Valency : Variable Valency

An element may exhibit a variable valency either in its compounds with the same element:

PCl₃ (3), PCl₅ (5) and SO₂ (4), SO₃ (6)

or in its compounds with different elements:

PH₃ (3), P₂O₅ (5) and SH₂ (2), SF₆ (6)

It will be noticed that the valency is usually either odd or even, but exceptions are known, e.g., WCl₅ (5), WCl₆ (6), and NH₃ (3), NO (2); HCl (1), ClO₂ (4).

The lowest valency is always shown in the hydrogen compounds, and the highest valency in the oxygen compounds. The sum of the hydrogen valency and the maximum oxygen valency is often equal to 8. This is Abegg and Bodländer's rule (1899).

SiH₄ (4), PH₃ (3), SH₂ (2), ClH (1), SiO₂ (4), P₂O₅ (5), SO₃ (6), Cl₂O₇ (7)

If an element, especially a metal, forms two or more series of compounds in which it has different valencies, the properties of the compounds in these series are usually different. As an example compare the properties of ferrous (bivalent iron) and ferric (tervalent iron) compounds:

Ferrous sulphate	Ferric sulphate
Green crystals, FeSO₄,7H₂O	Colourless crystals Fe₂(SO₄)₃,9H₂O
White precipitate Fe(OH)₂ with alkali	Brown precipitate Fe(OH)₃with alkali
White precipitate with potassium ferrocyanide	Dark-blue precipitate with potassium ferrocyanide
Dark blue precipitate with potassium ferricyanide	No precipitate, but dark brown colour, with potassium ferricyanide
No coloration with ammonium thiocyanate	Blood-red coloration with ammonium thiocyanate
Double salt, K₂SO₄,FeSO₄,H₂O, pale green crystals	Double salt, K₂SO₄, Fe₂(SO₄)₃,24H₂O, amethyst-coloured crystals

Unless we knew that ferrous and ferric sulphates were both salts of the same element, iron, these tests would lead us to conclude that we had to do with salts of two different elements.

Compounds of an element in which it has a particular valency may resemble compounds of another element with the same valency more closely than they resemble other compounds of the first element with a different valency. Thus, silver, mercurous, and cuprous chlorides are all white, sparingly soluble solids: AgCl, HgCl, CuCl. Mercuric and cupric chlorides, HgCl₂ and CuCl₂, are soluble; the latter is yellow when anhydrous. CuCl is, therefore, more analogous to HgCl and AgCl than to CuCl₂. Bivalent lead and tin compounds resemble each other more closely than compounds of bivalent lead resemble compounds of quadrivalent lead, or than compounds of bivalent tin resemble those of quadrivalent tin. Quadrivalent lead and tin are closely analogous:

SnCl₂ and PbCl₂ - white crystalline solid

SnCl₄ colourless fuming liquid, PbCl₄ yellow fuming liquid

In order to distinguish between the various valencies of an element, a Roman numeral representing the valency may be written over the symbol. Thus, H₂S^VIO₄ indicates that sulphur in sulphuric acid is sexivalent:

H^I, O^II, N^III, C^IV, P^V, S^VI, Cl^VII, Os^VIII.

Protein crystallography
Chemistry guide
Valency
-V-
<<<	>>>