Which of the following pairs of elements show diagonal relationship ?
1. Li and Mg 2. B and Si 3. Mg and Al Select the correct answer using the codes given below

Options

A. 1 and 3
B. 2 and 3
C. 1 and 2
D. 1,2 and 3

Answer

To determine which pairs of elements show a diagonal relationship, we need to understand what a diagonal relationship is in the context of the periodic table.

### Step-by-Step Solution:

1. **Understanding Diagonal Relationship**:
- A diagonal relationship occurs between certain pairs of elements in the second and third periods of the periodic table. Elements that are diagonally positioned often exhibit similar properties due to comparable atomic sizes and electronegativities.

2. **Identifying the Elements**:
- The elements mentioned in the question are:
- Pair 1: Lithium (Li) and Magnesium (Mg)
- Pair 2: Boron (B) and Silicon (Si)
- Pair 3: Magnesium (Mg) and Aluminium (Al)

3. **Analyzing Pair 1 (Li and Mg)**:
- Lithium (Li) is in Group 1 and Period 2, while Magnesium (Mg) is in Group 2 and Period 3.
- These two elements have comparable atomic sizes and some similar chemical properties, indicating a diagonal relationship.

4. **Analyzing Pair 2 (B and Si)**:
- Boron (B) is in Group 13 and Period 2, while Silicon (Si) is in Group 14 and Period 3.
- Similar to the first pair, Boron and Silicon also exhibit comparable properties and sizes, indicating a diagonal relationship.

5. **Analyzing Pair 3 (Mg and Al)**:
- Magnesium (Mg) is in Group 2 and Period 3, while Aluminium (Al) is in Group 13 and Period 3.
- Although they are in the same period, they do not show a diagonal relationship as they are not from different periods.

6. **Conclusion**:
- Based on the analysis, pairs that show a diagonal relationship are:
- Pair 1: Li and Mg
- Pair 2: B and Si
- Therefore, the correct answer is that pairs 1 and 2 show a diagonal relationship.

### Final Answer:
The correct answer is: **1 and 2 only**.