If an atom is reduced in a redox reaction. What must be happen to another atom in the system?
A redox reaction has two half-reactions; oxidation and reduction half-reactions.
Oxidation involves loss of electrons and increases in the oxidation state whereas reduction involves the gain of electrons and decrease in the oxidation state.
Both of these half-reactions take place at the same time as the species that reduces the other gets oxidized itself.
Thus, when an atom is reduced by gaining electrons, the other atom must have got oxidized by losing those electrons.