Oxidation State

The oxidation state of an atom in a compound refers to the hypothetical charge that an atom would have if all bonds to atoms of different elements were completely ionic. It describes the degree of oxidation or reduction of the atom.

1.0Oxidation State and Oxidation Number

Oxidation state and oxidation number are often used interchangeably, but they have distinct meanings: the oxidation state represents the hypothetical charge an atom would have if all bonds were completely ionic, useful for describing the degree of oxidation or reduction in a compound, whereas the oxidation number is a formal charge assigned to an atom in a molecule or ion according to a set of rules, particularly for tracking electron transfer in redox reactions. The oxidation state is a broader concept applicable in various contexts, while the oxidation number is more specifically defined and used for redox processes, reflecting actual or formal charges in compounds.

2.0General Rules for Determining Oxidation States

Calculating the oxidation number of an atom in a molecule or ion involves a series of steps based on a set of rules. Which are-

1. Elemental Form:

• An atom in its elemental form always has an oxidation state of zero.
• Example: O₂​, H₂​, and N₂​ have oxidation states of 0.

2. Monoatomic Ions:

• The oxidation state of a monoatomic ion is same to its ionic charge.
• Example: Na⁺ has an oxidation state of +1, Cl⁻ has an oxidation state of -1.

3. Oxygen:

• The oxidation state of oxygen is usually -2.
• Exceptions: In peroxides (e.g., H_2​O₂​), oxygen has an oxidation state of -1. In compounds with fluorine (e.g., OF₂), oxygen has an oxidation state of +2.

4. Hydrogen:

• The oxidation state of hydrogen is usually +1.
• Exception: In metal hydrides (e.g., NaH), hydrogen has an oxidation state of -1.

5. Fluorine:

• The oxidation state of fluorine is always -1 in its compounds.

6. Alkali Metals (Group 1):

• The oxidation state of alkali metals (e.g., Li, Na, K) in their compounds is always +1.

7. Alkaline Earth Metals (Group 2):

• The oxidation state of alkaline earth metals (e.g., Mg, Ca) in their compounds is always +2.

8. Sum of Oxidation States:

• The sum of oxidation states of all atoms in a neutral compound must be 0.
• The sum of oxidation states of all atoms in a polyatomic ion equals the charge of the ion.

3.0Solved examples of Oxidation state

1. Water (H₂O):

• Hydrogen: +1 (2 H atoms contribute +2)
• Oxygen: -2
• Sum: 2(+1)+(−2) = 02(+1) + (-2) = 02(+1)+(−2)=0

2. Sodium Chloride (NaCl):

• Sodium: +1
• Chlorine: -1
• Sum: +1+(−1)=0+1 + (-1) = 0+1+(−1)=0

3. Sulfuric Acid (H₂SO₄):

• Hydrogen: +1 (2 H atoms contribute +2)
• Oxygen: -2 (4 O atoms contribute -8)
• Sulfur: +6 (to balance the total to 0)
• Sum: 2(+1)+(+6)+4(−2)=02(+1) + (+6) + 4(-2) = 02(+1)+(+6)+4(−2)=0

4. Permanganate Ion (MnO₄⁻):

• Oxygen: -2 (4 O atoms contribute -8)
• Manganese: +7 (to balance the total to -1)
• Sum: +7+4(−2)=−1+7 + 4(-2) = -1+7+4(−2)=−1

4.0Trends in Oxidation States

1. Oxidation States of Transition Elements:

• Transition elements can exhibit multiple positive oxidation states.
• The maximum oxidation number of a transition element in the first two groups is the same as its group number.
• For other transition elements, the maximum oxidation number can be calculated as the group number minus 10 in the long form of the periodic table.

2. Oxidation States of p-Block Elements:

• p-Block elements (except fluorine and oxygen) have the highest oxidation number equal to their group number.
• The lowest oxidation state of p-block elements is the group number minus 8.

3. Calculation of Oxidation States in Transition Elements:

• The lowest oxidation number is determined by the number of electrons present in the ns orbital.
• The highest oxidation state corresponds to the number of unpaired electrons in ns and (n−1)d orbitals.

4. Trends in Oxidation States Across Periods:

• The highest oxidation number of an element increases across a period in the periodic table.
• For example, in the third period, the maximum oxidation number ranges from 1 to 7.

5. Stock Notation:

• In some cases, oxidation numbers or states are indicated using Stock notation, introduced by German chemist Alfred Stock.
• This notation uses a Roman numeral in parentheses after the metal’s symbol to represent its oxidation number in a compound.
• Examples:

Auric chloride (AuCl₃​) is written as Au(III)Cl_3​.

Aurous chloride (AuCl) is written as Au(I)Cl.

Stannous chloride (SnCl₂​) is written as Sn(II)Cl₂​.

Stannic chloride (SnCl₄​) is written as Sn(IV)Cl₄​.