A diagonal relationship is exists between elements which are diagonally adjacent to each other in the second and third periods (first 20 elements) of the periodic table. These pairs are:

• lithium (Li) and magnesium (Mg),
• beryllium (Be) and aluminium (Al),
• boron (B) and silicon (Si),

These pairs exhibit similar properties; for example, boron and silicon are both semiconductors, forming halides that are hydrolysed in water and have acidic oxides.

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