Oxidation number concept, how to get it out and examples

The oxidation number, Also called the oxidation state, it is one that describes the gain or loss of electrons in an atom, assuming that the compound of which it is a part has a purely ionic character. Therefore, when talking about oxidation number, it is assumed that all atoms are found as ions interacting electrostatically.

Although the real picture is more complicated than having ions everywhere, oxidation number is really useful for interpreting oxide-reduction (redox) reactions. Changing these numbers reveals which species have been oxidized or lost electrons, or if electrons have been reduced or gained..

The ionic charge of a monatomic ion matches its oxidation number. For example, the oxide anion, O^two-, one of the most abundant because it is found in innumerable minerals, it has an oxidation number of -2. This is interpreted as follows: it has two extra electrons compared to the ground state oxygen atom O.

Oxidation numbers are easily calculated from a molecular formula, and are often more useful and relevant when it comes to ion-packed inorganic compounds. Meanwhile, in organic chemistry it does not have the same importance, since almost all its bonds are essentially covalent..

Article index

• 1 How to get the oxidation number?
  • 1.1 Electroneutrality
  • 1.2 Valencias
  • 1.3 General rules
  • 1.4 Arithmetic operations
• 2 Examples
  • 2.1 Oxygen
  • 2.2 Nitrogen
  • 2.3 Chlorine
  • 2.4 Potassium
  • 2.5 Sulfur
  • 2.6 Carbon
  • 2.7 Phosphorus
• 3 References

How to get the oxidation number?

Electroneutrality

The sum of the ionic charges in a compound must equal zero for it to be neutral. Only ions can have positive or negative charges.

Therefore, it is to be assumed that the sum of the oxidation numbers must also equal zero. Keeping this in mind, and performing some arithmetic calculations, we can extract or determine the oxidation number of an atom in any compound.

Valencias

Valences are not reliable in determining the oxidation number of an atom, although there are several exceptions. For example, all the elements of group 1, the alkali metals, have a valence of 1, and therefore, an invariable oxidation number of +1. The same happens with alkaline earth metals, those of group 2, with an oxidation number of +2.

Note that positive oxidation numbers are always preceded by the '+' symbol: +1, +2, +3, etc. And in the same way the negatives: -1, -2, -3, etc..

General rules

There are some general rules that must be taken into account when determining the oxidation number:

-The oxidation number for oxygen and sulfur is -2: O^two- and S^two-

-Pure elements have oxidation number of 0: Fe⁰, P₄⁰, S₈⁰

-The hydrogen atom, depending on who it is bonded to, has an oxidation number of +1 (H⁺) or -1 (H^-)

-Halogens, as long as they are not bound to oxygen or fluorine, have an oxidation number of -1: F^-, Cl^-, Br^- and I^-

-For a polyatomic ion, such as OH^-, the sum of the oxidation numbers should not be equal to zero but to the charge of the ion, which would be -1 for OH^- (OR^two-H⁺)^-

-Metals under ordinary conditions have positive oxidation numbers

Arithmetic operations

Suppose we have the compound PbCO₃. If we identify the carbonate anion, CO₃^two-, calculating all oxidation numbers will be straightforward. We start with the same carbonate, knowing that the oxidation number of oxygen is -2:

(C^xOR₃^two-)^two-

The sum of the oxidation numbers must equal -2:

x + 3 (-2) = -2

x -6 = -2

x = +4

Therefore, the oxidation number of carbon is +4:

(C⁴⁺OR₃^two-)^two-

The PbCO₃ it would now look like:

Pb^zC⁴⁺OR₃^two-

Again, we add the oxidation numbers to equal zero:

z + 4 - 6 = 0

z = +2

Therefore, lead has an oxidation number of +2, so it is assumed that it exists as a Pb cation.^two+. Actually, it was not even necessary to do this calculation, because knowing that carbonate has a charge of -2, lead, its counterion must necessarily have a charge of +2 for there to be electroneutrality.

Examples

Here are some examples of oxidation numbers for various elements in different compounds..

Oxygen

All metal oxides have oxygen as O^two-: CaO, FeO, Cr_twoOR₃, BeO, Al_twoOR₃, PbO_two, etc. However, in the peroxide anion, O_two^two-, Each oxygen atom has an oxidation number of -1. Likewise, in the superoxide anion, O_two^-, each oxygen atom has an oxidation number of -1/2.

On the other hand, when oxygen binds to fluorine it acquires positive oxidation numbers. For example, in oxygen difluoride, OF_two, oxygen has a positive oxidation number. Which? Knowing that fluorine is -1 we have:

OR^xF_two^-1

x + 2 (-1) = 0

x -2 = 0

x = +2

Thus, oxygen has an oxidation number of +2 (O^two+) in the OF_two (OR^two+F_two^-).

Nitrogen

The main oxidation numbers of nitrogen are -3 (N^3-H₃⁺¹), +3 (N³⁺F₃^-) and +5 (N_two⁵⁺OR₅^two-).

Chlorine

One of the main oxidation numbers for chlorine is -1. But everything changes when it is combined with oxygen, nitrogen or fluorine, more electronegative elements. When this happens, it acquires positive oxidation numbers, such as: +1 (N^3-Cl₃⁺, Cl⁺F^-, Cl_two⁺OR^two-), +2, +3 (ClO_two^-), +4, +5 (ClO_two⁺), +6 and +7 (Cl_two⁷⁺OR₇^two-).

Potassium

Potassium in all its compounds has an oxidation number of +1 (K⁺); unless it is a very special condition, where it can acquire an oxidation number of -1 (K^-).

Sulfur

The case of sulfur is similar to that of chlorine: it has an oxidation number of -2, as long as it does not combine with oxygen, fluorine, nitrogen, or the same chlorine. For example, your other oxidation numbers are: -1, +1 (S_two⁺¹Cl_two^-), +2 (S^two+Cl_two^-), +3 (S_twoOR₄^two-), +4 (S⁴⁺OR_two^two-), +5 and +6 (S⁶⁺OR₃^two-).

Carbon

The main oxidation states of carbon are -4 (C^4-H₄⁺) and +4 (C⁴⁺OR_two^two-). This is where we begin to see the failure of this concept. Not in methane, CH₄, and neither in carbon dioxide, CO_two, we have carbon as C ions^4- or C⁴⁺, respectively, but forming covalent bonds.

Other oxidation numbers for carbon, such as -3, -2, -1 and 0, are found in the molecular formulas of some organic compounds. However, and again, it is not very valid to assume ionic charges on the carbon atom.

Match

And finally, the main oxidation numbers of phosphorus are -3 (Ca₃^two+P_two^3-), +3 (H₃⁺P³⁺OR₃^two-), and +5 (P_two⁵⁺OR₅^two-).

References

1. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
2. Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
3. Clark J. (2018). Oxidation States (Oxidation Numbers). Recovered from: chemguide.co.uk
4. Wikipedia. (2020). Oxidation state. Recovered from: en.wikipedia.org
5. Dr. Kristy M. Bailey. (s.f.). Assigning Oxidation Numbers. Recovered from: occc.edu