What is diagonal relationship? Give a pair of elements having diagonal relationship. Why do they show this relation?

Explanation

On moving diagonally across the periodic table the elements show certain similarities. An element of group in 2nd period has similar properties with second element of the next higher group in the 3rd period. This type of resemblance is called diagonal relationship e.g.: (Li, Mg), (Be, Al), (B, Si)

Diagonal relationship is due to similar sizes of atoms of atoms (or of ions) and similar electronegativities of the representative elements.
Diagonally similar elements posses the same polarizing power.
Polarizing power `= ("(ionic charge)")/(("ionic radius")^(2))`