Of the given alkali metals, the one with smallest size is

Options

A. Rb
B. Cs
C. K
D. Na

Explanation

To determine which alkali metal has the smallest size among the given options (potassium, sodium, rubidium, and caesium), we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Alkali Metals**: The alkali metals are located in Group 1 of the periodic table. The elements in this group include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).

2. **Understand the Trend in Atomic Size**: Atomic size increases as you move down a group in the periodic table. This is because as you go down the group, each successive element has an additional electron shell. The increased distance of the outermost electrons from the nucleus results in a larger atomic radius.

3. **List the Given Alkali Metals**: From the question, we have the following alkali metals to consider:
- Sodium (Na)
- Potassium (K)
- Rubidium (Rb)
- Caesium (Cs)

4. **Compare the Sizes**:
- Sodium (Na) has an electron configuration of [Ne] 3s¹.
- Potassium (K) has an electron configuration of [Ar] 4s¹.
- Rubidium (Rb) has an electron configuration of [Kr] 5s¹.
- Caesium (Cs) has an electron configuration of [Xe] 6s¹.

As we can see, sodium is the highest in the group, while caesium is the lowest. Therefore, the size increases from sodium to caesium.

5. **Determine the Smallest Size**: Since atomic size increases down the group, sodium (Na) being the highest up in the group will have the smallest atomic size among the given options.

### Final Answer:
The alkali metal with the smallest size among the given options is **Sodium (Na)**.

---