Carbon and its Compounds — Science STD 10 — Question

Mg has three outer shells thus electrons are closer to the nucleus of the atom and requires more energy in order to react to achieve ‘full outer shell’.
Ca has 4 shells and this 2⁺ electrons in the outer shell is less attracted to the nucleus making it more likely to want to achieve the ‘full outer shell’ so it takes less energy to lose those 2 electrons than magnesium.
When less energy is required for an atom to achieve its ‘full outer shell’ it is more reactive. When the electrons are being pulled by subatomic forces it takes a lot more energy to react. Thus Ca is more reactive than Mg.