Question

What is diagonal relationship and what cause it to be?

Answer 1

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 magnesium (Mg), beryllium (Be) and aluminium (Al), boron (B) and silicon (Si) etc.) exhibit similar properties; for example, boron and silicon are both semiconductors , forming halides that are hydrolysed in water and have acidic oxides.

In the second and third periods of the periodic table , diagonal relationship exists between two diagonally adjacent elements. Chemical properties and size of the atom are co related. On moving left to right in a row size decreases and on moving up to downwards size increases. Diagonal relationship between
Li Be
Na Mg
is such that as we go down the column, size increases and as we move left to right, size decreases thus size of Mg atom is same that of Li.
As you go across the row you add more protons but the electrons are present in the same shell and with each addition of proton this electron shell gets smaller. As we move down the column we are adding protons as well as electronic shells so the size increases.