What do you understand by diagonal relationship?

Certain elements resemble closely one-step down in the next higher group in the periodic table. The resemblance in properties of a member of the group and a diagonally situated element of the adjacent group is called diagonal relationship. For example, the first three elements of the second period (Li. Be. B) show diagonal similarity with elements (Mg, AI, Si) of the third period placed on the right-hand side.

Cause of diagonal relationship. The cause of the diagonal relationship is due to opposing trends in periodic properties along a period and down the group. For example,
(i) The electropositive character increases down a group and it decreases from left to right in a period. Consequently, diagonally opposite elements in the periodic table have more or less similar electropositive character and hence, in turn, have many similar properties also.
(ii) In case of metallic elements, the diagonally opposite metals possess nearly similar polarizing power [i.e. ionic charge/(ionic radius)²] because:
(a) ionic charge increases from left to right in a period and
(b) ionic radius decreases from left to right in a period but increases down a group.

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What do you understand by diagonal relationship?