Definition

Noun: - A numerical value assigned to an atom, ion, or molecule that represents its degree of oxidation or reduction in a chemical compound. It is a theoretical charge, indicating the number of electrons an atom has gained, lost, or appears to use when forming chemical bonds. For simple monatomic ions, the oxidation number is equal to the ionic charge.

Usage

The oxidation number is a fundamental concept used to: - Track electron transfer in redox reactions. - Balance chemical equations for redox reactions. - Name chemical compounds according to IUPAC nomenclature rules. - Understand the electronic state of an element within a compound.

Examples

• In water (H₂O), the oxidation number of hydrogen is +1 and of oxygen is -2.
• The oxidation number of a pure element, like O₂ or Fe, is always zero.
• To balance the redox equation, we first calculated the change in the oxidation number for manganese.
• The compound's name, iron(III) chloride, indicates that iron has an oxidation number of +3.

Advanced Usage

• "Assigning oxidation numbers": The process of determining the oxidation number for atoms in a compound based on a set of rules (e.g., the oxidation number of fluorine is always -1).
• "Change in oxidation number": An increase indicates oxidation (loss of electrons), while a decrease indicates reduction (gain of electrons). This change is central to identifying redox processes.
• "Fractional oxidation number": Some atoms in complex molecules, like in superoxides or certain metal clusters, can have oxidation numbers that are not whole numbers (e.g., the average oxidation state of iron in Fe₃O₄ is +8/3).

Variants and Related Words

• Oxidation state: A term often used interchangeably with oxidation number, though sometimes distinguished in advanced chemistry as a more precise description of electron density.
• Redox (adj): Pertaining to reactions involving oxidation and reduction.
• Oxidizing agent: A substance that causes oxidation by accepting electrons, itself being reduced (its oxidation number decreases).
• Reducing agent: A substance that causes reduction by donating electrons, itself being oxidized (its oxidation number increases).

Synonyms

• Oxidation state
• Electrovalence (in specific, ionic contexts)

Related Phrases and Concepts

• "Determine the oxidation number": To calculate or find the oxidation number of an element.
  • The first step is to determine the oxidation number of sulfur in this polyatomic ion.
• "Undergo a change in oxidation number": To be oxidized or reduced.
  • In this reaction, chlorine undergoes a change in oxidation number from 0 to -1.
• "Rule for oxidation numbers": One of the standard guidelines used for assignment (e.g., Group 1 metals are always +1).

Key Rules (Conceptual Summary)

While not idioms, the core rules for assigning oxidation numbers are essential: 1. The oxidation number of an atom in its elemental form is 0. 2. For monatomic ions, the oxidation number equals the ion charge. 3. Oxygen usually has an oxidation number of -2 (except in peroxides, where it is -1). 4. Hydrogen usually has an oxidation number of +1 (except in metal hydrides, where it is -1). 5. The sum of oxidation numbers in a neutral compound is zero; in a polyatomic ion, it equals the ion's charge.

Noun

1. the degree of oxidation of an atom or ion or molecule; for simple atoms or ions the oxidation number is equal to the ionic charge
   • the oxidation number of hydrogen is +1 and of oxygen is -2