some metal-metal bonds; and in the turmoil, at least some metal - oxygen bonds must also have been broken - even tho they might soon reform with O2 in air.

A similar, but chemical, film removal occurs when stainless steel, which is likewise protected by a very thin oxide coat, is treated with hydrochloric acid. The film is removed, and the steel corrodes. Because the reaction is so thorough, removing the whole oxide coat and a few atomic layers of metal as well, we don't stop to think about the transition layer. Different thicknesses can produce different colors as well as different degrees of protection.