Sudeep R

When magnesium ribbon is kept outside exposed to air, the outer layer of magnesium atoms begin to react with oxygen in the atmosphere and form magnesium oxide (this is a slow process). This can only happen to the outer thin layer of magnesium atoms since the magnesium oxide formed in this outer layer prevent the exposure of magnesium in the inner layers from oxygen. This outer layer of magnesium oxide does not burn in flame and this prevents the ribbon from catching fire. This is why we scrub off the layer of magnesium oxide that is formed before we burn it. (While burning, the same reaction occurs i.e. conversation if Mg to MgO but at a much faster rate and hence a huge amount of energy is realized (exothermic reaction) which makes it burn.)