Reactivity of Metals (OCR GCSE Chemistry A (Gateway Science Suite)): Revision Notes

4.1.4 Oxidation & Reduction in Terms of Electrons

Understanding Redox Reactions

A redox reaction involves the transfer of electrons between substances, where one substance is oxidised and another is reduced:

• Oxidation: The loss of electrons from an atom or ion. When a substance is oxidised, its oxidation state increases. Originally, oxidation was defined as the addition of oxygen to a substance, but now it more accurately refers to the loss of electrons.
• Reduction: The gain of electrons by an atom or ion. When a substance is reduced, its oxidation state decreases. Reduction is named so because it reduces the positive charge of the atom or ion by adding electrons.

• Sodium (Na):
  • Before the reaction: Sodium has an oxidation state of 0.
  • After the reaction: Sodium loses one electron to form $Na+$ with an oxidation state of +1 (oxidation).
• Chlorine (Cl₂):
  • Before the reaction: Chlorine has an oxidation state of 0.
  • After the reaction: Chlorine gains one electron per atom to form $Cl−$ with an oxidation state of −1 (reduction). In this redox reaction, sodium is oxidised (loses electrons) and chlorine is reduced (gains electrons).

	Right Hand Side		Left Hand Side
Element	Na	Cl	Na	Cl
Oxidation State	0	0	+1	−1

Oxidation States and Redox Identification

The oxidation state is a number that represents the total number of electrons an atom has gained or lost. By comparing the oxidation states of the reactants and products, we can determine whether a redox reaction has occurred. If there is a change in the oxidation state of any of the elements, the reaction is a redox process.