MCAT Question A Day - 2/14/14 - Answer!

Which of the following elements has an ionic radius that is larger than its atomic radius?

A. Na

B. K

C. Mg

D. Cl

D. This problem is best answered based on atomic properties and periodic trends. In general, a cation is smaller than a neutral atom of the same element because the removal of electrons decreases the electrostatic repulsion between the remaining electrons, thereby allowing the nucleus to pull them closer, decreasing the ionic radius. Conversely, anions are generally larger than the corresponding neutral atoms because of an increased number of electrons that repel one another. Thus the question can be translated into "Which of the following elements is most likely to form an anion?" This revised stem can be answered on the basis of periodic trends, in particular the trend in electronegativity which increases toward the upper right corner of the table. Chlorine, Cl, is the most electronegative of the elements listed; it would thus be expected to have the greatest electron affinity, and is thus most likely to take on an electron and become anionic. The four elements in the incorrect choices are all more likely to form cations than anions; their ionic radii will thus be smaller than their atomic radii.

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