A diagonal relationship is said to exist between certain pairs of diagonally adjacent elements in the 2nd and 3rd periods. (a) Li shows diagonal relationship with Mg. Due to comparable : Electronegativities Li=1.0⇒Mg=1.20 (b) Atomic size, i.e. atomic radii (in pm) H=152⇒Mg=160 (c) Atomic volume Li=12.97mL⇒Mg=13.97mL Thus, Li-resembles Mg in many respects.