What is the Diagonal Relationship in the Periodic Table? Explain with a few Examples

In this article, the diagonal relationship in the Periodic Table is explained with some examples.

Diagonal Relationship in the Periodic Table

The similarity in properties observed between two elements placed diagonally from left to right in two adjacent periods and two adjacent groups in the periodic table is known as the diagonal relationship.

The diagonal relationship is prominent among the lighter members of the second and third periods elements. Thus, Li (Lithium) of group IA shows the diagonal – relationship with Mg (Magnesium) of group IIA, Be (Beryllium) of group IIA shows the diagonal – relationship with Al (Aluminium) of group IIIA, and B (Boron) of group IIIA shows the diagonal – relationship with Si (Silicon) of group IVA respectively.

The diagonal relationship is well illustrated between Li and Mg in their following properties –

1. Melting – Unlike the alkali metals of Gr I but like Mg of Gr. II the melting and boiling points of Li are higher.
2. Decomposing – Li₂Co₃, LiNo_3, and LiOH all decomposed on heating to give Li₂O like the corresponding compounds of Mg.
3. Solid Bicarbon – Li does not form solid bicarbonate like Mg whereas Na – forms solid NaHCo₃.
4. Solubility – Li₂Co₃, Li₂Po₄, LiF are all insoluble in water like the corresponding Mg salts. LiOH is also sparingly soluble in water like Mg(OH)₂.
5. The halides and alkyls are co-valent like the Mg halides and alkyls. So these compounds are soluble in organic solvents.

Reason for the Diagonal Relationship

The reason for the diagonal relationship is their equal polarizing power or Ionic Potential. Polarizing Power or Ionic Potential = (Ionic charge)/(Ionic radius). For example, Li⁺ is a small size cation with a +1 charge and Mg²⁺ is a somewhat larger size cation with a +2 charge, so the ionic potential of each of the Li⁺ and Mg²⁺ ions is roughly the same.