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Wize University Chemistry Textbook > Chemical Reactions

Oxidation States

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Oxidation States

Oxidation state: A way to describe the degree of oxidation of a chemical

Wize Tip
Tips to Determine Oxidation State:

1. Free elements: oxidation state = 0
Examples: Cl2(g) has an oxidation state of 0, Na(s) has an oxidation state of 0

2. Monoatomic ions: oxidation state = ionic charge
Examples: Cl-(aq) has an oxidation state of -1, Na+(aq) has an oxidation state of +1.

3. Hydrogen: oxidation state = +1
a. Exception: hydrogen in hydrides (e.g., NaH) – oxidation state = -1

4. Oxygen: oxidation state = -2
a. Exception: oxygen in peroxides (e.g., H2O2) – oxidation state = -1

5. For neutral molecules, oxidation numbers add up to zero.
For polyatomic ions, oxidation numbers add up to the ion charge.
Examples: Cl2(g) has an oxidation state of 0, NaCl oxidation numbers add up to 0, SO42- oxidation numbers add up to 2-.



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Oxidation States - Additional Information

Other Low Yield Tips to Help Determine Oxidation States

  • Alkali metals: oxidation state= +1
  • Alkaline earth metals: oxidation state= +2
  • Nitrogen Group: oxidation state= -3
  • Oxygen family: oxidation state= -2
  • Halogens: oxidation state= -1

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Note on Transition Metals

Transition metals can have different charges (ex. Pb2+ and Pb4+)
As a result, transition metals can also have different oxidation states!

Example:
What is the oxidation state of Cu in CuCl?
+1
What is the oxidation state of copper in Cu(NO3)2?
2+



When naming compounds involving transition metals, the oxidation state of the transition metal is written in Roman numerals and put in brackets:
  • copper(I) chloride: CuCl
  • copper(II) nitrate: Cu(NO3)2
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Example: Oxidation State Practice

What is the oxidation state of:

S in S2O3:
+3

O in S2O3:
-2



What about Pb in Pb2O3?

This one is different because this compound is a mixed oxidation state compound, so although Pb should have 2+ or 4+, here, we get an oxidation state of
3+
and that is ok because both Pb2+ and Pb4+ are present in the compound.



What is the oxidation state of Br in BrO3- ?
What is the oxidation state for the following: (enter answer as -1 or +2 for example)

a) P in P2O5:

b) Cr in Na2Cr2O7:

c) Al in Al2(SO4)3:

d) Na in Na2O2:

e) O in Na2O2:

f) N in NH3:
Extra Practice
Practice: Lewis Structure: Formal Charge
Practice: Lewis Structure: Formal Charge
Calculate the formal charge of B in BH4⊝BH_4^{\circleddash }
Resonance Structure, Oxidation State
Choose the correct statement:
Electrochemistry: Oxidation state
What is the oxidation state of Br in BrO3- ?
Lewis Structure: Formal Charge
Calculate the formal charge of B in BH4⊝BH_4^{\circleddash }
Lewis dot structure
Consider the following structure and choose the correct statement:
Oxidation state
In which compound does carbon have an average oxidation state of −2?
Ionic Bonds
For the following pairs of atoms, determine the ion that will form for each and the resulting chemical formula of the ionic compound.
a) K, O
b) Ca, Cl
Electrochemistry: Oxidation state
Determine the oxidation number of each element in each of the following compounds:
a. H2CO3
b. N2
Electrochemistry: Oxidation states
Give the unit formula of beryllium chloride.
Give the unit formula for aluminum nitrite.
Oxidation State Practice Questions
1) Determine the oxidation state for each atom in the following complexes.
a) Mg3P2
b) Na2(SO4)
Electrochemistry: Oxidation states
When potassium metal reacts with chlorine the product(s) is(are):
Electrochemistry: Oxidation States
When magnessium reacts with oxygen the product(s) is(are):
Electrochemistry: Oxidation States
The SO32- ion can be oxidized to the SO42- ion. What is the formal charge on the sulfur atoms for SO32- and SO42- in there most stable resonance structure?
Electrochemistry: Oxidation State
What is the oxidation state for the following: (enter answer as -1 or +2 for example)
P in P2O5:
+5
Cr in Na2Cr2O7:
+6
Electrochemistry: Oxidation state
What is the oxidation state of Br in BrO3- ?