Diagonal Relationship

Diagonal relationship

A relationship within the periodic table by which certain elements in the second period have a close chemical similarity to their diagonal neighbours in the next group of the third period. This is particularly noticeable with the following pairs.

Lithium and magnesium:

(1) both form chlorides and bromides that hydrolyse slowly and are soluble in ethanol;

(2) both form colourless or slightly coloured crystalline nitrides by direct reaction with nitrogen at high temperatures;

(3) both burn in air to give the normal oxide only;

(4) both form carbonates that decompose on heating.

Beryllium and aluminium:

(1) both form highly refractory oxides with polymorphs;

(2) both form crystalline nitrides that are hydrolysed in water;

(3) addition of hydroxide ion to solutions of the salts gives an amphoteric hydroxide, which is soluble in excess hydroxide giving beryllate or aluminate ions [Be(OH)₄]²⁻ and [Al(OH)₄]⁻;

(4) both form covalent halides and covalent alkyl compounds that display bridging structures;

(5) both metals dissolve in alkalis.

Boron and silicon:

(1) both display semiconductor properties;

(2) both form hydrides that are unstable in air and chlorides that hydrolyse in moist air;