In which of the given compounds is the oxidation number of oxygen +2?
20172021
In which of the given compounds is the oxidation number of oxygen +2?
- A.
NaO2
- B.
Na2O
- C.
NaO3
- D.
OF2
Attempted by 2 students.
Show answer & explanation
Correct answer: D
In a neutral compound, the oxidation numbers of all atoms add up to zero. To find an unknown oxidation number, first fix the oxidation numbers of the atoms whose values are always fixed — alkali metals such as sodium are always +1, and fluorine, being the most electronegative element, is always -1 in every compound — then solve for the remaining atom.
Compound | Known oxidation numbers | Equation (sum = 0) | Oxygen's oxidation number |
|---|---|---|---|
NaO2 | Na = +1 | (+1) + 2x = 0 | x = -1/2 (superoxide) |
Na2O | Na = +1 (×2) | 2(+1) + x = 0 | x = -2 (normal oxide) |
NaO3 | Na = +1 | (+1) + 3x = 0 | x = -1/3 (ozonide) |
OF2 | F = -1 (×2) | x + 2(-1) = 0 | x = +2 |
Fluorine is more electronegative than oxygen, so in OF2 it is fluorine — not oxygen — that keeps its usual -1 oxidation number; oxygen is forced to the unusual positive value of +2 to keep the molecule neutral. This is the standard example used to show that oxygen can have a positive oxidation number, unlike in ordinary oxides, superoxides, or ozonides, where it stays negative.
Hence, the oxidation number of oxygen is +2 only in OF2.