Multiple Choice
Which ion has a larger ionic radius: Mg^{2+} or O^{2-}?
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10. Periodic Properties of the Elements
Periodic Trend: Ionic Radius
Multiple Choice
Which of the following elements forms an ion that is smaller in ionic radius than a calcium ion (Ca^{2+})?
A
Strontium (Sr^{2+})
B
Potassium (K^+)
C
Magnesium (Mg^{2+})
D
Cesium (Cs^+)
Verified step by step guidance1
Identify the charge and electron configuration of the calcium ion (Ca^{2+}). Calcium loses two electrons to form Ca^{2+}, resulting in the electron configuration of the noble gas argon (Ar), which has 18 electrons.
Compare the ions given (Sr^{2+}, K^{+}, Mg^{2+}, Cs^{+}) by considering their charges and positions in the periodic table. Ions with the same charge and fewer electron shells generally have smaller ionic radii.
Recognize that all the ions listed have a +2 or +1 charge, but their number of protons (nuclear charge) differs. A higher nuclear charge with the same electron configuration pulls electrons closer, reducing ionic radius.
Since Mg^{2+} has fewer electron shells than Ca^{2+} and a higher effective nuclear charge per electron, it will have a smaller ionic radius than Ca^{2+}.
Use periodic trends: moving up a group decreases ionic radius for ions with the same charge, and increasing positive charge also decreases ionic radius. This explains why Mg^{2+} is smaller than Ca^{2+}.
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