Magnesium ribbon is rubbed before burning because it comprises a layer of basic magnesium oxide. Magnesium is a reactive metal that creates a layer of magnesium oxide on its surface when exposed to air and further prevents reactions between magnesium and oxygen.

• The alkaline earth metal is called magnesium. The second group of metals in the periodic table is known as Alkaline earth metals. They only have two electrons in their outer electron shell, which are very reactive.
• Magnesium has a high reactivity nature, that is why magnesium ribbons need to be cleaned before use.
• When a magnesium ribbon is exposed to air for a long period of time, the oxygen in the air reacts with the magnesium ribbon and gets oxidized into magnesium oxide.

2Mg+ O2⟶2MgO 

(The above-given reaction is an example of a combination as well as an oxidation reaction)