Oxidation states of oxygen

Question

I'm given a question:

Oxidation number of $\ce{O}$ in $\ce{BaO2}$ is $x$ and in $\ce{OF2}$ is $y$; then the value of $x+y$ is what?

Now my main question is that if $\ce{F}$ has $-1$ valency in $\ce{OF2}$ then $\ce{O}$ must have valency of $+2$. But is that possible since $\ce{O}$ mainly has oxidation states of $-1$ & $-2$ or am I making any mistake?

Answer 1

The concept of oxidation state simply works on concept of electronegativity.The more electronegative atom acquire a negative charge while less electronegative atom acquire a positive charge. Depending on this hypothesis oxygen have 5 oxidation states.

1. In all the oxides,oxygen has an oxidation state of $-2$. Eg. $\ce{CO2,CO}$

2. In all peroxides (oxygen-oxygen linkage), oxygen has an oxidation state of $-1$. For example, consider $\ce{H2O2}$, here $\ce{H}$ is less electronegative so it will acquire a charge of $+1$ and to balance the $2$ positive charge of 2 H-atoms,each oxygen atom will acquire a charge of $-1$.

3. In all superoxides ($\ce{KO2,CsO2,RbO2}$), oxygen has an oxidation state of $-\frac{1}{2}$,this is because $\ce{K,Cs,Rb}$, being elements of the first group and less electronegative than oxygen acquire a charge of $+1$, to balance it, each oxygen atom acquires a charge of $-\frac{1}{2}$.

4. In one of the exceptions $\ce{OF2}$, the fluorine being more electronegative acquires a charge of $-1$ and to balance the $-2$ charge of 2 fluorine atoms oxygen acquires a charge of $+2$.

5. As last, there is $\ce{O2F2}$, similarly here to balance the $-2$ charge on 2 $\ce{F}$-atoms each oxygen atom acquire a charge of $+1$.

Answer 2

There can also be a $0$ oxidation state in compounds, not just elemental oxygen. Hypofluorous acid is known. This molecule with the structural formula $\ce{H - O - F}$ has (in the oxidation state formalism) oxygen gaining an electron from hydrogen but losing one to fluorine.

Answer 3

Oxygen can have 6 different oxidation states. Which oxidation state oxygen is in depends on which element oxygen is bonded to and what ratio the two elements are at:

• -2: This occurs in oxides e.g. $\ce{OsO4}$ and $\ce{RuO4}$
• -1: This occurs in peroxides e.g. $\ce{H2O2}$
• -0.5: This occurs in superoxides e.g. compounds that contain the $\ce{O^-2}$ ion such as $\ce{KO2}$
• 0: This occurs in $\ce{O2}$
• +1: This occurs in $\ce{O2F2}$ as $2 \cdot -1+2 \cdot 1=0$ and fluorine has a greater electronegativity than oxygen.
• +2: This occurs in $\ce{OF2}$ as $+2+2 \cdot -1=0$

Answer 4

Oxygen can have a range of oxidation states including -2,-1,0,+1,+2

So, yes oxygen can have positive oxidation states when present with more electronegative elements than it in a compound.