Oxidation-reduction (redox) reactions are reactions: a) in which oxidation numbers change.
b) Oxidation numbers are either real charges or formal charges which help chemists keep track of electron transfer.
c) In practice, oxidation numbers are best viewed as a bookkeeping device.
d) Oxidation cannot occur without reduction.
e) In a redox reaction the substance which is oxidized contains atoms which increase in oxidation number.
f) Oxidation is associated with electron loss (helpful mnemonic: LEO