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Magnesium atom precedes calcium atom in the same group, i.e., group 2. Magnesium atom has got three electron shells (2, 8, 2) whereas calcium atom has four electron shells (2, 8, 8, 2). So, calcium atom is larger than magnesium atom.

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magnesium is in period 3 and calcium is in period 4. Thus calcium atom has an extra orbit than magnesium. So the atomic radius of Mg is less than that of Ca