Which of the following has the highest electrode potential?
A. Be
B. Mg
C. Ca
D. Ba

Answer
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Hint: The elements beryllium, magnesium, calcium and barium belong to group 2 of the periodic table. These are called alkaline Earth metals. Electrode potential is defined as the tendency of a chemical species to gain or lose electrons.

Complete Step by Step Solution:
The general outer electronic configuration of alkaline earth metals is
\[{\rm{n}}{{\rm{s}}^{\rm{2}}}\].

These elements have two electrons in the S orbital of the valency cell. These elements lose two electrons to undergo oxidation.
\[{\rm{M}} \to {{\rm{M}}^{{\rm{2 + }}}}{\rm{ + 2}}{{\rm{e}}^{\rm{ - }}}\] where M = alkaline earth metal

These metals are strong reducing agents. Reducing agents are the chemical species that reduce other chemical species and undergo oxidation themselves. The oxidation potential is defined as the measure of the tendency of an element to lose electrons. Oxidation potential increases on moving from top to bottom in a group. This is because on moving down the group atomic size increases. Electrons are added to higher energy levels. Valence electrons are not closely held by the nucleus. The loss of electrons is easier as we move down the group.

Out of the given options, beryllium has the smallest atomic size. The loss of electrons or oxidation is difficult. Beryllium has the least oxidation potential.
We know that \[{\rm{oxidation potential = }}\left( {{\rm{ - reduction potential}}} \right)\].
As Be has the least oxidation potential, it has the highest reduction potential.
So, option A is correct.

Note: As the atomic size increases down the group, the electropositive character which is the tendency to lose electrons increases on moving from Be to Ba. The oxidation potential is defined as the measure of the tendency of an element to get oxidised i.e., to lose electrons. The reduction potential is defined as the measure of the tendency of an element to get reduced i.e., to lose electrons.