The correct answer is Option (3) → $Li^+$
An oxidizing agent is a substance that gains electrons (undergoes reduction). Its strength is measured by its standard reduction potential ($E^0$); the lower (more negative) the reduction potential, the weaker the oxidizing agent.
- Lithium (Li) has the lowest standard reduction potential $(\sim -3.04 \text{ V})$ of all elements.
- While Lithium metal is the strongest reducing agent, its conjugate ion $Li^+$ is the weakest oxidizing agent because it has the least tendency to gain an electron and return to its metallic state.
Why other options are incorrect
- 1. $F^-$: The fluoride ion is the product of the strongest oxidizing agent ($F_2$). While ($F_2$) is a powerful oxidizer, $F^-$ itself is a very weak reducing agent, not a weak oxidizing agent (it cannot gain more electrons).
- 2. $H_2$: Hydrogen gas typically acts as a reducing agent (it gets oxidized to $H^+$). In terms of reduction potentials, the $H^+ / H_2$ couple is defined as $0.00 \text{ V}$, which is significantly higher than that of Lithium.
- 4. $K^+$: Potassium has a very low reduction potential $(\sim -2.93 \text{ V})$, making it a very weak oxidizing agent. However, Lithium's potential is even lower $(\sim -3.04 \text{ V})$, making $Li^+$ the weakest among the choices provided.
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