For many elements, the valence of the element is equal to the number of unpaired valence electrons, as shown in the Lewis symbol of the element. An atom, with 2 unpaired electrons has a valence of two and forms two bonds. The valences derived from the number of unpaired electrons are the "normal" or "expected" valences of the elements.
However, some elements display a valence other than the normal valence. For example, when tin and chlorine react, one of two different products may form. The actual product depends on the conditions of the reaction, such as temperature and pressure. Tin has a valence of two and SnCl2 is the expected product. The other compound is SnCl4. In this compound, tin forms four bonds and thus has a valence of four. In other words, tin displays a valence of two in some of its compounds and a valence of four in others.Similarly, other elements, such as iron, copper, tin, nickel, and sulphur, display more than one valence.
When we name a compound containing one of these elements, how do we indicate which valence the element is displaying in that compound? How do we distinguish between SnCl2 and SnCl4, for example?
The method most widely used is the Stock System. It uses Roamn numerals to indicate the valence of the element which may have more than one valence.
For example, we see on the electrovalence chart that phosphorous can have a valence of +3 and +5. When it combines with chlorine two compounds may be possible:
1. manganese (II) chloride MnCl2
2. iron (II) oxide FeO
3. arsenic (III) nitride AsN
4. mercury (I) fluoride HgF
5. lead (IV) bromide PbBr4
6. mercury (II) phosphide Hg3 P2
7. tin (II) nitride Sn3 N2
8. phosphorus (III) oxide P2 O3
9. FeCl2 iron(II)chloride
10. SnF4 tin(IV) fluoride
11. CuS copper(II)sulphide
12. Cu3P2 copper(II) phosphide
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