Ca₂⁺ ion has a lower ratio of charge to volume.
The charge density of an ion is defined as the ratio of the charge on an ion to its volume, due to smaller volume and more charge, Ca have a higher charge density.
Charge density of an ion is defined as the ratio of charge on an ion to its volume. The properties of ion that affects its charge density are magnitude of charge and volume of ion. Volume is governed by the size. More is the size of ion, more will be its volume. It is directly proportional to magnitude of charge and inversely proportional to volume of the ion.
K and Ca belong to the first and second groups of the periodic table in the same period. On moving from K to Ca, the size decreases. So, the size of Ca is less than that of K. The magnitude of charge in Ca2+ is double that of K+ but the volume of Ca2+, due to its small size, is smaller than that of K+. Hence, due to smaller volume and more charge, Ca have a higher charge density.
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