Write the reason behind the digonal relationship among element.

Explanation

### Step-by-Step Solution

1. **Understanding Diagonal Relationships**:
Diagonal relationships refer to the similarities in properties between certain pairs of elements in the periodic table that are not in the same group or period but are diagonally positioned. For example, lithium (Li) and magnesium (Mg) or beryllium (Be) and aluminum (Al).

2. **Identical Size of Ions**:
The primary reason for diagonal relationships is the similar ionic sizes of these elements. As we move across a period from left to right, the atomic size decreases due to the increasing nuclear charge, which pulls the electrons closer to the nucleus.

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