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A species reduced is an atom or molecule that gains electrons in a redox reaction. In simple words, its oxidation state decreases as it accepts electrons. For example, in the reaction between zinc and copper sulfate, copper (Cu^2+) is the species reduced as it gains two electrons to become copper metal (Cu). A reducing agent is the substance that donates or loses electrons to another substance, thereby causing the reduction of the receiving species. In the same reaction mentioned above (between zinc and copper sulfate), zinc (Zn) is the reducing agent because it gives up its electrons to reduce the copper ions (Cu^2+). In summary, the species reduced undergoes a decrease in oxidation state by gaining electrons, while the reducing agent causes this change by sacrificing its electrons.

A species oxidized is an atom or molecule that loses electrons in a redox reaction. In other words, its oxidation state increases as it gives away electrons. Using the same example of the reaction between zinc and copper sulfate, zinc (Zn) is the species oxidized when it loses two electrons to become zinc ions (Zn^2+). An oxidizing agent is the substance that accepts or gains electrons from another substance, thereby causing the oxidation of the donating species. In our example, copper sulfate (CuSO_4) is the oxidizing agent because it gains electrons from zinc (Zn), leading to the formation of Cu and Zn^2+. In short, the species oxidized undergoes an increase in oxidation state by losing electrons, while the oxidizing agent causes this change by accepting the electrons.

The oxidation state (or oxidation number) denotes the number of electrons that an atom loses, gains, or apparently utilizes when forming a chemical bond with another atom. It is a theoretical concept that helps in the understanding of redox reactions. The oxidation state is represented by integers ranging from negative to positive values. For example, in H_2O, the oxidation state of hydrogen is +1, and that of oxygen is -2. The actual charge, on the other hand, is the net electric charge on a species in a compound or free state. It is the physical charge that results from the presence of more or fewer electrons than protons in an ion or unstable species. For example, in the same compound, H_2O, the actual charge on hydrogen atoms is zero, and the actual charge on the oxygen atom is also zero. The key difference between oxidation state and the actual charge is that the oxidation state is a conceptual value that helps us understand the behavior of atoms in a redox reaction, while the actual charge represents the real electric charge present on a particular species.