When calcium and magnesium metal are first purchased they are extremely shiny with metallic luster. The metal samples you received today however were not very shiny. During the past year these metals have been stored on a shelf until today's experiment/lab. Why wasn't the calcium metal or the magnesium metal as shiny as the aluminum metal when you first started today's lab?

Answer: The calcium and magnesium metal samples had undergone a chemical reaction (oxidation) with the oxygen present in the air around the shelf to form oxides which caused them to lose their metallic luster overtime. This process is known as tarnishing.

Aluminium also undergoes this process but at a slower rate due to its lower reactivity compared with calcium or magnesium.

Explanation:

Calcium and magnesium belong to the group 2 of the periodic table of metals, generally called alkaline earth metals. They are very reactive and on exposure to air, they react with the oxygen present in air to form coatings/films of calcium oxide and magnesium oxide respectively over the metals. This causes the metals to lose their metallic shine or luster overtime.

Aluminium also undergoes this process generally known as tarnishing forming aluminium oxide but at a slower rate when compared to calcium or magnesium due to their being more reactive than aluminium is.