In FeCl3, the oxidation state of chlorine is -1. Here's the explanation: The oxidation state of iron Fe in FeCl3 is +3, as iron has three electrons removed to form the Fe ion. There are three chlorine Cl atoms, each with an oxidation state of -1, as they each gain one electron to form the Cl ion. The sum of the oxidation states in a neutral compound must be zero, so in this case, the oxidation state of Fe +3 plus the combined oxidation states of the three Cl atoms -3 equals zero: +3 + 3 -1 = 0