How to Find Oxidation Number

In the field of chemistry, the terms “oxidation” and “reduction” are used to refer to reactions in which an atom (or group of atoms) loses or gains one or more electrons. An oxidation number is a number assigned to an atom (or group of atoms) by which we can keep track of the number of electrons exchanged when a substance is oxidized or reduced in a reaction. The process of determining the oxidation number of an atom can be very simple, but it can also be complex, depending on the charge of the atom as well as the chemical composition of the compound of which the atom is composed. For complex forms of matter, some atoms can have more than one oxidation number. However, the determination of oxidation numbers has been laid out in clear rules that are easy to understand if you already have a basic background in math and chemistry.

Table of Contents

Determination of oxidation number based on chemical rule

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Determines whether the analyte is a single element or a compound. Atoms in the free state, not bonded to other atoms, always have an oxidation number of 0. This rule holds true for a single substance composed of one or more atoms of the same element.

For example, Al _(s) and Cl ₂ both have an oxidation number of 0 because they are in unbound form.
Note that _S8 sulfur, a rare form of sulfur, also has an oxidation number of zero.

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Determine whether the analyte is an ion. For ions, the oxidation number is equal to the charge number of the ion itself. This rule applies to both free ions and ions in compounds.

For example, the Cl ion ^– has an oxidation number of -1.
In the compound NaCl, the Cl ion still has an oxidation number of -1. Since the Na ion has a charge of +1 by definition, we also know that the Cl ion has a charge of -1, so the oxidation number of the Cl ion is also -1.

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Metal ions can have many oxidation numbers. Metal ions can have one or more charges. For example, iron (Fe) ion can have a charge of +2 or +3. ^{[1] X Source of Study} The charge of a metal (also an oxidation number) can be determined in relation to the charge of another atom in the compound, or by the roman numeral notation in literal form (e.g. in the sentence “Iron(III) ion has a charge of +3.”)

For example, consider a compound containing the metal aluminum ion. AlCl ₃ compound has a total charge of 0. Since we know that Cl ion ^– which has a charge of -1 and has 3 Cl ions ^– in the compound, therefore, for the total charge of the compound to be zero, the Al ion must be zero. has a charge of +3. Therefore, the oxidation number of Al is +3.

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Oxygen has an oxidation number of -2 (except in some special cases). In the ‘most’ cases, the oxygen atom has an oxidation number of -2, except in the following cases:

When oxygen is in its elemental state (O ₂ ), the oxidation number is 0, the same for other elemental atoms.
When oxygen is in the compound ”peroxide”, the oxidation number of oxygen is -1. Peroxides are a group of compounds with a single bond between two oxygen atoms (or O ₂^-2 anion). For example, in the molecule H ₂ O ₂ (hydrogen water), oxygen has an oxidation number (and charge) of -1. Similarly, when the oxygen is in the highly reactive oxidation state (superoxide), the oxidation number of oxygen is -0.5.
When oxygen is bonded to fluorine, its oxidation number is +2. We will consider the rule for fluorine-containing compounds later. In the compound (O ₂ F ₂ ), the oxidation number of oxygen is +1.

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Hydrogen has an oxidation number of +1 (except in special cases). Similar to oxygen, the oxidation number of hydrogen also varies with a few exceptions. Normally, the oxidation number of hydrogen is +1 (except in the case of hydrogen in its element H ₂ ). However, in hydroxide compounds, hydrogen has an oxidation number of -1.

For example, in the compound H ₂ O, hydrogen has an oxidation number of +1 because oxygen has a charge of -2 and we need two +1 charges to neutralize the total charge. However, in the compound Sodium Hydride, NaH, hydrogen has an oxidation number of -1 because the Na ion has a charge of +1, and the total charge of NaH is 0, so the charge of hydrogen is -1 and also the oxidation number of hydrogen in this case.

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Fluorine always has an oxidation number of -1. As stated above, the oxidation numbers of some elements can change for many reasons (as is the case with metal ions, oxygen atoms in peroxides, etc.). However, the oxidation number of Fluorine does not change and is equal to -1 in all compounds containing this element. This is because fluorine is the most electronegative element – in other words, the Fluorine atom is the hardest to lose electrons compared to the atoms of other elements, on the contrary, it is easy to attract electrons from the element’s atom. other. Therefore, the charge of Fluorine does not change.

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The oxidation number of a compound is equal to the charge of that compound. The total oxidation number of all atoms in a compound must be equal to the charge of that compound. For example, if a compound is uncharged, the total oxidation number of each atom in that compound must be zero; if the charge of a compound composed of many ions is equal to -1, the total oxidation number of the ions constituting the compound must be -1.

This is a pretty cool way to double-check your results. If the sum of the oxidation numbers of the atoms in the compound is not equal to the sum of the charges of the substance, then you probably assigned or miscalculated somewhere.

Find oxidation numbers for atoms that don’t have their own rules for oxidation numbers

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Find atoms that do not have their own rules for oxidation numbers. Some atoms do not follow a certain rule about oxidation numbers. If the atom doesn’t follow the rule outlined above and you don’t know the charge of the atom (for example, the atom is part of a larger compound, so the charge of the atom is not given), you can find the oxidation number of that atom by subtraction. First you need to determine the oxidation numbers of all the other atoms in the compound, then just solve the problem of finding the unknown number based on the total charge of the compound.

For example, in the compound Na ₂ SO ₄ , the charge of sulfur (S) is not known – and the sulfur atom is not in the elemental state at this time, so its oxidation number is different from zero. This is examples that we will use in this section.

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Find the oxidation numbers of the other elements in the compound. Using the rules for assigning oxidation numbers above, we can find the oxidation numbers of other elements in a compound. Remember to consider exceptions for O, H, etc.

In Na ₂ SO ₄ , based on the principles stated, we know that the Na ion has a charge (also an oxidation number) of +1, the oxygen atom has an oxidation number of -2.

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Multiply the atomic number of each element by their oxidation number. After determining the oxidation numbers of all atoms except the unknown we need to find, we need to consider how many atoms of the same element in the compound. Multiply this number (the number written at the bottom and after the chemical symbol for that element in the compound) by the oxidation number.

In Na ₂ SO ₄ , we see that there are 2 Na atoms, 4 O atoms. So we have 2 × +1, the oxidation number of Na, get 2, and 4 × -2, the oxidation number of O, get – 8.

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Add the sum of the products found. Adding the products found we will determine the oxidation number of the compound that “doesn’t” consider the oxidation number to be found.

In the example of Na ₂ SO ₄ , the sum of 2 and -8 is -6.

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Calculate the required oxidation number based on the charge of the compound. With the calculated values, we only need to perform simple addition and subtraction to find the oxidation number of the element in question. Make an equation where the charge of the compound is equal to the sum of the results obtained in the previous step plus the desired oxidation number. Or in other words: (Sum of known oxidation numbers) + (oxidation number to find) = (total charge of the compound).

In the example of Na ₂ SO ₄ , we have:
- (Sum of known oxidation numbers) + (oxidation number to find) = (total charge of compound)
- -6 + S = 0
- S = 0 + 6
- S = 6. So S has an oxidation number of +6 in the compound Na ₂ SO ₄ .

Advice

In an uncharged compound, the sum of all the oxidation numbers of the atoms in the compound must be zero. If there is an ion consisting of 2 atoms, the sum of the oxidation numbers must equal the charge of that ion.
Knowing how to read the periodic table of the chemical elements and how to locate elements in the periodic table is very helpful in determining oxidation numbers.
The atoms in a single element have an oxidation number of 0. A monatomic ion has an oxidation number equal to the charge of that ion. Atoms of group 1A metals in their elemental state, such as hydrogen gas, elemental lithium and sodium, have an oxidation number of +1; Atoms of group 2A metals in their elemental state, such as magnesium and calcium metals, have an oxidation number of +2. The oxidation number of a hydrogen atom and an oxygen atom depends on the atom to which it is bonded and the type of bond.
Here are some tips to help you identify the difference between oxidation and reduction:
- O (oxidation) donor, reduction accept.
- Metal atoms tend to lose electrons and form positive ions (oxidation).
- Nonmetals and nonmetals tend to gain electrons and form negative ions (reduction).
- An ion can also accept or give away electrons to become an ion with a different charge than the original ion, or to become an electrically neutral atom.

Things you need

Periodic table of chemical elements
Internet connection, chemistry textbook
Paper, pen
Computer

Steps

Determination of oxidation number based on chemical rule

Find oxidation numbers for atoms that don’t have their own rules for oxidation numbers

Advice

Things you need

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