What Do We Need to Know About Oxidation Number and Percent Composition?

Do you read the list of ingredients while buying packed food? If you read the ingredients list on a jam jar, then you would see that every ingredient is mentioned with its percent composition in the jam. The percent composition of all elements present in a compound helps indicate the nature and properties of that compound. The oxidation number is also an important concept in chemistry particularly while writing chemical equations. We balance a chemical equation to find reactants and products. Here I have listed some important things that you should know about oxidation number and percent composition.

What is the oxidation number?

The oxidation number of an atom in a molecule or ion is the amount of charge it would carry after the complete transfer of electrons. The oxidation number is positive if the atom has donated its electrons and is negative if the atom has accepted the electrons.

Rules of assigning oxidation number:

In chemistry, finding the oxidation number of elements in compounds and ions is a key skill. You can excel in this skill by simply remembering the following rules for assigning oxidation numbers;
  • The sum of the oxidation number of each atom in a neutral compound is always zero. For example, H2o is a neutral molecule so its overall oxidation number is zero.
  • In the case of a polyatomic ion, the total oxidation number of the ion must be equal to the charge on the ion. For example, the sum of oxidation numbers of Sulphur and oxygen in SO4-2 is -2.
  • Oxidation number and ionic charge are not the same things. Ionic charge is fixed for each element while oxidation numbers can be different for the same element in different compounds. e.g., the oxidation number of nitrogen in N2o is +4 while the oxidation number for nitrogen in NO is +2.
  • The oxidation number for free elements or monoatomic species is always zero and in the case of free ions/monoatomic ions, it is equal to the overall charge of the ion. e.g., Ca2+ and Fe have the oxidation number of +2 and 0 respectively.
  • Some elements in the periodic table exhibit the same oxidation number which helps in the calculation of the O.N of other atoms in the compound. for example, the oxidation number of fluorine is always -1 while that of group 1 elements is always +1.
  • Hydrogen has an oxidation number of +1 when bonded with non-metals and -1 when bonded with metals.
  • The oxidation number of oxygen is generally -2 except with fluorine and peroxides.

The formula of percent composition:

Percent composition is the percentage amount of every element that is present in a compound calculated by mass. For example, If the compound is Cr2o3 then the percent composition would tell what percent of Cr2o3 is chromium by mass and what percentage of the compound is oxygen by mass.

You can find the percent composition of any element present in a compound by using the given formula;

Percent composition of x = (mass of element/mass of the compound) x 100

Example to calculate percent composition:

Let us calculate the percent composition by mass of Sulphur in H2s4o8 for practice;

Mass of Sulphur = 32 x 4 = 128 g/mol

Mass of compound = 258 g/mol

% composition by mass = (mass of Sulphur/mass of H2s4o8) x 100

% composition by mass = (128/258) x 100

% composition by mass = 49.6%

Solution; the percent composition of Sulphur in H2s4o8 is calculated as 49.6%.


Percent composition and oxidation number are two important concepts in chemistry that every student must learn and understand. I have listed below the important points of this post that you must remember;
  • The oxidation number is always zero for a neutral molecule and is always equal to the overall charge for a polyatomic ion.
  • The oxidation number of hydrogen is always 1 and that of oxygen is -2 (except in peroxides and fluorine).
  • The oxidation number is positive if the electrons are donated and negative if the electrons are accepted.
  • Percent composition by mass is the percentage of an element present in a compound and helps identify the nature and properties of a compound.
This was all you need to know about oxidation number and percent composition. I hope this content is helpful for you and let us know if you have any further queries.
Topic revision: r1 - 22 Nov 2022, AnnyDavid
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