Which one of the following cations has the maximum hydration number?

Question

Question

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UP LT Grade Teacher (Science) 2018 Official Paper

Answer 1

CONCEPT:

Hydration Number

Hydration number refers to the number of water molecules that surround a cation in an aqueous solution.
The hydration number is influenced by the size and charge density of the cation. Smaller and highly charged cations tend to have a higher hydration number because they have a stronger attraction for water molecules.
Charge density is calculated as the ratio of charge to ionic radius. Higher charge density leads to a higher hydration number.

EXPLANATION:

Li⁺ is the smallest and most highly charged among the alkali metals (with charge density). Its smaller size allows it to attract more water molecules, giving it a higher hydration number compared to other alkali metal cations like K⁺, Rb⁺, and Cs⁺.
$C$ s $^{+}$ is highly charged and has a small ionic radius, which suggests that it would have a very high hydration number, possibly the highest among the listed options.
K⁺ and Rb⁺ have much larger ionic radii and thus lower charge densities, leading to fewer water molecules being associated with them compared to Li⁺.
Thus, the cation with the maximum hydration number is Li⁺ (option 4).

Therefore, the cation with the maximum hydration number is Li⁺.