Diagonal relationship exists between certain pairs of diagonally adjacent elements in the second and third periods of the periodic table.

These pairs (Li & Mg, Be & Al, B & Si etc.) exhibit similar properties.

REASON -

Such a relationship occurs because crossing and descending the periodic table have opposing effects. On moving across a period of the periodic table, the size of the atoms decreases, and on moving down a group the size of the atoms increases. Similarly, on moving across the period, the elements become progressively more covalent, less basic and more electronegative, whereas on moving down the group the elements become more ionic, more basic and less electronegative. Thus, on both descending a group and crossing by one element the changes "cancel" each other out, and elements with similar properties which have similar chemistry are often found - the atomic size, electronegativity, properties of compounds (and so forth) of the diagonal members are similar