Diagonal relationship is formed due to the identical size of ions. As we move towards right across a period in the periodic table, then the size of the atom reduces, and when we move downward across a group then the size of the atoms gradually increases.

Why Be and Al Show diagonal relationship?

Diagonal relationship of Be with Al Because of its small size Be differs from other earth alkaline earth metals but resembles in many of its properties with Al on account of diagonal relationship. (1) Be2+ and Al3+ have almost same and smaller size and thus favour for covalent bonding.

What is meant by diagonal relationship give example?

A diagonal relationship in S-block elements exists between adjacent elements which are located in the second and third period of the periodic table. For example, Lithium of group 1A and second period shows similarities with the properties of magnesium which are located in the 2nd group and 3rd period.