When the oxidation number of an element decreases in a chemical reaction, the process is known as reduction. This term originates from the concept of "reducing" the amount of positive charge on an ion, or "reducing" the oxidation state of an element.
In the context of redox (reduction-oxidation) reactions, the element that undergoes reduction is said to be reduced and acts as an oxidizing agent because it gains electrons and causes the oxidation of another species. Conversely, the element that loses electrons and increases its oxidation number is said to be oxidized and acts as a reducing agent.
For example, in the reaction of sodium chloride formation, the sodium atom is oxidized, and the chlorine atom is reduced:
\[ \text{Na (s)} \rightarrow \text{Na}^+ \text{(aq)} + e^- \] (Oxidation: Na goes from 0 to +1)
\[ \text{Cl}_2 \text{(g)} + 2e^- \rightarrow 2\text{Cl}^- \text{(aq)} \] (Reduction: Cl goes from 0 to -1)
Overall reaction:
\[ 2\text{Na (s)} + \text{Cl}_2 \text{(g)} \rightarrow 2\text{NaCl (aq)} \]
In this reaction, sodium (Na) is oxidized, and chlorine (Cl_2) is reduced.
Occurs when the oxidation number of an element decreases in chemical reaction
Oxidation
Reduction
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