Which among the following elements has the largest atomic radii ?

Options

A. Na
B. Mg
C. K
D. Ca

Explanation

To determine which among the given elements has the largest atomic radii, we will analyze the elements: sodium (Na), magnesium (Mg), and potassium (K).

### Step-by-Step Solution:

1. **Identify the Elements and Their Positions**:
- Sodium (Na) and Magnesium (Mg) are in Period 3 of the periodic table.
- Potassium (K) is in Period 4 and belongs to Group 1, just below Sodium.

2. **Understand Atomic Radius Trends**:
- Atomic radius decreases as you move from left to right across a period due to increasing nuclear charge, which pulls the electrons closer to the nucleus.
- Atomic radius increases as you move down a group because additional electron shells are added, which increases the distance of the outermost electrons from the nucleus.

3. **Compare Sodium and Magnesium**:
- Since Sodium is to the left of Magnesium in Period 3, Sodium has a larger atomic radius than Magnesium.
- **Conclusion**: Na > Mg in terms of atomic radius.

4. **Compare Sodium and Potassium**:
- Potassium is in Period 4, below Sodium in Group 1. As we move down the group, the atomic radius increases.
- **Conclusion**: K > Na in terms of atomic radius.

5. **Compare Magnesium and Potassium**:
- Since Potassium is below Magnesium in Group 1, it also has a larger atomic radius than Magnesium.
- **Conclusion**: K > Mg in terms of atomic radius.

6. **Final Comparison**:
- From the comparisons made:
- K > Na
- K > Mg
- Therefore, Potassium has the largest atomic radius among the given elements.

### Final Answer:
Potassium (K) has the largest atomic radius.