The correct answer to the question is Magnesium.
Calcium is a group 2 element. Every element in this group has a valence electron of 2. Other elements in this group are: beryllium (Be) , magnesium (Mg), strontium (Sr), barium (Ba) and radium (Ra)
The ion of magnesium, Mg²⁺ is smaller than the ion of calcium, Ca²⁺ because magnesium, Mg²⁺ has 2 shells while calcium ion, Ca²⁺ has 3 shells.
Potassium ion, K⁺ and calcium ion, Ca²⁺ both have 3 shells.
Strontium ion, Sr²⁺ has 4 shells while caesium ion Cs²⁺ has 5 shells.
Therefore, we can conclude that magnesium ion, Mg²⁺ is smaller than calcium ion, Ca²⁺.
The correct answer to the question is Magnesium.
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