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

Options

A. Melting point : `Li gt Na gt Kgt Rb`
B. `Density : Rb gt Na gt K gt Li`
C. Metallic radius: `Rb gt K gt Na gt Li`
D. lonization enthalpy : `Li gt K gt Na gt Rb`

Answer

To solve the question regarding the trends of alkali metals and identify the incorrect trend, we will analyze the properties of alkali metals: melting point, density, metallic radius, and ionization energy.

### Step-by-Step Solution:

**Step 1: List the Alkali Metals**
- The alkali metals include Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), and Cesium (Cs).

**Step 2: Analyze the Melting Point Trend**
- As we move down the group from Lithium to Cesium, the melting point decreases.
- Correct trend: Li > Na > K > Rb > Cs (decreasing melting point).

**Step 3: Analyze the Density Trend**
- As we move down the group from Lithium to Cesium, the density increases.
- Correct trend: Li < Na < K < Rb < Cs (increasing density).

**Step 4: Analyze the Metallic Radius Trend**
- As we move down the group from Lithium to Cesium, the metallic radius increases.
- Correct trend: Li < Na < K < Rb < Cs (increasing metallic radius).

**Step 5: Analyze the Ionization Energy Trend**
- As we move down the group from Lithium to Cesium, the ionization energy decreases.
- Correct trend: Li > Na > K > Rb > Cs (decreasing ionization energy).
- However, the order given in the question states that potassium has higher ionization energy than sodium, which is incorrect.

**Step 6: Identify the Incorrect Trend**
- The incorrect trend is related to ionization energy. The correct order should be Lithium > Sodium > Potassium, but the provided order suggests otherwise.

### Conclusion:
- The incorrect trend among the options provided is the order of ionization energy, where potassium is incorrectly stated to have a higher ionization energy than sodium.

### Final Answer:
- The incorrect trend is regarding ionization energy, where the order should be Li > Na > K, but it is incorrectly presented in the options.