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Oxidation Number Exercise

Rule 0 The following rules are in the form of a hierarchy; that is, the first stated rule takesprecedence over subsequent rules if a conflict arises.

Rule 1 The oxidation numbers for all the atoms in a neutral molecule must add up to 0.Similarly, the oxidation numbers for all the atoms of an ion must add up to thecharge of the ion. (You are expected to recognize polyions. For the commonpolyions, know their charges and their names. A summary of the commonpolyions appears at the end of this exercise. The first step is, always, to determinewhat polyions are present.)

Rule 1a The oxidation number of elements in the elemental form is 0. (Note - this rule is adirect consequence of rule 1. How so?)

Rule 1b The oxidation number of any monatomic ion is the same as its charge. (Seecomment in 1a.)

Exercises - Give the oxidation number for the following atoms:

O2 O = 0 F2 F = 0

S8 S = 0 Cl2 Cl = 0

N2 N = 0 Al Al = 0 _

Co2+ Co = +2 Mn2+ Mn = +2

Cl!

Cl =!

!!

!

1 Cr3+

Cr = +3

I3! I = !!!!1/3 Hg2

2+ Hg = +1

Rule 2 Fluorine has an oxidation number of!1.

NaF Na = +1 IF3 I = +3 ClF2! Cl = +1

S = +4 PF3 P = +3 SF62! S = +4

P = +5 PF63! P = +3

SF4

PF5

OF2 O = +2 NF3 N = +3

W2F93! W = +3

F2 F = 0

Rule 3 The metal of groups IA have an oxidation number of +1The metal of groups IIA have an oxidation number of +2Sc, Y and Al have an oxidation number of +3.

Exercises - Give the oxidation number for the following atoms:

Na2O Na = +1 Na2O2 O = !!!!1 KO2 K = -1/2

NaOH Na = +1 ScH3 H = !!!!1 LiH H = !!!!1

CaC2 C = !!!!1 CaMgO2 O = !!!!2 MgH2 H = !!!!1

MgF2 Mg = +2 RbO2 O = -1/2 MgSF6 S = +4

NaPF6 P = +5 LiBF4 B = +3

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Rule 3 The metal of groups IA have an oxidation number of +1The metal of groups IIA have an oxidation number of +2Sc, Y and Al have an oxidation number of +3.

Exercises - Give the oxidation number for the following atoms:

Na2O Na = +1 Na2O2 O = !!!!1 KO2 K = -1/2

NaOH Na = +1 ScH3 H = !!!!1 LiH H = !!!!1

CaC2 C = !!!!1 CaMgO2 O = !!!!2 MgH2 H = !!!!1

MgF2 Mg = +2 RbO2 O = -1/2 MgSF6 S = +4

NaPF6 P = +5 LiBF4 B = +3

Rule 4 Hydrogen has an oxidation number of +1 when combined with elements on the rightside of the periodic chart (non-metals) and a !1 when combined with elements on theleft side of the periodic chart (metals).

Exercises - Give the oxidation number for the following atoms:

HCl Cl = !!!!1 HF F = !!!!1 rule 2(why) NaOH O = !!!!2

HI I = !!!!1 UH3 U = +3 NH4+ N = !!!!3

PH3 P =!!!!

3 H2SF6 S = +4 NH3 N =!!!!

3

AsH3 As = !!!!3 ScH3 Sc = +3 HPF4 P = +3

H3O+ O = !!!!2 OH! O = !!!!2

Rule 5 Oxygen has an oxidation number of!2. (Note: Your knowledge of the polyions isnow needed.

Cl = +1 O = !!!!1Co(ClO)2 Co = +2

CoCrO4 Cr = +6 Co = +2 N = +5

Mg(OH)2 Mg = +2

Na2O2 Na = +1

AgNO3 Ag = +1

H2SO4 S = +6

ClO4! Cl = +7RbO2 Rb +1

KMnO4= Mn = +7

O = -2

O = !!!!

K = +1

OF2 F = !!!!1

KO2 K = +1 O = !!!!

IO2! I = +3

NH4OHN = !!!!3

IO3! I = +5

K2Cr2O7 Cr = +6

BrO2! Br = +3

Zn(NO2)2 Zn = +2

Rule 6Rule 7Rule 8

Group VIIA atoms have an oxidation number of -1.Group VIA atoms have an oxidation number of -2.Group VA atoms have an oxidation number of -3.

Exercises - Give the oxidation number for the following atoms:

PH3 P = !!!!3 CH3NH2 C = !!!!2 CN! C = +2

BF3NH3 B = +3 MnCl4 Mn = +4 W2Cl93! W = +3

Co3N2 Co = +2 NCl3 N = +3 KSCN C = +4

HCN C = +2 POCl3 P = +5 V3N4 V = +4

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NH3 N = !!!!3 As2O5 As = +5 SiF4 Si = +4

HNO3 N = +5 N2H4 N = !!!!2 PCl6! P = +5

MnO2 Mn = +4 CrCl3 Cr = +3 Cr2O72! Cr = +6

AgCH3COO Ag = +1 N2O N = +1 N2O5 N = +5

Au2O Au = +1 AuO Au = +2 CuSO4 Cu = +2

Os2O5 Os = +5 Fe3O4 Fe = +8/3 Fe2O3 Fe = +3

FeO Fe = +2 FePO4 Fe = +3 SiO2 Si = +4

H2S S = !!!!2 FeS S = !!!!2 NaHCO3 C = +4

AuHCO3 Au = +1 ScAsO4 As = +5 NH4OH N = !!!!3

SO3 S = +6 H2CrO4 Cr = +6 P = +5

Cl2 Cl = 0 S2O32- S = +2 C = !!!!1

S2Cl2 S = +1 Cr2(CO3)3 Cr = +3 Fe = +5

S8 S = 0 B = +3 Al = +3

I = !!!!1/3

BO2!

RbO2

Th4H11

O = !!!!1/2

Th = +11/4 S = +6

Ag2CrO4 Ag = +1

RbH H = !!!!1

Na2HPO4 P = +5 Eu3(PO3)2 P = +3 B = +3

P4O6 P = +3 BeF2 Be = +2

H4P2O7

MgC2

K3FeO4

Al2O3

I3!

NaHSO4

B4O72!

P4O10 P = +5

OF2 F = !!!!1

Ce(ClO3)2 Ce = +2

Fe(MnO4)2 Mn = +7

NaSCN S = !!!!2

O = +2

Cl = +5

Fe = +2

C = +4 N = !!!!3

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