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diagonal relationship


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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;(3) both form acidic oxides with covalent crystal structures, which are readily incorporated along with other oxides into a wide range of glassy materials. The reason for this relationship is a combination of the trends to increase size down a group and to decrease size along a period, and a similar, but reversed, effect in electronegativity, i.e. decrease down a group and increase along a period.

(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.

(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.

(1) both display semiconductor properties;

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

(3) both form acidic oxides with covalent crystal structures, which are readily incorporated along with other oxides into a wide range of glassy materials.

Subjects: Chemistry.


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