Chemistry · Periodic Classification Of Properties · NEET
It depends on who pulls electrons harder. The rule is: the more electronegative atom gets the negative oxidation state. The electronegativity order is F > O > Na. In OF₂, fluorine is stronger than oxygen, so F takes the shared electrons (F = -1 each) and oxygen is left positive (+2). In Na₂O, oxygen is stronger than sodium, so oxygen takes the electrons and becomes -2. Same oxygen atom, two different answers, because the partner is different.
Oxygen is -2 in almost every compound, so it is a good default. But it is a rule of thumb, not a law. Oxygen becomes positive only when it is bonded to fluorine, because fluorine is the single most electronegative element in the whole table. So the two exceptions to remember are: OF₂ (oxygen = +2) and O₂F₂ (oxygen = +1). Everywhere else, treat oxygen as -2. In peroxides like H₂O₂ it is -1, and in superoxides it is -1/2, but those are O-O bonds, a separate case.
Step 1: Fluorine is ALWAYS -1 (it is the most electronegative element, so it can never be positive). Step 2: The molecule OF₂ is neutral, so the sum of all oxidation states = 0. Step 3: Let oxygen = x. Then x + 2(-1) = 0, so x - 2 = 0, so x = +2. Oxygen is +2. The trick for NEET: fix fluorine at -1 first, then solve for oxygen.
Valence is how many bonds an atom forms; oxidation state is the charge it would have if the shared electrons went fully to the more electronegative atom. In OF₂ the valence of oxygen is 2 (it makes 2 bonds), and its oxidation state is +2. NCERT says the term oxidation state is now frequently used in place of valence, but they are not the same idea. Valence is never negative; oxidation state can be positive, negative, or zero.
Fluorine has the highest electronegativity of all elements (4.0 on the Pauling scale). In any bond, it always pulls the shared electrons toward itself. Because there is no element that can pull electrons away from fluorine, fluorine's oxidation state is -1 in every compound and 0 only as free F₂. This is why fluorine forces oxygen to become positive in OF₂. Remember: fluorine is the king of pulling electrons.
Yes, the logic is the same but the answer flips. In Cl₂O, oxygen (electronegativity 3.5) is more electronegative than chlorine (3.0), so oxygen takes the electrons and is -2, and each chlorine is +1. Compare: in OF₂, oxygen is beaten by fluorine and turns +2; in Cl₂O, oxygen beats chlorine and stays -2. Always compare the two electronegativities and give the negative state to the stronger one.
The correct order of N-compounds in its decreasing order of oxidation states is
Among Group 16 elements, which one does NOT show the -2 oxidation state?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
+2. Fluorine is fixed at -1, and the molecule is neutral, so oxygen must be +2: x + 2(-1) = 0 gives x = +2.
-2. Oxygen is more electronegative than sodium, so it takes the electrons. Sodium is +1 each, and 2(+1) + x = 0 gives x = -2.
No. It is -2 in normal oxides like Na₂O, MgO and CO₂, but -1 in peroxides (H₂O₂), -1/2 in superoxides, and +2 or +1 when bonded to fluorine (OF₂, O₂F₂).
Fluorine has the highest electronegativity of all elements, so it always pulls shared electrons to itself. No element can make fluorine positive, so it is -1 in every compound.
Electronegativity is a periodic property. It increases across a period and up a group, so F is the top. The oxidation state an element shows depends on its electronegativity compared to its partner, which is why the same oxygen can be +2 or -2.
