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Arrangement (a) contains a cation that is lesser in size than its surrounding anions. Due to a high difference in size and charge, the ions are more tightly packed, and the close proximity of anions in (a) decreases the stability of the molecule. On the other hand, (b) features cations and anions much closer in size and more spread apart. Ions that are closer in charge exert forces of repulsion on each other, causing them to be less tightly packed. Because there is more distance between the ions, there is also more distance between each anion, making the molecule in (b) more stable.