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The properties of an element and its compounds in the periodic table are similar to those of another element located diagonally above or below it; this phenomenon is known as the diagonal rule or diagonal relationship. This is an objective reflection of a large number of facts. This relationship is clearly evident among the following pairs of typical elements: for example, beryllium and aluminum, whose standard electrode potential values (E) are similar (EBe2+/Be = –1.85 V, EAl3+/Al = –1.66 V). Both beryllium and aluminum become passivated in concentrated nitric acid; they can also dissolve in strong alkaline solutions, releasing hydrogen gas. Beryllium oxide (BeO) and aluminum oxide (Al2O3) both have high melting points and high hardness. Beryllium hydroxide [Be(OH)2] and aluminum hydroxide [Al(OH)3] are amphoteric hydroxides. Beryllium chloride (BeCl2) and aluminum chloride (AlCl3) are dimeric substances that exhibit covalent properties, can sublimate, and are soluble in organic solvents. The diagonal rule is an empirical rule for which there is no unified theoretical explanation. It is generally believed that Li(Ⅰ), Be(II), and B(III) have smaller ionic radii than the ions of their respective group elements, and due to their two-electron configuration, they exhibit a greater polarizing effect than the ions of those elements. While Mg(II), Al(III) and Si(VI), which are located on the diagonal opposite to them, have slightly larger ionic radii, their ionic charges are also one unit higher; as a result, their polarizing powers are similar, and they exert similar ionic field forces. Therefore, the properties of their elements and compounds are quite similar.