CBSE Class 12-science Answered

The oxidation state is an indication of the degree of oxidation of an atom in a chemical compound. Oxidation state shows the no. of electrons gained or lost by an atom.

The guidelines for assigning oxidation states (numbers) are given below:

1. The oxidation state of an uncombined element is zero.
2. The sum of the oxidation states of all the atoms or ions in a neutral compound is zero.
3. The sum of the oxidation states of all the atoms in an ion is equal to the charge on the ion.
4. The oxidation state of any element such as Fe, H₂, O₂, P₄, S₈ is zero (0).
5. The oxidation state of oxygen in its compounds is -2, except for peroxides like H₂O₂, and Na₂O₂, in which the oxidation state for O is -1.
6. The oxidation state of hydrogen is +1 in its compounds, except for metal hydrides, such as NaH, LiH, etc., in which the oxidation state for H is -1.
7. The oxidation states of other elements are then assigned to make the algebraic sum of the oxidation states equal to the net charge on the molecule or ion.
8. The following elements usually have the same oxidation states in their compounds:

• +1 for alkali metals - Li, Na, K, Rb, Cs;
• +2 for alkaline earth metals - Be, Mg, Ca, Sr, Ba;
• -1 for halogens except when they form compounds with oxygen or one another;

 Examples:

1. Find oxidation state of chromium in CrCl₃

Oxidation state of Cl = -1

Oxidation state of Cr = x

So, x + 3 (-1) = 0

i.e. x = +3

So, oxidation state of Cr = +3

2. Find the oxidation state of Co in [CoCl(NH₃)₅]²⁺

Oxidation state of Cl = -1

Oxidation state of NH3 = 0

Oxidation state of Co = x

So, x + (-1) + 5 (0) = +2

i.e. x = +3     

So, oxidation state of Co = +3