Oxidation number

What is the oxidation number?

The oxidation number also known as oxidation state is the fictitious electrical charge that is assigned to an atom. It is used to indicate the possible net charge of the same, and a possible method of accounting of electrons. For example, it is used to know whether a reaction is redox or not. Only in the case of elemental ions can say that the oxidation numbers match with the electric charge actual same.

The oxidation number it is necessary to be able to make and name correctly to a compound.

Valence

La valence it is a term (obsolete) that indicates the number of links that presents an element in a chemical compound. Is the number of electrons that are missing or have to assign a chemical element to complete its last energy level. Normally, the chemical elements display one or more valences. Currently, it has been replaced this term for the concept of oxidation numbers and that really means the same thing.

Rules to assign oxidation number

Outlined below are a number of rules to assign oxidation numbers to different atoms:

  • The oxidation number of an element in its free state is zero, 0. For example: H2O2, P4, etc = 0.
  • The oxidation number of iónes, monatomic is equal to the charge of these ions. For example: Na+ = +1, Ca2+ = +2; Cl = -1.
  • The oxidation number of the hydrogen is +1 in all its compounds, except in the metal hydrides that is -1. For example: NaH.
  • The oxidation number of the oxygen is -2 in all its compounds, except in the peroxides gift of is -1. For example: H2O2 or Na2O2. The oxygen combined with fluorine has an oxidation number of -2. For example: OF2.
  • The oxidation number for the alkali metals (Li, Na, K, Rb,...) is +1, and for the alcalinoterreos (Be, Mg, Ca, Sr ...) +2. Fluorine is the most electronegative element of all with an oxidation number of -1.
  • In the combinations between non-metals (without hydrogen or oxygen), the item less metal presents an oxidation number of negative (being the numeric value equal to the charge of the negative ion to be more frequent). For example: in the carbon tetrachloride, CCl4, the chlorine presents a number of oxidation of -1 and the carbon +4. (see next rule).
  • The oxidation numbers of the remaining elements, calculated from the above rules, taking into account the algebraic sum of the oxidation numbers must be zero for neutral molecules, and is equal to the net charge for polyatomic ions.

Periodic table with oxidation numbers

In the following figure are collected the oxidation numbers of the elements of the periodic table.

number of oxidation of table periodic of elements

As a general rule, the alkali metal atoms (group 1) have charge +1the alkaline earth metals (group 2), number of oxidation +2, and the halogens when they have an oxidation number of negative -1.

Calculation of oxidation number to a single carbon atom

In compounds with a single carbon oxidation number can be calculated from the molecular formula.

Are often used as reference values +1 and -2 for the oxidation number of hydrogen and oxygen, respectively.

Therefore, in molecules such as CH4 or CO2 due to the total charge is 0, the oxidation number of carbon in methane is -4, and in the carbon dioxide +4.

  • La oxidation a carbon corresponds to the increase the number of bonds C-O.
  • La reduction a carbon corresponds to the decrease the number of bonds C-O, or the increase the number of bonds carbon-hydrogen C-H.

For example, in the following reaction:

number of oxidation calculation of the number of oxidation state of oxidation

the molecule of RCOH is oxidized to RCOOH (increases the number of bonds (C-O), while also reduces until RCH2O (decreases the number of bonds C-O, or, increases the number of bonds (C-H).

Therefore, in this example the reagent is oxidized and also reduced; it is the call reaction Cannizzaro.

Examples

  • What is the oxidation number of each atom in the molecule Zinc chloride, ZnCl2 ?

The total charge on the molecule must be zero, 0. Therefore, if the number of atoms of Chlorine it is two and its oxidation number -1 (to be negative ion and belong to the group 17), the result for the cloro is (-1) × 2 = -2.

That is to say, the only one atom of Zinc must have the opposite charge (oxidation number) +2 so that the total sum is zero:

Cl = 2 × (-1) = -2

Zn = +2

Cargo net = 0 = (-2) + (+2)

In consequence, the oxidation number the Zn = +2 and the chlorine Cl = -1.

  • What is the oxidation number of each atom in the molecule sodium sulfate, Na2SO4 ?

As in the case of ZnCl2, The total charge on the molecule must be zero 0. Unfold one side, for the sodium and on the other the sulfate. Therefore, if the number of atoms of Na is two and its oxidation number +1 (to be positive ion and belong to the group 1, alkaline) the result for the Na is 2 ×(+1) = +2. That is to say, the load +2, you must compensate negatively, the negative ion sulfate that is why it is written as (SO4=).

The oxygen have an oxidation number of oxide -2, and therefore 4 atoms × (-2) charge each for a total of (-8) if we subtract the two negative charges of the ion total, the sulfur you must have the same magnitude but of opposite sign. That is to say, (-8) – (-2) = -6, and this is why the oxidation number of the sulfur you must be in this compound is +6.

So that the total sum is zero in the molecule must fulfill that:

Na = 2 × (+1) = +2

O = 4 × (-2) = -8

S = 1 × (+6) = +6

Cargo net = 0 = (+2) + (-8) + (+6)

In consequence, the oxidation number the Na = +2 and the oxygen O = -2, and the sulfur S = +6.

Summary

The oxidation number it is necessary to be able to make and name correctly to a compound and is used to indicate the net charge of the same and count the number of electrons. Only in the case of ions elementary this coincides with the electrical load. In consequence, the oxidation numbers of the atoms most commonly used you must memorize, and for the rest of the items, you could easily deduce from their molecular formulae, as we have seen in the previous examples.

Frequently asked questions about the Oxidation Number

  • What is the oxidation number?

The oxidation number refers to the degree of oxidation (or also the loss of electrons) of an atom in a chemical compound. Conceptually, the oxidation state is the hypothetical charge an atom would have if all bonds to other atoms of different elements were 100% ionic, with no covalent component. This is never completely true for actual bonds.

  • Is it the same oxidation number and oxidation state?

Yes, both oxidation number and oxidation state are terms used interchangeably and refer to the same concept.

  • What is the oxidation number of Cr?

Chromium (Cr) has three oxidation numbers: +2, +3 and +6, the most frequent value being +3. For example, in the inorganic salt chromium (III) sulfate with the formula Cr2(SO4)3, Cr has an oxidation number of +3.