Find Oxidation Number

Step 1: Re-emphasis the following points

• oxidation state is a number that is assigned to an element in a chemical combination. This number represents the number of electrons that an atom can gain, lose, or share when chemically bonding with an atom of another element.
• The terms “oxidation state” and “oxidation number” are often used interchangeably.
• The transfer of electrons is described by the oxidation state of the molecule.
• formal charge and oxidation state are different.
• Oxidation state is used to determine the changes in redox reactions and is numerically similar to valence electrons.
• Formal charge determines the arrangement of atoms and the likelihood of the molecule existing.

Step 2: Introduce the seven Rules to calculating oxidation:

To effectively assign oxidation states to a compound, the seven basic rules must be followed in order. Remember to use the rule that comes first if two rules conflict with each other.  These rules hold true for most compounds..

• RULE 1: Any individual atom uncombined with other elements has the oxidation state of 0 (zero).

Ex.) The OS for Ag is 0. The oxidation state for O (oxygen) or O₂ is 0 as long as it is uncombined with any other element.

• RULE 2: The total sum of the oxidation state of all atoms in any given species is equal to the net charge on that species.

a.) In neutral species, the total sum of the oxidation state of all atoms is 0.

Ex.) The sum of OS for NaCl is 0 since the OS of Na = +1 and the OS of Cl = -1, therefore NaCl total OS = 0

b.) In ions, the total sum of the oxidation state is the charge of the ion.

Ex.) The OS of Ca²⁺ (Calcium) is = +2. The total sum of the OS of all atoms in CrO₄²⁻  (Chromate ion) is -2. The total sum of the oxidation states of all atoms in CH₃COO^– (Acetate ion) is -1.

• RULE 3: In a compound, the Oxidation state for Group 1(1A) metal is +1 and for Group 2(2A) metal, the oxidation state is +

Ex.) In NaCl, Na has the oxidation state of +1 since it is a Group 1 Alkali metal. Cl would have an oxidation number of -1 to make the sum of the oxidation states 0 (Rule 2). In MgCl₂, Mg has the oxidation state of +2, since it is a Group 2 Alkaline Earth metal. Cl would have an oxidation state of -1, and since there are 2 Cl atoms, the overall charge of the species would again be 0 (Rule 2).

• RULE 4: The oxidation state of FLOURINE is -1 in a compound. 

Ex.) OS of F is -1 in HF, SF₆

• RULE 5: The oxidation state of HYDROGEN is +1 in a compound.

Ex.) OS of H is +1 in HI, CH₄, NH₄⁺

• RULE 6: The oxidation state of OXYGEN is -2 in a compound. 

Ex.) OS of Oxyen is -2 in OH^–, H₂O, CO₃^2–

• RULE 7: In two-element compounds with metals, Group 15(3A) elements have the oxidation state of -3, Group 16(6A) elements have the oxidation state of -2, and Group 17(7A) elements have the oxidation state of -1. 

Ex.) In MgBr₂, Br has the oxidation state of -1, since it is a Group 17 element. In Li₂S, S has the oxidation state of -2, since it is a Group 16.

Fractional Values for Oxidation States 

The oxidation states are usually in whole numbers, but in some cases, they are in fractional numbers. Consider Fe₃O₄.

• Using rule #2, the sum of the oxidation state of all atoms is 0
• Using rule #6, the O has the oxidation state of -2.  4x(-2) = -8
• Equation: 3Fe + (-8) =  0
• 3Fe = 8
• Fe = 8/3