What determines the strength of reducing and oxidizing agents?

The best reducing agents are located at the bottom left of the periodic table (low electronegativity) and the best oxidizing agents are located at the top right of the periodic table (high electronegativity), excluding noble gases. Is electronegativity the factor that determines the strength of reducing agents and oxidizing agents?

$\begingroup$ The strength of reducers and oxidizers depends upon the thermodynamic favorability of their reactions. The strongest elemental reducing agent is lithium, which is not the least electronegative element. When Li acts as a reducing agent, metallic bonds are broken and one electron is removed from each Li atom. These processes are endothermic. The Li is solvated (exothermic). For an oxidizer like chlorine, the Cl-Cl bond is broken (endothermic) then the Cl gains an electron (exothermic). The chloride is solvated (exothermic). All this adds up to how favorable the reduction is. $\endgroup$

$\begingroup$ There are ionic oxidizing agents that are not at the top right of the periodic table that are good oxidizing agents. For example, silver and gold ions. $\endgroup$

$\begingroup$ It's all about electron energy levels. Populated orbitals with high energy mean the substance tends to be a reducing agent, while unpopulated orbitals with low energy mean the substance tends to be oxidising. $\endgroup$

1 Answer 1

Because compounds can be oxidizing agents like potassium permangenate (KMnO₄) and reducing agents LiH₄, what makes a compound an oxidizing or reducing agent is oxidation & reduction tables. As oxidation is the giving off of electrons and reduction is the acceptance of electrons, like plus vs minus relationship, if you have an oxidation table you can turn it into a reduction table by putting the flipping the table, changing the signs and reversing the equations. Anyway, reduction tables are more standard, where the strongest oxidizing agents have the most positive/largest standard reduction potentials and the strongest reducing agents have the most negative/smallest standard reduction potentials.

The standard reduction potentials are determined with a volt-meter connecting two cells together as electrons pass through the salt-bridge.

But there are some patterns such that

Cesium reacts more violently (in an explosive reaction which occurs as hydrogen gas is ignited from the heat of the strongly exothermic reaction) than all the metals above it because it is a stronger reducing agent reducing agent being oxidized itself more than the metals above it because it has the the lowest ionization energy due to shielding as the many shells of electrons around the nucleus of Cesium reduce the pull of the positive charged nucleus on the electrons due to the electrons in the electron shells repelling the valence electrons further from the nucleus by like charge repulsion. Ionization energy is simply the measurement of the heat energy required to cause an atom to lose an electron in the gas phase.

This makes sense, but there are anomalies to consider not going by the electrochemical series from experiments and just trying to rationalize off of stuff like ionization energies, electronegativities, etc that I started off my answer with. The reason that you might find anomalies is because the reduction potentials in voltages of elements are calculated in aqueous solution, while ionization energies are calculated in the gas phase, though many oxidation-reduction reactions occur in liquid solutions, one potential difference being the failure to consider enthalpy of solvation when considering ionization energies. Furthermore, Pauling's electronegativites can be calculated from running some equations through elements considering the physics of ionization energies, so ionization energies and electronegativites give you a similar perspective.