What are Oxidation States?
Oxidation states (OS) represent the number of electrons lost or gained by an atom in a compound. They help track electron transfer in redox reactions and predict which elements will oxidize or reduce.
Oxidation states are numbers assigned to atoms showing electrons lost (positive OS) or gained (negative OS) versus the neutral atom. Key rules: elements in elemental form are 0; oxygen is usually −2; hydrogen is usually +1; alkali metals are +1.
- •Cl₂ (elemental): 0
- •Cl⁻ (chloride): −1
- •ClO⁻: +1
- •ClO₃⁻: +5
- •ClO₄⁻: +7
- •N₂ (elemental): 0
- •NH₃: −3
- •NO₂: +4
- •NO₃⁻: +5
Step-by-step worked examples
What is the oxidation state of Cr in K₂Cr₂O₇?
Let Cr OS = x. Potassium +1, oxygen −2. (2)(+1) + (2)(x) + (7)(−2) = 0 2 + 2x − 14 = 0 2x = 12 → x = +6
Find the OS of S in SO₄²⁻.
Let S OS = x. Oxygen −2, charge −2. (x) + (4)(−2) = −2 x − 8 = −2 x = +6
What is the OS of N in NO₃⁻?
Let N OS = x. Oxygen −2, charge −1. (x) + (3)(−2) = −1 x − 6 = −1 x = +5
Flashcards
Quick quiz
Q1.In H₂O, what is the OS of oxygen?
Q2.What is the OS of Mn in KMnO₄?
Q3.In Cl₂, each Cl atom has OS =
Q4.Which statement is true?
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Common mistakes
Oxidation states and ion charges are the same. — Correct: Oxidation states are assigned numbers; ion charges are actual net charges.
Hydrogen always has OS +1. — Correct: Hydrogen is +1 in most compounds but −1 in metal hydrides (e.g. NaH).
Oxidation states must be whole numbers. — Correct: Oxidation states are assigned integers to individual atoms, not fractional values (average OS can appear fractional in polyatomic ions).
Oxygen always has OS −2. — Correct: Oxygen is −2 usually, but −1 in peroxides (H₂O₂) and +2 in OF₂.
FAQ
What is an oxidation state definition?
A number assigned to an element in a chemical compound representing the number of electrons lost or gained by an atom.
How do I assign oxidation states?
Use the rules: elements = 0, O = −2 (usually), H = +1 (usually), alkali metals = +1. Verify the sum equals zero (or the ion charge).
What are oxidation state examples?
In H₂SO₄: H is +1, S is +6, O is −2. In Cr₂O₇²⁻: Cr is +6, O is −2.
Why do oxidation states matter?
They show which atoms lose or gain electrons in reactions, helping identify redox processes and predict reaction products.