Question

Which of the following has the highest tendency to give the following reaction?
${M^ + }(g)\xrightarrow[{medium}]{{aqueous}}{M^ + }(aq)$
A. \[Na\]
B. \[Li\]
C. \[K\]
D. \[Rb\]

Answer 1

Hint: The size of ions will decide here which metal ion will have the highest tendency to replace and do the required reaction. And we know that the size of the element increases down the group.

Step by step answer: A hydration reaction is a chemical reaction in which a substance combines with water.
In this given reaction we can see that the metal ion is in a gaseous state and reacting in aqueous medium is converted into a liquid state.

${M^ + }(g)\xrightarrow[{medium}]{{aqueous}}{M^ + }(aq)$

Now, the metals given in the option are Sodium, Lithium, Potassium and Rubidium. These all are alkali metals and group one elements. Now we know the order of size of the atoms. Atomic size increases down the group due to the increase in number of electrons. Therefore, the smallest size is of Lithium and largest size is of rubidium. We know that smaller the size of cation, more is the tendency to get hydrated. As lithium ions have the smallest size, more is its stability of hydrated ions. So, it has the highest tendency to give the following reaction.

Therefore, the correct answer is option B.

Additional Information: Group 1 elements are lithium, sodium, potassium, rubidium, caesium and francium. These alkali metals are reactive as they have one electron in its outermost shell. They are also called lithium families. Because of one valence electron they have weak metallic bonding. They are soft and have low density, melting point, boiling point as well as heats of sublimation, vaporization and dissociation. It has low 1st ionisation energy and very high second ionisation energy.

Note: The hydration tendency here is more for the smallest element. But it may not be the same for every first element for the periodic table. It must be taken care that while moving along a period the size decreases so students may go wrong here.