The corresponding two elements and their compounds have many similarities in properties, such as the electronegativity of elements. This similarity is called diagonal rule.

In the same period, the polarization of ions with the same outermost electronic configuration increases from left to right with the increase of ion charge number.

The ionic polarization of the same group of elements gradually weakens from top to bottom.

Therefore, the charge number and radius of the two elements in the diagonal position of the upper left and lower right in the periodic table have opposite effects. Their ionic potentials are similar, so their properties are similar.