O NOX, or Oxidation Number, is a positively or negatively charged number that indicates whether a particular atom is deficient or greater. number of electrons when it establishes a chemical bond with another atom, the same or different from it, or in a chemical reaction. Thus, we can say that:
positive NOX: indicates that the atom is deficient in electrons;
negative NOX: indicates that the atom has a greater amount of electrons.
Knowing the type of chemical bond between atoms, we can know whether the NOX of the atom will be negative or positive. See some cases:
The) In the Ionic bond
Ionic bonding always occurs between metal atoms with non-metal or metal and hydrogen atoms. As metals have as their main characteristic the tendency to lose electrons, the non-metal or hydrogen bonded to it will receive electrons.
Thus, in the cases below:
Case 1: KI
Potassium is metal and Iodine is non-metal, therefore, Potassium loses electron and Iodine gains electron. We conclude, then, that:
Potassium: Has positive NOX.
Iodine: Has negative NOX.
Case 2: NaH
Sodium is metal and therefore loses an electron. Hydrogen, on the other hand, which is not classified as a metal or non-metal, receives the electron lost by sodium. We conclude, then, that:
Sodium: Has positive NOX
Hydrogen: Has negative NOX
B) Covalent bond
Covalent bonding occurs between:
Ametal with Ametal
Ametal with Hydrogen
hydrogen with hydrogen
As the covalent bond does not have the presence of metal, none of the atoms involved lose electrons. However, as there is a difference in electronegativity (the ability to attract electrons from another atom) between the atoms, the electrons of one may be closer to the other.
The descending order of electronegativity of atoms is:
F>O>N>Cl>Br>I>S>C>P>H
So, in cases:
Case 1: HCl
Because chlorine has greater electronegativity than hydrogen, it attracts electrons from hydrogen toward it. Thus, we can say that chlorine has more electrons and hydrogen is deficient in electrons. We conclude, then, that:
Chlorine: Has negative NOX
Hydrogen: Has positive NOX
Case 2: H2 it's the2
Since in both molecules we have the same atoms interacting with each other, we cannot assess the difference in electronegativity. Therefore, we concluded that both in H2 how much in the O2, the NOX of each atom is zero.
In addition to determining whether an atom will have positive or negative NOX, we can also determine the number of electrons that he lost or gained in the ionic bond or the amount of electrons he approached or moved away in the bond covalent. To do this, we use the following rules:
1st) Simple substances
your atoms will always have NOX zero, as they are formed by equal atoms. Examples: Cl2 and on.
2nd) Simple ionic substances
The NOX of the atom of a simple ionic substance is always the charge itself. For example:
Example 1: the ion Al+3 features NOX +3.
Example 2: the ion Cl-1 features NOX -1.
3rd) Compound substances
Compound substances, ionic or covalent, are those that have atoms of different chemical elements. Here's what we should consider to determine the NOX of each element present:
If you have Alkaline Metal (IA) or the element Silver (Ag) at the far left of the formula: this will always have NOX +1.
if you have Alkaline Earth Metal (IIA) or the element Zinc (Zn) at the far left of the formula: this will always have NOX +2.
If you have Boron family metal (IIIA) at the far left of the formula: this will always have NOX +3.
If you have chalcogen (VIA), with the exception of metals in this family, at the far right of the formula: this will always have NOX -2.
if you have halogen (VIIA) at the far right of the formula: this will always have NOX -1.
The NOX of any other chemical element present in the formula of the compound will be determined from the knowledge that the sum of the NOX of all atoms will always equal 0.
Let's follow the NOX determination of elements in some compound substances:
Example 1: PbI2.
Iodine, which is a halogen, has NOX -1. To determine the NOX of lead (Pb), just use the following expression:
NOX of Pb + NOX of I (multiplied by 2) = 0
NOXPb + 2.(-1) = 0
NOXPb – 2 = 0
NOXPb = +2
Example 2: Au2s
Sulfur is a chalcogen and therefore has NOX -2. To determine the NOX of the element Gold (Au), which appears with index 2 in the formula, just use the following expression:
NOX of Au (multiplied by 2) + NOX of S = 0
2.NOXAu + (-2) = 0
2.NOXAu – 2 = 0
2.NOXAu = +2
NOXAu = +2
2
NOXAu = +1
Example 3: Al2(ONLY4)3
Oxygen (with an index 4.3) is a chalcogen and therefore has NOX -2. Aluminum belongs to the boron family and therefore has NOX +3. To determine the NOX of the element sulfur (S), which appears with index 1.3 in the formula, just use the following expression:
NOX of Al (multiplied by 2) + (multiplied by 2) + NOX of O (multiplied by 12) = 0
2.(+3) + 3.NOXs + 12.(-2) = 0
+6 + 3.NOXs – 24 = 0
3.NOXs = +24 – 6
3.NOXs = +18
NOXs = +18
3
NOXs = +6
4th) Compound ion
The difference between a compound ion and a compound substance is the fact that it has a charge in the composition of the formula. See an example:
ONLY4-2
The rules that we will use to determine the NOX of all its elements are the same as those used previously for composite substances. The difference is that the sum of the NOX of each atom present is always equal to the charge in the formula.
Let's follow the determination of NOX of elements in composite ions:
Example 1: ONLY4-2
Oxygen, which has an index of 4, is a chalcogen and, therefore, has NOX -2. To determine the NOX of sulfur (S), just use the following expression:
NOX of S + NOX of O (multiplied by 4) = -2 (compound ion charge)
NOXs + 4.(-2) = -2
NOXs – 8 = -2
NOXs = -2 + 8
NOXs = + 6
Example 2: P2O7-4
Oxygen, which has an index of 7, is a chalcogen and, therefore, has NOX -2. To determine the NOX of phosphorus (P), just use the following expression:
NOX of P (multiplied by 2) + NOX of O (multiplied by 7) = -4 (compound ion charge)
2.NOXP + 7.(-2) = -4
2.NOXP – 14 = -4
2.NOXs = -4 + 14
NOXs = +10
2
NOXs = + 5
By Me. Diogo Lopes Dias
Source: Brazil School - https://brasilescola.uol.com.br/o-que-e/quimica/o-que-e-nox.htm