Name _____________________________
Date
_____________ Section _______
Oxidation Number Worksheet 1-Write the rule # next to your answer
1. What is the oxidation number of sulfur in each of the following?
a) S2-
e) S8
b) H2SO4
f) Na2S2O6
c) S2O32-
g) S2Cl2
d) CS2
2. In H2AsO4-1, what is the oxidation number of:
a) Hydrogen
c) Arsenic
b) Oxygen
3. State the oxidation number of the underlined element in each of the following:
a) P2O74-
d) VO2+
b) MnO4-
e) UO22+
c) MnO42-
f) ClO3-
4. Calculate the oxidation number of the underlined element in the following:
a) SO32-
c) O3
b) Na2Cr2O7
5. Determine the oxidation number of the underlined element in:
a) K4P2O7
d) Ba2XeO6
b) NaAuCl4
e) Ca(ClO2)2
c) ICl
Rules for Assigning Oxidation Numbers to Elements
1. In free elements, (not combined with other elements) each atom has an oxidation
number of ZERO (0).
Example: Each oxygen atom in O2 has an oxidation number of ZERO.
2. For ions composed of only one atom, the oxidation number is equal to the charge
(+/-) on the ion.
Example The oxidation number of Ca+2 is +2
The oxidation number for K+ is +1
3a. All alkali metals (group 1 excluding Hydrogen) have an oxidation number of +1.
Example The oxidation number of Lithium (Li) is +1
3b. All alkaline earth metals (group 2) have an oxidation number of +2.
Example: The oxidation number of Magnesium (Mg) is +2
3c. The oxidation number of Aluminum is always +3
3d. The oxidation number for Fluorine is always -1
3e. The oxidation number of hydrogen is +1, except when bonded to a metal as a
negative ion, in which it is -1.
Example: In H2O, hydrogen has an oxidation number of +1
Example: In NaH, hydrogen has an oxidation number of -1
3f. The oxidation number of oxygen in most compounds is -2.
In peroxides (O22-) it is -1
Example: In Na2O, oxygen has an oxidation number of -2
Example: In H2O2, oxygen has an oxidation number of -1
3g. When halogens (group 17) form negative ions, they will have oxidation numbers
of -1. (F, Cl, Br, I)
Example: In NaCl and CaCl2, chlorine has an oxidation number of -1
4. The oxidation numbers of elements in compounds are written PER ATOM
5. IN a NEUTRAL MOLECULE, The sum of all the oxidation numbers must equal
ZERO.
Example: In H2O, each hydrogen is +1 and oxygen is -2.
So, 2(+1) + (-2) = 0
6. IN A POLYATOMIC ION, The sum of the oxidation numbers for ALL of the
elements in the ion must equal the net charge on the ion.
In the polyatomic ion known as hydroxide (OH-), the oxygen is -2 and the hydrogen is +1
So, (-2) + (+1) = -1, the same as the charge on the hydroxide ion (OH-)
1. The oxidation number of an atom in an
uncombined element is zero.
2. The oxidation number of any
monoatomic ion equals its ionic charge
3a. Elements in Group 1 are always +1
3b. Elements in Group 2 are always +2
3c. Aluminum is always +3
3d. Fluorine is always -1
3e. Hydrogen is +1 when combined with
nonmetals
3f. Oxygen is -2 in most compounds and
ions
3g. The halogen group (group 17) have an
oxidation number of -1
4. Oxidation numbers of elements in a
compound are written per atom
5. The sum of the individual oxidation
numbers of all the atoms in a compound
is zero.
6. The sum of the individial oxidation
numbers of all the atoms in a polyatomic
ion is equal to the charge of the ion