How to determine the oxidation state of elements in a compound
Main principles of identifying oxidation state
The fascinating science of chemistry has the concept of oxidation state, which is a number for formulating oxidation-reduction reactions.
To be concise, in this science the oxidation state means the conditional charge in the atom which loses or gains electrons, and this figure is a method of calculating the transfer of electrons. This number is given to one atom or group of atoms, and characterizes the number of redistributed electrons, and shows the principle of transfer of electrons as a result of a certain chemical reaction.
Determining the oxidation state is easy and difficult at the same time – it depends on the atoms and the molecules that constitute them. It often happens that the atoms of some chemical elements can have an absolutely different oxidation state.
In order to simplify the process of determining the oxidation state, special simple rules are used, and anyone who knows the basics of chemistry and mathematics will be able to use them to find a certain oxidation state without difficulty. We should always remember that often the oxidation state and the valency of the element are equal to each other.
This scientific topic is widely studied at school, so in order to understand how to find an oxidation state, we propose that you read this article.
The first stage: we determine whether the chemical substance is elementary.
The oxidation state of atoms which do not interact with other atoms in any way as a result of chemical processes is zero.
This principle is applied to a group of substances which was formed from individual free atoms. This rule is also applied for chemical elements which consist of diatomic or polyatomic molecules of just one element.
For example, iron Fe and calcium Ca have an oxidation state of zero, because they consist of one element that is not chemically bonded with others, and so do polyatomic molecules with the same type of atom, for example for ozone O₃ the oxidation state will also be 0.
Calculating the oxidation state in ionic compounds
The oxidation state is identical to the charge on the atoms or group of atoms. This principle is applicable both for free ions and for those which are part of the structure of chemical compounds.
For example, the oxidation state of the chlorine ion is -1, and if we examine chlorine in a chemical compound, for example in hydrochloric acid HCl, the oxidation state of this element will also be -1. As a hydrogen ion has an oxidation state of +1, the charge of the chlorine ion is -1, which means that its oxidation state is -1.
Metal ions can have many oxidation states
We will examine this using the example of iron (Fe), because its ion can have a charge of +2 and +3. The charge of ions of metallic elements can be determined by the charge of other ions in the chemical compound, and in writing formulas this charge is indicated by Roman numerals, for example iron (II) has an oxidation state of +2. Here you’ll find stunning safe experiments with iron.
How do we find the oxidation state in a compound?
As we have already established, the compound should be neutral. We will examine AlCl₃.
As we said above, the charge of the ions in chlorine is -1, and in this compound there are three chlorine atoms. Accordingly, to compensate the minuses, the charge of aluminum must be +3.
How do we determine the oxidation state of O₂?
When oxygen is in a free state (without entering into a reaction with any elements), the oxidation state is zero (like other elementary elements, in fact).
If oxygen is a component of any hydroxide, for example hydrogen hydroxide H2O2, it will have an oxidation state of -1.
If oxygen interacts with fluorine (F), it will have an oxidation state of +2.
Let us examine the oxidation state of hydrogen H
This chemical element has an oxidation state of +1 (except the molecular state of hydrogen), but there are exceptional cases.
For example, in water H₂O the oxidation state of hydrogen will be +1, because the oxidation state of oxygen is -2, and so the whole compound, according to the rules, has a neutral charge.
But if we take NaH, the oxidation state of H will be -1, as sodium has a charge of +1.
How to determine the oxidation state of fluorine (F)
Although the oxidation state of chemical elements depends in most cases on numerous factors, fluorine will always have an oxidation state of -1. This is because fluorine has a low electrical negativity, i.e. F atoms unwillingly break free from their own electrons, but intensively attract electrons of other elements.
Rule: the sum of oxidations states is equal to the charge of the chemical element.
The sum of oxidation states of all atoms of the compound should be neutral. With this rule, we can test whether or not we have made a mistake in solving a chemistry problem.
How do we determine oxidation states? Here’s some useful advice which will help in solving problems:
The periodic table will come in handy to make the calculation precise. You must learn to use it properly and distinguish where metals and non-metals are located.
To find the oxidation state of metals, which often have several corresponding values, you must determine them by the oxidation states of other atoms in the compound.
If you add up all the values of oxidation of atoms in a chemical bond, you will always get a zero oxidation state.
The highest oxidation state of an element is determined using the periodic table by the group in which it is located.
Metals in all compounds have a positive oxidation state.
In compounds with non-metals, hydrogen has an oxidation state of +1, and an oxidation state of -1 with metals.
In compounds, oxygen has an oxidation state of -2, apart from in H₂O₂, OF₂, K₂O₂.
The oxidation states of non-metals when joined to metal atoms will always be negative, but on interactions with atoms of non-metals they may have a positive or negative oxidation state.
To find the highest oxidation state in non-metals, from the number 8 subtract the number of the group in which the element is located, and the highest oxidation state with a plus sign will be equal to the number of electrons on the outer layer. To find the number of electrons in the outer layer, look at the number of the group in the periodic table.