CBSE Class 11-science Answered

A diagonal relationship is said to exist between certain pairs of diagonally adjacent elements in the second and third periods of the periodic table. These pairs (lithium (Li) and Mg, Be and Al, boron (B) and silicon (Si) etc.) exhibit similar properties; for example, boron and silicon are both semiconductors

It is found that the chemistry of a first-row (second period) element often has similarities to the chemistry of the second-row (third period) element being one column to the right of it in the Periodic Table. Thus, the chemistry of Li has similarities to that of Mg, the chemistry of Be has similarities to that of Al, and the chemistry of B has similarities to that of Si. The reasons for why diagonal relationships exist are not fully understood, but charge density is a factor. For example, Li⁺ is a small cation with a 1+ charge and Mg²⁺ is somewhat larger with a 2+ charge, so the charge density on each ion is roughly the same.