Valency is the combining capacity of an atom of an element with atoms of other elements to form molecules.
Some elements exhibit more than one valency. They're said to have variable valency.
Examples : Iron, copper, tin, lead, sulphur, phosphorus.
In the case of the metals exhibiting
variable valency, we represent the lower
valency by adding the suffix 'ous' to the
name of the metal; to represent the
higher valency the suffix 'ic' is attached
to the name of the metal.
For example , the metal iron has valencies +2 and +3. For the lower valency (+2) we write ferrous (Fe2 +) and for the higher valency (+3) we write ferric (Fe3 +).
In the modern method the variable
valency of the element is represented by
Thus ferrous ion is represented. as Fe (II) and ferric ion as Fe (III).
The advantage of the modern convention is that neither the name of the element nor its symbol changes.
In the case of a non-metallic atom
the number of the other types of
atoms attached to it determines its
Phosphorus has valencies 3 and 5. With chlorine it forms two compounds, PCl3 and PCl5. Therefore the molecule of phosphorus trichloride, which has three chlorine atoms in it, has the lower valency (3), and the molecule of phosphorus pentachloride, which has five chlorine atoms in it, has the higher valency (5) for phosphorus atom.
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