Oxygen has an oxidation number of -2 in _. O2 NO2 Na2O2 MgO2
The Correct Answer and Explanation is:
Correct Answer: NO₂
Explanation:
Oxidation numbers are used to keep track of the transfer of electrons in chemical compounds. In most compounds, oxygen typically has an oxidation number of -2. However, there are exceptions based on the type of compound and the elements involved.
Let’s analyze each compound:
- O₂ (Oxygen gas):
In its elemental form, oxygen has an oxidation number of 0. This is true for all elements in their natural state. Therefore, oxygen is not -2 here. - NO₂ (Nitrogen dioxide):
This compound consists of nitrogen and oxygen. We assume oxygen is -2 unless it is in a peroxide or elemental form.
Let the oxidation number of nitrogen be x.
There are two oxygen atoms, each contributing -2, so:
x + 2(-2) = 0 ⇒ x – 4 = 0 ⇒ x = +4
This confirms oxygen is -2 in NO₂. - Na₂O₂ (Sodium peroxide):
This is a peroxide compound. In peroxides, oxygen has an oxidation number of -1.
Sodium (Na) is an alkali metal, so it always has an oxidation number of +1.
The total oxidation must be zero:
2(+1) + 2(x) = 0 ⇒ 2 + 2x = 0 ⇒ x = -1
So oxygen is -1, not -2 here. - MgO₂ (Magnesium peroxide):
Like sodium peroxide, this is also a peroxide, so oxygen has an oxidation number of -1 here as well.
Magnesium (Mg) has an oxidation number of +2, and the two oxygen atoms together must balance this with 2(-1) = -2.
Conclusion:
The only compound among the choices where oxygen has an oxidation number of -2 is NO₂. In elemental oxygen (O₂) the number is 0, and in both peroxides (Na₂O₂ and MgO₂) the number is -1.
