Animated Solution for Chemistry - Redox Reactions: Oxidation number of potassium in $K_{2}O$, $K_2{O}_2$ and $K{O}_2$, respectively, is

When you first look at the compounds $K_{2}O$, $K_2{O}_2$, and $K{O}_2$, it is incredibly tempting to assume that because the ratio of potassium to oxygen is changing, the oxidation state of potassium must be changing too. This is a classic trap set by examiners.

Many students instinctively assign oxygen a fixed oxidation state of $-2$ and try to calculate the oxidation state of potassium from there. If you do that for $K{O}_2$, you would get an oxidation state of $+4$ for potassium, which is chemically absurd! To solve this problem correctly, we must rely on a fundamental rule of the periodic table.

Potassium ($K$) is an alkali metal, proudly sitting in Group 1 of the periodic table. The defining characteristic of alkali metals is their electron configuration, which ends in $n{s}^1$. They have exactly one valence electron.

Because these metals are highly electropositive, they are desperate to lose that single outer electron to achieve a stable, noble gas electron configuration. Once they lose that electron, they form a $+1$ cation. Removing a second electron would require breaking into a highly stable, full inner shell, which requires an immense amount of ionization energy. Therefore, in all of their compounds, alkali metals exhibit a fixed oxidation state of $+1$. There are absolutely no exceptions to this rule in standard chemistry.

Let's apply our golden rule to the three compounds given in the question:

1. Potassium Oxide ($K_{2}O$) This is the standard oxide. It dissociates into two potassium ions ($2K^{+}$) and one oxide ion ($O^{2-}$). The oxidation state of potassium is $+1$.

2. Potassium Peroxide ($K_2{O}_2$) Here, the oxygen exists as a peroxide linkage. The compound dissociates into two potassium ions ($2K^{+}$) and one peroxide ion ($O_2^{2-}$). Even though the oxygen species has changed, the potassium remains steadfastly at $+1$.

3. Potassium Superoxide ($K{O}_2$) This is a fascinating and highly reactive compound. It dissociates into one potassium ion ($K^{+}$) and one superoxide ion ($O_2^{-}$). Once again, the oxidation state of potassium is $+1$.

In all three compounds, the oxidation state of potassium is $+1$. It is the oxygen that is changing its oxidation state to accommodate the alkali metal, not the other way around. In $K_{2}O$, oxygen is $-2$. In $K_2{O}_2$, oxygen is $-1$. And in $K{O}_2$, oxygen has a fractional oxidation state of $-1/2$.

Thus, the correct sequence of oxidation states for potassium is $+1,+1$, and $+1$.