Oxidation and reduction in terms of electrons
One particle loses electrons. The other gains them. Both happen at once.
OIL
Oxidation Is Loss
Loses electrons. Charge goes up.
Mg → Mg²⁺ + 2e⁻
RIG
Reduction Is Gain
Gains electrons. Charge goes down.
Cu²⁺ + 2e⁻ → Cu
The electrons one particle loses are the same electrons the other gains.