This is because to reduce something you add electrons, thus reducing its overall charge
Stephan reduction is the reduction of -NO2 to -NH2 via 1) SnCl2 2) EtOH
Reduction, according to Lewis description of acids and bases is a donation of electrons to an atom/molecule, whereas oxidation is while taking electrons. In organic chemistry, reduction is...
In Chem, reduction is the gain of electrons and I think it's the same in Bio.
Because it gains electrons ,therefore its valency decreases : it becomes Negative