Which one of the following alkali metals has lowest melting point ?

Options

A. Sodium
B. Potassium
C. Rubidium
D. Caesium

Explanation

To determine which alkali metal has the lowest melting point, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Alkali Metals**: The alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). We need to compare their melting points.

2. **Melting Point and Lattice Energy Relationship**: The melting point of a metal is influenced by its lattice energy. Lattice energy is the energy required to separate one mole of a solid ionic compound into its gaseous ions. It is directly proportional to the melting point.

3. **Lattice Energy and Atomic Size**: Lattice energy is inversely proportional to the size of the ions. This means that larger ions will have lower lattice energy and, consequently, lower melting points.

4. **Compare the Sizes of Alkali Metals**: Among the alkali metals, the atomic size increases as we move down the group from lithium to cesium. Therefore, cesium (Cs) is the largest alkali metal.

5. **Determine the Melting Points**: The melting points of the alkali metals are as follows:
- Lithium (Li): 453 K
- Sodium (Na): 371 K
- Potassium (K): 336 K
- Rubidium (Rb): 312 K
- Cesium (Cs): 302 K

6. **Identify the Lowest Melting Point**: From the melting points listed, cesium has the lowest melting point at 302 K.

### Conclusion:
Therefore, the alkali metal with the lowest melting point is **cesium (Cs)**.