Q.Lithium show diagonal relationship with

Li and Mg resemble each other because they have very similar ionic potential (charge/radius ratio).

Lithium (Group 1) and magnesium (Group 2) show a 'diagonal relationship' — despite being in different groups, their properties are unusually similar (e.g. both form a nitride, both carbonates decompose on heating, both show covalent character in their compounds). This is because their ionic potential (the ratio of ionic charge to ionic radius, z/r), which …

Lithium shows a diagonal relationship with magnesium.

The first element of a group often resembles the second element of the next group diagonally, because moving down a group increases size while moving across a period decreases it, and the two trends can cancel to give similar size and polarising power (charge/radius ratio). Lithium therefore resembles magnesium: both form covalent, thermally unstable compounds, both give normal oxides (Li2O, MgO) rather than peroxides, both …