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Knowledge database: Basics: Oxidation states

Oxidation states are values which tell us how a particular atom bonds with other atoms. Oxidation states are based on the number of unmatched electrons which an element/atom has. Here again, we will need the scheme that shows the way electrons are filled into orbitals (picture below).

chemistry tutorials - electron placement scheme

Electron placement scheme.

To also explain the creation of a simple chemical compound, we will take sodium chloride (table salt) as an example. Sodium has 11 electrons, which are filled into orbitals in this order: 1s holds 2 electrons, the 2s also 2, while the 2p orbitals hold 6 electrons. That makes 10 electrons, so there is still one left. This one is located in the next, 3s orbital. So in the last orbital (which is located in the 3rd shell), we have an unpaired electron. There is a general rule, that tells us that atoms always have a tendency to fill all their orbitals with electrons. That way the ideal electron configuration with maximal stability is achieved. In the case of sodium, in order to achieve the most stable configuration, sodium will release an electron (this way, the whole second shell is full).

In the same way, all other alkali elements also release 1 electron during the formation of chemical compounds that include them. Now let us explain chlorine as an element which is combined with sodium. Chlorine has 17 electrons. By filling the electrons into the orbitals using the scheme shown above, the last of the electrons are located in the 3p orbitals. From a total of 6 available positions, only 1 remains free. To achieve the ideal configuration, chlorine should receive an electron. Now it probably becomes clear that this needed electron is the one that sodium should release to achieve its ideal configuration. Because of this, it is possible to bond these two elements, and the result is a very strong ionic bond (more about that later).

Now lets get back to the oxidation numbers. In the case described above, the oxidation number of sodium is 1 (because it has one electron to release), and the oxidation number of chlorine -1 (because it has one electron to receive). Now that this has been explained, we can state the oxidation numbers of some other elements as well:

Hydrogen: 1 (or -1 in metallic, or other kinds of hydrides)
All alkali metals: 1
All alkaline earth metals: 2
All halogens: -1
Oxygen: -2 (except when forming peroxides, when it is -1)

For most other elements, the oxidation number may have more than one values.


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