In reactions, metals act as reducing agents, that is, elements that donate electrons. It is easier to donate those electrons that are farther from the nucleus. In the periodic table, the radius of an atom (the distance from the nucleus to the electrons of the outer layer) increases from right to left and from top to bottom. Mg and Ca are in the same group, but in different periods. Mg is higher than Ca, which means that Ca has more energy levels, that is, the radius of the atom is larger, which means that it is easier for it to donate electrons than Mg, therefore its metallic properties are more pronounced.
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