Difference between oxidation state and formal charge

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Difference Between Oxidation State and Formal Charge

Oxidation State

Definition:
The oxidation state (or oxidation number) of an atom in a molecule or ion is the hypothetical charge that the atom would have if all its bonds to other atoms were completely ionic. It indicates the degree of oxidation (loss of electrons) of an atom in a chemical compound.

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Key Points:

1. Hypothetical Charge: It is a theoretical charge assigned to an atom in a molecule, assuming complete ionic character of bonds.
   

   

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2. Electron Transfer: It reflects the number of electrons an atom has gained or lost in forming a bond.
   

   

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3. Rules for Assignment:
   • The oxidation state of a free element is zero.
   • The sum of the oxidation states of all atoms in a neutral compound is zero.
   • The sum of the oxidation states of all atoms in a polyatomic ion equals the charge of the ion.
   • More electronegative elements are assigned negative oxidation states, while less electronegative elements are assigned positive oxidation states.

Applications:

• Naming Compounds: Used in naming compounds, especially inorganic ones, to indicate the oxidation state of the central atom (e.g., iron(III) oxide).
• Balancing Redox Reactions: Helps in balancing redox reactions by ensuring that the total charge on both sides of the equation is equal.
• Predicting Reactivity: Can predict the reactivity of elements and compounds based on their oxidation states.

Formal Charge

Definition:
The formal charge of an atom in a molecule is the charge assigned to that atom assuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electronegativity.

Key Points:

1. Equal Sharing Assumption: It assumes that all bonding electrons are shared equally between atoms.
2. Calculation: It is calculated using the formula: $$\text{Formal Charge} = \text{Valence Electrons} - (\text{Non-bonding Electrons} + \frac{1}{2} \times \text{Bonding Electrons})$$
3. Stability Indicator: It helps in determining the most stable Lewis structure by minimizing the formal charges on atoms within a molecule.

Applications:

• Lewis Structures: Used to draw and evaluate Lewis structures by ensuring that the formal charges on atoms are minimized, which often corresponds to the most stable structure.
• Resonance Structures: Helps in identifying and comparing different resonance structures by evaluating their formal charges.
• Predicting Reactivity: Can provide insights into the reactivity of molecules by indicating regions of high or low electron density.

Summary of Differences

1. Purpose:
   • Oxidation State: Indicates the degree of oxidation or reduction of an atom in a compound.
   • Formal Charge: Indicates the charge distribution within a molecule, assuming equal sharing of bonding electrons.
2. Calculation Basis:
   • Oxidation State: Based on the hypothetical ionic character of bonds.
   • Formal Charge: Based on the equal sharing of bonding electrons.
3. Applications:
   • Oxidation State: Used in naming compounds, balancing redox reactions, and predicting reactivity.
   • Formal Charge: Used in drawing and evaluating Lewis structures, identifying resonance structures, and predicting reactivity within molecules.