1. Hydration energy of alkali metal ions (order).
+
Correct Answer
Li+ > Na+ > K+ > Rb+ > Cs+
Explanation
Hydration energy is the energy released when one mole of gaseous ions becomes
surrounded by water molecules to form hydrated ions.
M+(g) → M+(aq)
For alkali metal ions, all ions have the same charge, +1.
Therefore, hydration depends mainly on their ionic size and charge
density.
Smaller ions have greater charge density and attract the polar water molecules
more strongly.
Smaller ionic radius → Higher charge density → Stronger hydration → More negative hydration enthalpy
Hence:
Li+ > Na+ > K+ > Rb+ > Cs+
Here the order is for the magnitude of hydration energy.
If hydration enthalpy is expressed with its sign, the values become
progressively less negative from Li+ to Cs+.
Why is Li+ Highly Hydrated?
Li+ has the smallest ionic radius among the alkali metal ions.
Its charge is concentrated over a very small volume, giving it a very high
charge density. Therefore, it strongly attracts the negative
oxygen end of water molecules.
Li+ = smallest size + highest charge density → maximum hydration
Cs+, on the other hand, is much larger and has the lowest charge
density, so its interaction with water is weakest.
JEE / NEET Shortcut
For Group 1 cations:
Ionic size:
Li+ < Na+ < K+ < Rb+ < Cs+
Therefore:
Hydration energy magnitude:
Li+ > Na+ > K+ > Rb+ > Cs+
Hydration enthalpy:
Li+ < Na+ < K+ < Rb+ < Cs+
The last order is because hydration enthalpies are negative; for example,
a more strongly hydrated ion has a more negative value.
30 Related MCQs with Solutions
1. The correct order of hydration energy of alkali metal ions is:
A. Cs+ > Rb+ > K+ > Na+ > Li+
B. Li+ > Na+ > K+ > Rb+ > Cs+
C. Na+ > Li+ > K+ > Cs+ > Rb+
D. K+ > Li+ > Na+ > Rb+ > Cs+
Answer: B
2. Which alkali metal ion has the highest hydration energy?
A. Li+
B. Na+
C. K+
D. Cs+
Answer: A. Li+
3. Which ion has the lowest hydration energy among Group 1 cations?
A. Li+
B. Na+
C. Rb+
D. Cs+
Answer: D. Cs+
4. Hydration energy mainly depends on:
A. Ionic size and charge
B. Atomic mass only
C. Number of neutrons
D. Melting point
Answer: A
5. As ionic radius decreases, hydration energy generally:
A. Decreases
B. Increases in magnitude
C. Becomes zero
D. Remains constant
Answer: B
6. The highest charge density among the following is possessed by:
A. Li+
B. Na+
C. K+
D. Cs+
Answer: A. Li+
7. Strong hydration of Li+ is due to its:
A. Large radius
B. Small radius and high charge density
C. Low charge
D. High atomic mass
Answer: B
8. The hydration process is represented by:
A. M+(aq) → M+(g)
B. M+(g) → M+(aq)
C. M(g) → M−(aq)
D. M+(s) → M(g)
Answer: B
9. Hydration enthalpy is generally:
A. Positive
B. Negative
C. Always zero
D. Independent of ion size
Answer: B. Negative
10. Which ion has the most negative hydration enthalpy?
A. Li+
B. Na+
C. K+
D. Cs+
Answer: A. Li+
11. Moving down Group 1, ionic radius:
A. Decreases
B. Increases
C. Remains constant
D. Becomes zero
Answer: B
12. Moving down Group 1, hydration energy magnitude:
A. Increases
B. Decreases
C. Remains constant
D. First increases then decreases
Answer: B
13. Which has greater ion–dipole attraction with water?
A. Li+
B. Cs+
C. Both equal
D. Neither
Answer: A. Li+
14. The oxygen end of water molecules is attracted strongly toward:
A. M+
B. Electrons only
C. Neutrons
D. H2 molecules
Answer: A
15. Which ion is most strongly solvated in water?
A. Cs+
B. Rb+
C. Li+
D. K+
Answer: C. Li+
16. High charge density means:
A. Charge spread over a large volume
B. Charge concentrated over a small volume
C. No charge
D. Only high atomic mass
Answer: B
17. Which order represents increasing ionic radius?
A. Cs+ < Rb+ < K+ < Na+ < Li+
B. Li+ < Na+ < K+ < Rb+ < Cs+
C. K+ < Li+ < Cs+ < Na+
D. Na+ < K+ < Li+ < Cs+
Answer: B
18. Which ion has the greatest ion–dipole attraction per unit charge?
A. Li+
B. Na+
C. K+
D. Cs+
Answer: A. Li+
19. Hydration energy is associated with formation of:
A. Hydrated ions
B. Metallic bonds
C. Covalent molecules only
D. Neutrons
Answer: A
20. Which statement is correct?
A. Larger monovalent ions hydrate more strongly
B. Smaller monovalent ions hydrate more strongly
C. Size has no effect
D. Hydration depends only on mass
Answer: B
21. Which ion has the lowest charge density?
A. Li+
B. Na+
C. K+
D. Cs+
Answer: D. Cs+
22. Which ion is expected to have the greatest hydrated radius?
A. Li+
B. Cs+
C. Both equal
D. Na+
Answer: A. Li+
23. The unusually high hydration of Li+ is important in explaining:
A. Its strong reducing character in aqueous solution
B. Its noble-gas nature
C. Its zero ionisation energy
D. Its inability to react with water
Answer: A
24. If hydration enthalpy becomes less negative, hydration strength:
A. Increases
B. Decreases
C. Becomes infinite
D. Remains unchanged
Answer: B
25. Which ion has the strongest interaction with the oxygen atom of water?
A. Li+
B. Cs+
C. Rb+
D. K+
Answer: A. Li+
26. Hydration energy of Na+ compared with K+ is:
A. Greater
B. Smaller
C. Equal
D. Zero
Answer: A. Greater
27. Which trend is correct for hydration enthalpy values?
A. Li+ is most negative
B. Cs+ is most negative
C. All are positive
D. All are zero
Answer: A
28. Hydration energy is an example of interaction between an ion and:
A. Polar solvent molecules
B. Neutrons
C. Metals only
D. Nonpolar atoms only
Answer: A
29. The best reason for Li+ having maximum hydration energy is:
A. Maximum ionic radius
B. Maximum charge density
C. Maximum atomic mass
D. Minimum charge
Answer: B
30. The key relationship for Group 1 ions is:
A. Larger ion → stronger hydration
B. Smaller ion → stronger hydration
C. Larger ion → higher charge density
D. Size has no effect
Answer: B
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