The correct arrangement of decreasing order of atomic radius among `Na, K, Mg` and `Rb` is

Options

A. `Rb gt Na gt N gt Mg`
B. `K gt Rb gt Na gt Mg`
C. `Rb gt K gt Na gt Mg`
D. `Mg gt Rb gt K gt Na`

Explanation

To determine the correct arrangement of decreasing order of atomic radius among Na (Sodium), K (Potassium), Mg (Magnesium), and Rb (Rubidium), we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Group and Period of Each Element**:
- Sodium (Na) and Potassium (K) are both in Group 1 (alkali metals).
- Magnesium (Mg) is in Group 2 (alkaline earth metal).
- Rubidium (Rb) is also in Group 1 (alkali metal).

2. **Understand the Trend in Atomic Radius**:
- As we move down a group in the periodic table, the atomic radius increases. This is because additional electron shells are added, which increases the distance between the nucleus and the outermost electrons.
- As we move from left to right across a period, the atomic radius decreases due to increasing nuclear charge, which pulls the electrons closer to the nucleus.

3. **Compare the Elements**:
- **Rubidium (Rb)**: Being the lowest in Group 1, Rb has the largest atomic radius due to having more electron shells compared to Na and Mg.
- **Potassium (K)**: Also in Group 1 but above Rb, K has a smaller atomic radius than Rb but larger than Na.
- **Sodium (Na)**: Higher up in Group 1 than K, Na has a smaller atomic radius than both K and Rb.
- **Magnesium (Mg)**: In Group 2, Mg has a smaller atomic radius than Na because it has a higher nuclear charge and is to the right of Na in the periodic table.

4. **Arrange in Decreasing Order**:
- Based on the above comparisons, we can arrange the elements in decreasing order of atomic radius:
- Rb > K > Na > Mg

### Final Answer:
The correct arrangement of decreasing order of atomic radius among Na, K, Mg, and Rb is:
**Rb > K > Na > Mg**