Arrange the following in proper order of size

Options

A. Mg` lt Si lt A1 lt P`
B. `P gt A1 gt Mg gt Si`
C. `A1 lt Mg lt Si lt P`
D. `Mg gt A1 gt Si gt P `

Explanation

To arrange the elements magnesium (Mg), aluminum (Al), silicon (Si), and phosphorus (P) in order of size, we need to consider their atomic radii and the periodic trends.

### Step-by-Step Solution:

1. **Identify the Elements and Their Positions**:
- Magnesium (Mg) is in Group 2 (alkaline earth metals).
- Aluminum (Al) is in Group 13 (p-block).
- Silicon (Si) is in Group 14 (p-block).
- Phosphorus (P) is in Group 15 (p-block).
- All these elements are in the third period of the periodic table.

2. **Understand Atomic Size Trends**:
- Atomic size generally decreases across a period from left to right due to increasing nuclear charge (more protons) which pulls the electrons closer to the nucleus.
- Therefore, as we move from magnesium to phosphorus, the atomic size will decrease.

3. **Order the Elements Based on Their Size**:
- Magnesium (Mg) has the largest atomic radius because it is the furthest left in the period.
- Aluminum (Al) is next, as it has one more proton than magnesium, pulling the electrons in closer.
- Silicon (Si) comes after aluminum, as it has even more protons.
- Phosphorus (P) has the smallest atomic radius among these four elements due to the highest nuclear charge.

4. **Final Arrangement**:
- The order of size from largest to smallest is:
1. Magnesium (Mg)
2. Aluminum (Al)
3. Silicon (Si)
4. Phosphorus (P)

### Conclusion:
Thus, the correct order of the elements based on their atomic size is:
**Mg > Al > Si > P**