which element has an ion that is smaller than an ion of calcium?\no potassium\no strontium\no cesium\no…

Explanation:

Step1: Recall ionic - size trends

Ionic size decreases across a period and increases down a group. Calcium ($Ca^{2 + }$) is in Group 2.

Step2: Analyze each option

• Potassium ($K^{+}$) is in Group 1, one group to the left of calcium. But its ion is larger than $Ca^{2+}$ due to its lower effective nuclear charge.
• Strontium ($Sr^{2 + }$) is in the same group as calcium but in a lower - period, so its ion is larger.
• Cesium ($Cs^{+}$) is in Group 1 and a much lower period than calcium, so its ion is larger.
• Magnesium ($Mg^{2 + }$) is in the same group as calcium but in a higher period. Ions of elements in the same group decrease in size as you go up the group. So, $Mg^{2+}$ is smaller than $Ca^{2+}$.

Answer:

magnesium