Paul J. answered • 07/09/21
Bachelor's of Science in Forensic Chemistry
The oxidation number of an atom will tell you if it was oxidized or reduced during a redox reaction. If an atom is reduced, it will gain electrons. This number also represents the theoretical charge of an atom in a neutral molecule. If an atom is oxidized, it will lose electrons.
A simple way to remember is with the pneumonic LEO goes GER (Loses Electrons Oxidized, Gains Electrons Reduced).
The easiest way to determine whether an atom gains or loses electrons is by looking at the sign of the charge (aka the oxidation number). If an atom has a positive charge or oxidation state, that is indicative that it has lost electrons. The reason for this is because the number of protons an atom has is equal to the total number of electrons that are found in the electron cloud of that atom. When an atom loses electrons, it will have a higher ratio of protons to electrons. Since protons are positively charged and there is a higher proton:electron ratio, the atom will have a positive charge. If the charge of the atom is negative, that means it has gained electrons. In its neutral state, an atom will not have a charge. When an atom gains electrons, it will then have a higher ratio of electrons to protons and as a result, will have a more negative charge since electrons are negatively charged.
In your problem, your proposed atom initially has a charge/oxidation state of -2 and the oxidation state decreases to -5 when the reaction completes. Since your atom is becoming more negatively charged, you know that it is gaining electrons rather than losing. So in order for your atom to experience a decrease in oxidation state from -2 to -5, it would need to gain 3 electrons. Since it gains electrons, it is reduced.
Answer: D
As a general rule, if you see an increase in oxidation state, that means the atom is losing electrons and being oxidized. If you see a decrease in oxidation state, that means the atom is gaining electrons and being reduced.
