Reduction is when the atoms or ions of an element become more negative or less positive than they were before. In other words, when an atom or ion undergoes a change in oxidation state, in the negative direction it is reduced. Take a look at this reaction right here between sodium and chlorine. The Chlorine becomes a chloride ion, because it’s becoming negative here.
That’s why it has the negative symbol right here. The element chlorine has an oxidation state of zero. That’s because all elements in their natural state have oxidation numbers equal to zero. A chloride ion has an oxidation state of -1. The net change of oxidation for chlorine is in the negative direction. By definition, chlorine is reduced here.
Whenever an atom in a reaction gets reduced, some other atom gets oxidized. That’s because what determines reduction is how many electrons something has, or if the atom is gaining electrons. When an atom gains electrons, that means some other atom had to give up electrons. Electrons didn’t come out of nowhere.
Another atom had to give up electrons, becoming less negative. That’s oxidation. Oxidation is the opposite of reduction. There’s a balance here when one atom gains electrons and another one loses them. The atom that gains electrons is considered to be reduced.