In chemistry, the terms oxidation and reduction refer to reactions in which an atom (or group of atoms), successively, loses or gains electrons. An oxidation number is a number assigned to an atom (or group of atoms) that helps chemists track how many electrons are available for transfer and if a given reactant is oxidized or reduced in a reaction. The process of assigning oxidation numbers to atoms can vary from very easy to quite complex, based on the charge in the atom and the chemical composition of the molecules that make it up. To make things more complicated, some atoms have more than one oxidation number. Fortunately, the determination of the oxidation number is done with rules that are clear and easy to follow, although a knowledge of basic chemistry and algebra will make explaining these rules much easier.
Step
Method 1 of 2: Determining Oxidation Numbers Based on Chemical Regulations
Step 1. Determine if the substances in question are elements
Atoms of free elements always have an oxidation number of 0. This applies to atoms whose elemental form consists of a single atom, as well as atoms whose elemental form is diatomic or polyatomic.
- For example, both Al(s) as well as Cl2 have an oxidation number of 0 because they are forms of elements that are not bound to other elements.
- Note that the elemental form Sulfur, S8, or octasulfur, although abnormal, also has an oxidation number of 0.
Step 2. Determine if the substances in question are ions
Ions have the same oxidation number as their charge. This is true for ions that are not bound to other elements, as well as ions that are part of ionic compounds.
- For example, the Cl. ion- has an oxidation number of -1.
- The Cl ion still has an oxidation number of -1 when Cl is part of the NaCl compound. Since the Na ion, by definition, has a charge of +1, we know that the Cl ion has a charge of -1, so the oxidation number remains -1.
Step 3. Recognize that metal ions may have multiple oxidation states
Many metallic elements have more than one charge. For example, the metal Iron (Fe) can be an ion with a +2 or +3 charge. The charge of a metal ion (and thus its oxidation number) can be determined, either in terms of the charges of the other constituent atoms in the compound, or, when written in text form in Roman numeral notation (as in the sentence, The iron(III) ion has a charge of + 3.).
For example, let's examine a compound containing the metal ion aluminum. AlCl. compound3 has an overall charge of 0. Since we know that the Cl. ion- has a charge of -1 and there are 3 Cl. ions- in the compound, the Al ion must have a charge of +3 so that the total charge of all the ions is 0. Thus, the oxidation number of Al is +3.
Step 4. Assign the oxidation number of -2 to oxygen (without exception)
In almost all cases, the oxygen atom has an oxidation number of -2. There are some exceptions to this rule:
- When oxygen is in its elemental form (O2), the oxidation number is 0, because this is the rule for all atoms of the element.
- When oxygen is part of a peroxide, its oxidation number is -1. Peroxides are a class of compounds containing oxygen-oxygen single bonds (or the peroxide anion O2-2). For example, in the H. molecule2O2 (hydrogen peroxide), oxygen has an oxidation number (and charge) of -1. Also, when oxygen is part of superoxide, its oxidation number is -0.5.
- When oxygen is bound to fluorine, its oxidation number is +2. See Fluorine regulations below for more information. In (O2F2), its oxidation number is +1.
Step 5. Assign an oxidation number of +1 to hydrogen (without exception)
Like oxygen, the oxidation number of hydrogen is a special case. In general, Hydrogen has an oxidation number of +1 (except, as above, in its elemental form, H2). However, in the case of special compounds called hydrides, hydrogen has an oxidation number of -1.
For example, in H2O, we know that hydrogen has an oxidation number of +1 because oxygen has a charge of -2 and we need to need a charge of 2+1 to make the compound's charge zero. However, in sodium hydride, NaH, hydrogen has an oxidation number of -1 because the charge on the ion has a charge of +1, and for the sum of the charges on the compound to be zero, the charge on the hydrogen (and thus its oxidation number) must equal -1.
Step 6. Fluorine always has an oxidation number of -1
As noted above, the oxidation numbers of certain elements can differ due to several factors (metal ions, oxygen atoms in peroxides, etc.) However, Fluorine, has an oxidation number of -1, which never changes. This is because fluorine is the most electronegative element – in other words, it is the element that is least likely to give up its electrons and most likely to take up atoms of other elements. Thus, the charge does not change.
Step 7. Make the oxidation number in the compound equal to the charge on the compound
The oxidation number of all atoms in a compound must equal the charge on the compound. For example, if a compound has no charge, the oxidation number of each atom must add up to zero; if the compound is a polyatomic ion with a charge of -1, the oxidation number must add up to -1, etc.
This is a good way to check your work – if the oxidation numbers in your compound don't add up to the charge on your compound, you know you've set one or more of the wrong oxidation numbers
Method 2 of 2: Assigning Numbers to Atoms Without an Oxidation Number Rule
Step 1. Find the atoms without the rule of oxidation number
Some atoms have no specific rules about oxidation numbers. If your atom doesn't appear in the rules above and you're not sure what its charge is (for example, if the atoms are part of a larger compound and thus don't show their respective charges), you can find the atom's oxidation number by a process of elimination. First you will determine the oxidation state of all the atoms in the compound, then you will only solve the unknown atoms based on the total charge of the compound.
For example, in the compound Na2SO4, the charge of Sulfur (S) is unknown – the atom is not in elemental form, so its oxidation number is not 0, but that is all we know. This is a good example of this algebraic way of determining the oxidation number.
Step 2. Find the known oxidation numbers of other elements in the compound
Using the rules for assigning oxidation numbers, determine the oxidation numbers of the other atoms in the compound. Watch out for special cases like O, H, etc.
In Na2SO4, we know that, according to our rules, the Na ion has a charge (and thus its oxidation number) +1 and the oxygen atom has an oxidation number of -2.
Step 3. Multiply the number of atoms by their oxidation number
Now that we know the oxidation numbers of all of our atoms except the unknown, we must consider the fact that some of these atoms may appear more than once. Multiply each coefficient number of each atom (written in small below after the chemical symbol of the atom in the compound) by its oxidation number.
In Na2SO4, we know that there are 2 Na atoms and 4 O atoms. We will multiply 2 × +1, the oxidation number of Na, to get the answer 2, and we will multiply 4 × -2, the oxidation number O, to get the answer -8.
Step 4. Add up the results
Adding up the results of your multiplication will give you the oxidation number of the compound without calculating the unknown oxidation number of your atom.
In the Na. example2SO4 us, we'll add 2 by -8 to get -6.
Step 5. Calculate the unknown oxidation number based on the charge of the compound
Now, you have everything you need to find unknown oxidation numbers using simple algebra. Create an equation: your answer in the previous step, plus the unknown oxidation number equals the overall charge of the compound. In other words: (Amount of known oxidation number) + (unknown oxidation number, which is sought) = (charge of compound).
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In the Na. example2SO4 us, we'll solve it as follows:
- (sum of known oxidation number) + (unknown oxidation number, which is sought) = (charge of compound)
- -6 + S = 0
- S = 0 + 6
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S = 6. S has an oxidation number
Step 6. in Na2SO4.
Tips
- Atoms in the elemental form always have an oxidation number of 0. A monatomic ion has an oxidation number equal to its charge. Metal 1A in its elemental form, such as hydrogen, lithium, and sodium, has an oxidation number of +1; 2A metals in elemental form, such as magnesium and calcium, have an oxidation number of +2. Both hydrogen and oxygen have two different oxidation states which may depend on the bond.
- In a compound, the sum of all the oxidation numbers must equal 0. If an ion has 2 atoms, for example, the sum of the oxidation numbers must equal the charge on the ion.
- It is very helpful to know how to read the periodic table of elements and the location of metals and non-metals.
