To find the valency of a substance, you need to know its chemical formula and the valence number of each element in the substance.

The valence number is the number of electrons that an element gains or loses in order to achieve a stable outer electron shell.

To calculate the valency of an element in a compound, first determine its oxidation number using the rules of oxidation numbers. The oxidation number represents the charge an atom would have if all the bonds were ionic.

Then, compare the oxidation number of the element to its standard valency number. This number is based on the element's position on the periodic table and can be determined by looking at the group or column number of the element.

If the oxidation number is different from the standard valency number, the element's valence number will equal the difference between the two values. If the oxidation number matches the standard valency number, then the element has a zero valence.

For example, if you want to find the valency of iron (Fe) in iron sulfide (FeS), you would first determine the oxidation number of iron, which is +2. Then, you would compare that to its standard valency number, which is +3 since iron is in Group 3 of the periodic table. The difference between +3 and +2 is 1, so the valency of iron in iron sulfide is 1.