Question

the diagram above represents four cations, all shown to the same scale. which cation would be predicted by coulombs law to have the strongest ion - dipole attraction to water, and why? a li⁺, because it is the smallest group 1 metal ion. b mg²⁺, because it has the largest charge - to - size ratio. c na⁺, because it has the smallest charge - to - size ratio. d ca²⁺, because it is the largest group 2 metal ion.

Explanation:

Step1: Recall Coulomb's law concept

The strength of ion - dipole attraction between a cation and water depends on the charge - to - size ratio of the cation. A higher charge - to - size ratio leads to a stronger ion - dipole attraction.

Step2: Analyze charges and sizes of given cations

$Li^+$ is a group 1 metal ion with a + 1 charge. $Na^+$ is also a group 1 metal ion with a + 1 charge, but it is larger than $Li^+$ due to more electron shells. $Mg^{2 + }$ is a group 2 metal ion with a + 2 charge and is relatively small. $Ca^{2+}$ is a group 2 metal ion with a + 2 charge but is larger than $Mg^{2+}$ as it has more electron shells.

Step3: Compare charge - to - size ratios

$Mg^{2+}$ has a relatively high charge (+ 2) and a small size among these cations. The high charge and small size give it the largest charge - to - size ratio, resulting in the strongest ion - dipole attraction to water.

Answer:

B. $Mg^{2+}$, because it has the largest charge - to - size ratio.