Valency is the combining capacity of an atom of an element with atoms of other elements to form molecules.

Variable Valency

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 Roman numbers.

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 valency.

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.

Access All Content

Video Explanations, Practice Worksheets and Solved Examples for all Chapters.
Textbook solutions, Formulae and Mindmaps across all boards.

More from Grade 6 Chemistry


Combining, reacting, exploding and much more. Learn what all elements can do with their atoms!


Learn about the element twins, radicals, and how they react!

Chemical Formula

Chemical Formula is the language of Chemistry. Learn how to write the chemical formula of a compound from valencies of the elements using the criss-cross method.