In chemistry, the terms "oxidation" and "reduction" refer to reactions in which an atom (or group of atoms) loses or acquires electrons, respectively. Oxidation numbers are numbers assigned to atoms (or groups of atoms) that help chemists keep track of how many electrons are available for transfer and check whether certain reactants are oxidized or reduced in a reaction. The procedure for assigning oxidation numbers to atoms ranges from simple examples to very complex ones, based on the charge of the atoms and the chemical composition of the molecules they are part of. To complicate the matter, some atoms may have more than one oxidation number. Fortunately, the assignment of oxidation numbers is characterized by well-defined and easy-to-follow rules, although knowledge of basic chemistry and algebra will make the task easier.
Steps
Method 1 of 2: Part 1: Assign the Oxidation Number Based on Simple Rules
Step 1. Determine if the substance in question is an element
The atoms of the free, non-combined elements always have an oxidation number equal to zero. This occurs for elements composed of an ion, as well as for diatomic or polyatomic forms.
- For example, Al(s) and Cl2 they both have oxidation number 0, because they are both in their non-combined element form.
- Note that the elemental form of sulfur, S8, or octasulfide, although irregular, also has an oxidation number of 0.
Step 2. Determine if the substance in question is an ion
The ions have oxidation numbers equal to their charge. This is true for free ions as well as for ions that are part of an ionic compound.
- For example, the Cl ion- has an oxidation number equal to -1.
- The Cl ion still has an oxidation number of -1 when it 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 its oxidation number is still -1.
Step 3. For metal ions, you need to know that multiple oxidation numbers are still possible
Many metallic elements can have more than one charge. For example, the metal iron (Fe) can be an ion with a charge of +2 or +3. The metallic charges of the ions (and thus the oxidation numbers) can be determined in relation to the charges of the other atoms present in the compound of which they are part or, when they are written, through the Roman numeral notation (as in the sentence, "The Iron ion (III) has charge +3 ").
For example, let's look at a compound containing the metal ion of aluminum. The AlCl compound3 has a total charge of 0. Since we know that the ions Cl- they have a charge of -1 and there are 3 Cl ions- in the compound, the ion Al must have a charge of +3 so that the total charge of all ions gives 0. Thus, the oxidation number of aluminum is +3.
Step 4. Assign an oxidation number of -2 to oxygen (with some exceptions)
In almost all cases, oxygen atoms have oxidation number -2. There are some exceptions to this rule:
- When oxygen is in its elemental state (O2), its oxidation number is 0, as in the case of all atoms of elements;
- When oxygen is part of a peroxide, its oxidation number is -1. Peroxides are a class of compounds that contain a single oxygen-oxygen bond (or the peroxide anion O2-2). For example, in the molecule H.2OR2 (hydrogen peroxide), oxygen has an oxidation number (and charge) of -1;
- When oxygen binds to fluorine, its oxidation number is +2. Check the Fluorine rules for more information.
Step 5. Assign an oxidation number of +1 to hydrogen (with exceptions)
Like oxygen, the oxidation number of hydrogen has exceptions. Generally, hydrogen has an oxidation number of +1 (unless, as above, it is in its element form, H2). However, in the case of special compounds called hydrides, hydrogen has an oxidation number of -1.
For example, in H.2Or, we know that hydrogen has an oxidation number +1 since oxygen has a charge of -2 and we need 2 +1 charges to make the compound's charge go zero. However, in sodium hydride, NaH, the hydrogen has an oxidation number of -1 because the Na ion has a +1 charge and, since the compound's total charge must be zero, the hydrogen charge (and so the oxidation number) must give -1.
Step 6. Fluorine always has an oxidation number of -1
As noted above, the oxidation number of certain elements can vary due to several factors (metal ions, oxygen atoms in peroxides, and so on). 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 least willing to lose its electrons and most likely to accept them from the other atom. Furthermore, his office does not change.
Step 7. Set the oxidation numbers of the compound equal to the charge of the compound
The oxidation numbers of all the atoms present in a compound must equal its charge. For example, if a compound has no charge, that is, it is neutral, the oxidation numbers of each of its atoms must give zero; if the compound is a polyatomic ion with a charge equal to -1, the added oxidation numbers must give -1, etc.
Here's how to check your work: If the oxidation in your compounds is not equal to the charge of your compound, then you know you have assigned one or more oxidation numbers incorrectly
Method 2 of 2: Part 2: Assign Oxidation Numbers to Atoms Without Using Rules
Step 1. Find the atoms with no oxidation number rules
Some atoms have no specific rules on oxidation numbers. If your atom does not appear in the rules presented above and you are unsure of its charge (for example, if it is part of a larger compound and thus its specific charge is not identifiable), you can find the oxidation number of the atom by proceeding by elimination. First, you need to determine the oxidation number of each atom in the compound; then you will simply have to solve an equation based on the total charge of the compound.
For example, in the Na compound2SO4, the charge of sulfur (S) is not known since it is not in element form, so it is not 0: that's all we know. It is an excellent candidate for determining the oxidation number by the algebraic method.
Step 2. Find the known oxidation number for the other elements in the compound
Using the rules for assigning oxidation numbers, identify those of the other atoms in the compound. Be careful if there are exceptions for O, H, etc.
In the compound Na2SO4, we know, based on our rules, that the Na ion has a charge (and thus an oxidation number) of +1 and that oxygen atoms have an oxidation number of -2.
Step 3. Multiply the quantity of each atom by its oxidation number
Keep in mind that we know the oxidation number of all our atoms except one; we need to take into consideration the fact that some of these atoms can appear more than once. Multiply the numerical coefficient of each atom (written in subscript after the chemical symbol of the atom in the compound) and its oxidation number.
In the compound Na2SO4, we know that there are 2 atoms of Na and 4 of O. We should multiply 2 by +1, oxidation number of sodium Na, to get 2, and we should multiply 4 by -2, oxidation number of oxygen O, to get -8.
Step 4. Add the results
By adding the results of your multiplications you get the current oxidation number of the compound without taking into account the oxidation number of the atom about which nothing is known.
In our example, Na2SO4, we should add 2 to -8 to get -6.
Step 5. Calculate the unknown oxidation number based on the charge of the compound
You now have everything you need to find your unknown oxidation number using simple algebraic calculations. Set up an equation like this: "(sum of known oxidation numbers) + (oxidation number you need to find) = (total compound charge)".
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In our example Na2SO4, we can proceed as follows:
- (sum of known oxidation numbers) + (oxidation number you need to find) = (total compound charge)
- -6 + S = 0
- S = 0 + 6
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S = 6. S to an oxidation number equal to
Step 6. in the Na compound2SO4.
Advice
- Atoms in their element form always have zero oxidation number. A monatomic ion has an oxidation number equal to its charge. The metals of group 1A in the form of element, such as hydrogen, lithium and sodium, have an oxidation number equal to +1; the group of metals 2A in their form of elements, such as magnesium and calcium, have an oxidation number equal to +2. Hydrogen and oxygen both have two possible oxidation numbers, which depend on what they are attached to.
- It is very useful to know how to read the periodic table of elements and where the metals and non-metals are located.
- In a compound, the sum of all oxidation numbers must equal zero. If there is an ion that has two atoms, for example, the sum of the oxidation numbers must equal the charge of the ion.
