Question

Comment on each of the following
I) The mobilities of alkali metal ions in aqueous solution are
Li^+< Na^+ ii) Lil is more soluble than KI in ethanol
iii) Beryllium and Magnesium do not impart colour to the flame while other
members of the group do so.

Answer 1

Comment on (2)

Lil has a a covalent character because Li ion has high polarizing power due to its small size. Therefore, Lil is soluble in alcohol. On the other hand, k i is predominantly an ionic compound, therefore is insoluble in ethanol.

Comment on (1)

On moving down the alkali group, the ionic and atomic sizes of the metals increase. The given alkali metal ions can be arranged in the increasing order of their ionic sizes as :

$l {i}^{ + } < n {a}^{ +} < {k}^{ + } < r {b}^{ + } < c {s}^{ + }$

Smaller the size of an ion, the more highly is it hydrated. Since Li^+ is the smallest, it gets heavily hydrated in an aqueous solution. On the other hand, Cs^+ is the largest and so it is the the least hydrated. The given alkali metal ions can be arranged in in decreasing order of their hydrations as :

$l {i}^{ + } > n {a}^{ + } > {k}^{ + } > r {b}^{ + } > c {s}^{ + }$

Greater the mass of a hydrated ion the lower is its ionic mobility. Therefore, hydrated Li^+ is the list mobile and hydrated Cs^+ is the most mobile. Thus the given alkali metal ions can be arranged in the increasing order of their mobilities as :

$l {i}^{ + } < n {a}^{ + } < {k}^{ + } < r {b}^{ + } < c {s}^{ + }$

Comment on (3)

Beryllium and magnesium do not impart any colour to the the flame because they have small size and very high ionization enthalpies. The energy of the flame is not sufficient to excite the electrons to the higher energy levels. Hence, they do not impart any characteristic colour to the flame.

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