Question

The mobilities of the alkali metal ions in aqueous solution are \[L{i^ + } < N{a^ + } < {K^ + } < R{b^ + } < C{s^ + }\] because:
A. Greater is the degree of hydration, lesser is the mobility in aqueous medium
B. Larger the size of cation, greater is the mobility in aqueous medium
C. Larger the size of cation, lesser is the mobility in aqueous medium
D. Lesser the degree of hydration, lesser the mobility of ions in aqueous medium

Answer 1

Hint: For solving the given question, we will first understand the comparison in the atomic sizes of the given elements. Then we must identify the trend in degree of hydration. After this, we have to identify the trend in the mobility of their corresponding ions. In the end, we must establish a relation between all these trends.

Complete Step-by-Step answer:
The elements in the given series are Lithium, Sodium, Potassium, Rubidium and Caesium. All these elements belong to the first group of the periodic table and are known as alkali metals. The order in which the atomic number, and hence atomic radii of the ions of these elements is increasing is:
\[L{i^ + } > N{a^ + } > {K^ + } > R{b^ + } > C{s^ + }\]
Addition of alkali metals to aqueous medium means the ability of the corresponding atoms to get hydrated. The size of the ion determines the extent of hydration of an atom. Atoms with smaller sizes or small atomic radii get hydrated easily as compared to larger atoms. Hence, the trend for hydration observed in the given elements is:
\[L{i^ + } > N{a^ + } > {K^ + } > R{b^ + } > C{s^ + }\]
Also, mobility of a given atom or ion is inversely proportional to its size. Hence, from the previous results, the trend observed in mobility of the ions of the given elements is:
\[L{i^ + } < N{a^ + } < {K^ + } < R{b^ + } < C{s^ + }\]
From the above explanations, we can observe that the mobilities of the alkali metal ions in aqueous solution are \[L{i^ + } < N{a^ + } < {K^ + } < R{b^ + } < C{s^ + }\] because

Hence, Options A and Care the correct options.

Note: The mobilities of the ions of these metals can be defined as the ability of charged particles to transverse through a given medium because of an external electrical field. The separation of ions in accordance with their mobility in their gas phase is known as ion mobility, while it is known as electrophoresis in the liquid phase.