For alkali metals, which one of the following trends is incorrect ?

Options

A. Hydration energy `: Li^(+) gt Na^(+) gt K^(+) gt Rb^(+)`
B. Ionisation energy `: Li gt Na gt K gt Rb`
C. Electronegativity`: Li lt Na lt K lt Rb`
D. Atomic size `: Li lt Na lt K lt Rb`

Answer

To determine which trend for alkali metals is incorrect, we need to analyze the given properties: hydration energy, ionization energy, electronegativity, and atomic size.

### Step-by-Step Solution:

1. **Understanding Hydration Energy**:
- Hydration energy generally decreases down the group as the size of the alkali metal ions increases.
- Correct order: Li+ > Na+ > K+ > Rb+.

2. **Analyzing Ionization Energy**:
- Ionization energy decreases down the group due to the increasing atomic size and shielding effect.
- Correct order: Li > Na > K > Rb.

3. **Evaluating Atomic Size**:
- Atomic size increases down the group as the number of electron shells increases.
- Correct order: Li < Na < K < Rb.

4. **Examining Electronegativity**:
- Electronegativity generally decreases down the group. However, the provided order may be incorrect.
- The correct order of electronegativity should be: Li > Na > K > Rb, meaning that lithium is the most electronegative and rubidium is the least.

5. **Identifying the Incorrect Trend**:
- The question states that the order of electronegativity given is incorrect. The correct order should reflect that electronegativity decreases down the group, which contradicts the provided order in the question.

6. **Conclusion**:
- The incorrect trend among the options given is related to electronegativity. The correct order should be Li > Na > K > Rb, while the provided order was incorrect.

### Final Answer:
The incorrect trend for alkali metals is the order of electronegativity.

---