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Oxidation and reduction reactions are complementary processes that involve the transfer of electrons between chemical species. In a redox (reduction-oxidation) reaction, one substance loses electrons while another gains electrons. - Oxidation is the process in which an element loses one or more electrons. The species that loses electrons is said to be oxidized. - Reduction is the process in which an element gains one or more electrons. The species that gains electrons is said to be reduced. Example: In the reaction between magnesium and oxygen, magnesium loses two electrons to form magnesium ions while oxygen gains two electrons to form oxide ions. \[ \text{Mg} \rightarrow \text{Mg}^{2+} + 2\text{e}^-\] \[ \frac{1}{2}\text{O}_2 + 2\text{e}^- \rightarrow \text{O}^{2-}\] In this reaction, magnesium is oxidized (loses electrons) and oxygen is reduced (gains electrons).

Oxidation numbers (or oxidation states) are assigned to elements or ions to help keep track of the number of electrons gained or lost during redox reactions. - Oxidation is the process in which the oxidation number of an element increases. - Reduction is the process in which the oxidation number of an element decreases. Example: In the reaction between hydrogen gas and chlorine gas to form hydrogen chloride: \[\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}\] Before the reaction, hydrogen has an oxidation state of 0 (in its elemental form), and chlorine also has an oxidation state of 0 (as diatomic Cl2). After the reaction, hydrogen chloride (HCl) is formed, and each hydrogen atom now has an oxidation state of +1, while the oxidation state of each chlorine atom is -1. In this reaction, the oxidation number of hydrogen increases from 0 to +1 (oxidation), while the oxidation number of chlorine decreases from 0 to -1 (reduction).