Which of the following statements about Na$$_2$$O$$_2$$ is not correct?

We are given the question: Which of the following statements about Na$$_2$$O$$_2$$ is not correct? We will evaluate each option step by step.

First, recall that Na$$_2$$O$$_2$$ is sodium peroxide, which contains the peroxide ion O$$_2^{2-}$$.

Option A: "It is diamagnetic in nature." The peroxide ion O$$_2^{2-}$$ has a bond order of 1 and all electrons are paired. Therefore, Na$$_2$$O$$_2$$ is diamagnetic. This statement is correct.

Option B: "It is derivative of H$$_2$$O$$_2$$." Sodium peroxide can be prepared from hydrogen peroxide (H$$_2$$O$$_2$$) by reaction with sodium hydroxide: $$2NaOH + H_2O_2 \rightarrow Na_2O_2 + 2H_2O$$. Additionally, Na$$_2$$O$$_2$$ reacts with water to produce H$$_2$$O$$_2$$: $$Na_2O_2 + 2H_2O \rightarrow 2NaOH + H_2O_2$$. Thus, it is indeed a derivative of H$$_2$$O$$_2$$. This statement is correct.

Option C: "Na$$_2$$O$$_2$$ oxidises Cr$$^{3+}$$ to CrO$$_4^{2-}$$ in acid medium." In acidic medium, Na$$_2$$O$$_2$$ reacts to form hydrogen peroxide: $$Na_2O_2 + 2H^+ \rightarrow 2Na^+ + H_2O_2$$. Hydrogen peroxide then oxidizes Cr$$^{3+}$$ to chromate (CrO$$_4^{2-}$$). The balanced reaction is: $$2Cr^{3+} + 3H_2O_2 + 10OH^- \rightarrow 2CrO_4^{2-} + 8H_2O$$. However, in acid medium, OH$$^-$$ is not present. But sodium peroxide provides an alkaline medium when dissolved, and the oxidation can occur after acidification is compensated. Typically, Cr$$^{3+}$$ is oxidized to CrO$$_4^{2-}$$ in alkaline conditions using peroxides. Despite the medium, it is known that sodium peroxide can oxidize Cr$$^{3+}$$ to CrO$$_4^{2-}$$. This statement is correct.

Option D: "It is the super oxide of sodium." Sodium superoxide is NaO$$_2$$, which contains the superoxide ion O$$_2^-$$. Sodium peroxide, Na$$_2$$O$$_2$$, contains the peroxide ion O$$_2^{2-}$$, not the superoxide ion. Therefore, Na$$_2$$O$$_2$$ is not the superoxide of sodium. This statement is not correct.

Hence, the correct answer is Option D.