The oxidation number (NOX) of an atom in a molecule or a polynuclear ion corresponds to the charge that atom “appears” to possess on these chemical species. That is, it represents the tendency of an atom to attract the electrons involved in the reaction it is carrying out. Therefore, Nox is not the actual charge of the atom, except in the case of monoatomic ions.
This brings us to the fact that Nox must be determined for each atom alone. There are some rules that make this determination easier. Two of them were mentioned in the text "Oxidation Number Concept (NOX)", that are:
1. In the case of simple ions, that is, in monoatomic ions, NOX is equivalent to its own electrical charge. Examples:
O2-: Nox = -2
Cl-: Nox = -1
At+: Nox = +1
Faith2+: Nox = +2
Al3+: Nox = +3
2. In the case of an element or simple substances, which are those composed of atoms of only one type of element, we have to NOX is equal to zero.Some examples of such substances are: O2, no2, H2, He, etc.
Other rules are listed below:
3. The NOX of certain elements of some families in the periodic table are fixed in their compounds. Examples:
- Alkali metals (family 1 or IA): NOX equal to +1;
- Alkaline earth metals (family 2 or IIA): NOX equal to +2;
- Zinc: NOX equal to +2;
- Silver: NOX equal to +1;
- Aluminum: NOX equal to +3.
4. Generally the NOX of hydrogen (H) in composite substances is +1, and that of oxygen is -2;
5. In binary compounds in which the halogens (elements of family 17 or VII A) are the most electronegative, their charge is -1;
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6. The sum of the NOx of all atoms that make up an ionic or molecular compound is always equal to zero.
Observation: This last rule is the most important, because through it it is possible to verify if the NOX of the elements are correct.
Now see how to use this information to determine the NOX of atoms of various compounds:
- H3DUST4:
- H's NOX is +1 and from the O is -2;
- This compound is molecular, so the sum of NOx will equal zero;
- To determine the NOX of phosphorus (P) we perform the following calculation:
H3 DUST4
3. (+1) + x + 4. (-2) = 0
3 + x -8 = 0
x = +8 - 3
x = +5
- Al2(ONLY4)3:
- The NOX of Al is equal to +3;
- NOX of O is equal to -2;
- The sum of NOX is equal to zero.
Al2 (S)3 (O4)3
2. (+3) + 3. x + 4. 3. (-2) = 0
6 + 3. x - 24 = 0
3. x = +24 - 6
X = 18/3
x = +6
- Cr2O72-
- NOX of O is equal to -2;
- This case constitutes an ionic grouping with a total charge equal to -2, therefore, the sum of the NOx of the constituent atoms must not be equal to zero, but equal to this charge.
Cr2 O72-
2. x + 7. (-2) = -2
2. x -14 = - 2
2. x = -2 +14
x = 12/2
x = 6
By Jennifer Fogaça
Graduated in Chemistry
Would you like to reference this text in a school or academic work? Look:
FOGAÇA, Jennifer Rocha Vargas. "Determination of the Oxidation Number (NOX)"; Brazil School. Available in: https://brasilescola.uol.com.br/quimica/determinacao-numero-oxidacao-nox.htm. Accessed on June 28, 2021.
