DIAGONAL RELATIONSHIPS
On moving diagonally some members show similar properties with the members of next higher group which is particularly seen in the elements of second and third periods of the periodic table. However the similarities shown are far less pronounced than the similarities with in a group.
The main reasons for the diagonal relationship are
(i) Similarity in electropositive character
The electropositive character decreases along a period, but increases down a group. Hence on moving diagonally the two opposing trend partially cancels out. As a result diagonally related elements have similar electropositive character and hence exhibit similar properties.
(ii) Similarity in polarizing power
On moving along a period from left to right, the charge on the ions increases while ionic size decreases, hence polarizing power increases. On moving down the group, the ionic size increases and hence polarizing power decreases. On moving diagonally, these two trends partially cancel out. As a result thus diagonally related elements have same polarizing power and thus exhibit similar properties.
(iii) Similarity in atomic or ionic radii
The atomic and ionic radii decrease across a period and increase in a group. Evidently on moving diagonally, the two trends partially cancel out. As a result, diagonally related elements have similar atomic & ionic radii and hence have similar properties
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