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The species reduced is the molecule or atom that gains electrons during a redox reaction. The reducing agent is the molecule or atom that donates electrons during a redox reaction. So the species reduced undergoes the reduction while the reducing agent causes this reduction to happen. For example, in the following reaction: \[Zn(s) + Cu^{2+}(aq) \rightarrow Zn^{2+}(aq) + Cu(s)\] Copper ion (\(Cu^{2+}\)) is the species reduced (it gains electrons to become copper solid), and zinc solid is the reducing agent (it donates electrons to copper ion).

The species oxidized is the molecule or atom that loses electrons during a redox reaction. The oxidizing agent is the molecule or atom that accepts these electrons during a redox reaction. Consequently, the species oxidized goes through oxidation, while the oxidizing agent induces this oxidation. Continuing with the previous example: \[Zn(s) + Cu^{2+}(aq) \rightarrow Zn^{2+}(aq) + Cu(s)\] Zinc solid is the species oxidized (it loses electrons to become zinc ion), and copper ion (\(Cu^{2+}\)) is the oxidizing agent (it accepts electrons to become copper solid).

Oxidation state is a hypothetical charge assigned to an atom in a molecule or ion, assuming that all bonds were ionic rather than covalent. It helps to keep track of electron transfer in redox reactions. It is just a number (positive, negative or zero) and does not necessarily represent the actual charge on the atom. Actual charge, on the other hand, is the electric charge possessed by an ion or present on an atom due to its environment (for example, in a crystal lattice or bound within a molecule). It is determined experimentally and reflects the real charge distribution on an atom or ion. For example, in the molecule \(H_2O\), the oxygen atom has an oxidation state of -2, and the hydrogen atoms have an oxidation state of +1. However, in reality, the molecule is neutral, and the actual charges on the oxygen and hydrogen atoms are not equal to their oxidation states. Oxidation states serve as a useful tool in predicting redox reactions, but they may not always reflect the actual charge distribution in a molecule or ion.