Question

Hey guys what is diagonal relationship

Answer 1

A diagonal relationship is said to exist between certain pairs of diagonallyadjacent elements in the second and third periods (first 20 elements) of the periodic table. ... Moving rightward across the period decreases the atomic radii of atoms, while moving down the group will increase the atomic radii

Answer 2

the similarities between the first element of a group and the second element of the next higher group..
for example..
lithium is the first element in the first group and magnesium is the second element of the next higher group..hence they show diagonal relationship.