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.

1. First, determine the oxidation number of nitrogen (N) in the given compound or molecule. This can be done by considering its electronegativity and the number of bonds it has with other elements. For example, in the compound NH3, nitrogen has an oxidation number of -3 because it is more electronegative than hydrogen and forms three bonds with hydrogen.
2. Next, use the oxidation number of nitrogen to determine its valence electrons. Valence electrons are the outermost electrons of an atom that are involved in chemical bonding. For nitrogen, the number of valence electrons can be found using its group number, which is 5 in the periodic table. This means nitrogen has 5 valence electrons.
3. Now, determine the valence of nitrogen by looking at its electron configuration. Nitrogen has 2 electrons in its first energy level and 5 electrons in its second energy level. Therefore, it has a total of 7 electrons and since it needs 8 electrons to have a full outer shell, it's valence is -3 or 5 (since 5 - (-3) = 8).
4. Finally, consider the overall charge of the compound or molecule. If the compound is neutral, then the valence of nitrogen is equal to its oxidation number. However, if the compound has a charge, then the valence of nitrogen needs to be adjusted accordingly to maintain the compound's overall charge. For example, in the compound NH4+, nitrogen's valence is now +1 because it has to balance out the +1 charge of the ammonium ion.