Oxidation Number — Core Principles

Core Principles

The oxidation number, or oxidation state, is a hypothetical charge assigned to an atom in a compound or ion, assuming all bonds are ionic and electrons are fully transferred to the more electronegative atom.

It's a bookkeeping tool to track electron distribution and transfer in chemical reactions. Key rules include: elements have an ON of 0; monatomic ions have ON equal to their charge; Group 1 metals are +1, Group 2 are +2; fluorine is always -1; hydrogen is usually +1 (except -1 in metal hydrides); oxygen is usually -2 (except -1 in peroxides, -1/2 in superoxides, +2 in $OF_2$).

The sum of oxidation numbers in a neutral compound is zero, and in a polyatomic ion, it equals the ion's charge. An increase in ON signifies oxidation (electron loss), while a decrease signifies reduction (electron gain).

This concept is fundamental for identifying redox reactions, determining oxidizing/reducing agents, and balancing complex chemical equations.

Important Differences

vs Valency

Aspect	This Topic	Valency
Definition	Oxidation Number (Oxidation State): Hypothetical charge assigned to an atom if all bonds were 100% ionic, with electrons assigned to the more electronegative atom.	Valency: The combining capacity of an element, representing the number of bonds an atom can form.
Sign	Can be positive (+), negative (-), zero (0), or even fractional.	Always a positive integer (e.g., 1, 2, 3, 4).
Nature	A bookkeeping tool for electron transfer; indicates the degree of oxidation/reduction.	A measure of bonding capacity; indicates how many other atoms an atom can combine with.
Context	Primarily used in redox reactions to track electron movement and balance equations.	Used to determine chemical formulas and predict bond formation.
Example (Oxygen)	In $H_2O$, ON of O is -2. In $H_2O_2$, ON of O is -1. In $OF_2$, ON of O is +2.	The valency of oxygen is always 2 (it forms two bonds).
Example (Carbon)	In $CH_4$, ON of C is -4. In $CO_2$, ON of C is +4. In $C_2H_6$, ON of C is -3.	The valency of carbon is typically 4.

While both oxidation number and valency relate to an atom's combining ability, they are distinct concepts. Oxidation number is a signed, hypothetical charge reflecting electron distribution in a bond, crucial for understanding redox processes. Valency, conversely, is an unsigned integer representing an atom's bonding capacity, fundamental for writing chemical formulas. An element can have multiple oxidation numbers but usually a fixed valency (or a few common valencies). Understanding this distinction is vital for accurate chemical analysis and problem-solving in NEET.