OX NUMBERS

DETERMINING OXIDATION NUMBERS

Summary of Rules from pages 284 - 285

For all compounds, whether covalent, polar covalent, or ionic, we treat as ionic for counting electrons and for oxidation-reduction reactions.

Rule 1: Sum of the oxidation numbers of all the atoms in the chemical species equals the charge on the species.
Neutral compounds: Sum of oxidation numbers = 0
Ionic species: Sum of oxidation numbers = charge of the ion

Rule 2: In Binary Compounds, the more Electronegative (EN) element is assigned to have a negative oxidation number. (See EN trends.)

Rule 3: Atoms may have only certain oxidation numbers. The range is:

Maximum oxidation number possible = + Group number.
Minimum oxidation number possible = (Group number - 8) (this number will be negative)

Atoms which will have known oxidation numbers are:

Atoms as Elements: Ex. H₂, O₂, P₄, Fe
Oxidation number = 0

Monoatomic Ions:
Cations: Ex. Na⁺, Al³⁺ (main group metals)
Oxidation number = + Group Number
Anions: Cl^-, O^2-
Oxidation number = Group Number - 8

Hydrogen
Combined with Nonmetals: Ex. NH₃, H₂O, HCl
Oxidation number = +1
Combined with Metals: Ex. NaH, CaH₂ (hydrides)
Oxidation number = -1

Oxygen (Unless O₂^2-, peroxide)
Oxidation number = -2

Examples:

CO: (Sum will equal 0 since it is a neutral molecule)
O will have a -2 ox. number.

1 C + 1 O = 0
(C?) + (-2) = 0
C? = +2

Oxidation number of C in CO is +2
Oxidation number of O in CO is -2 (known)

Check ox. number to see if it falls within range:
+2 is in between the maximum value of C, +4, (Gr#) and the minimum value of C, - 4, (Gr# - 8).
So okay.

Cr₂O₇^2-: (Sum of all oxidation numbers will equal -2 since it is an ion.)

2 Cr + 7 O = -2
2(Cr?) + 7(-2) = -2
2(Cr?) + (-14) = -2
2(Cr?) = +12
Cr? = +6

Oxidation number of each Cr in Cr₂O₇^2- is +6
Oxidation number of each O in Cr₂O₇^2- is -2 (known)

Check ox. number to see if it falls within range:
+6 is the maximum value that Cr can have (Gr#). So okay.

CS₂: (Sum will equal 0 since it is a neutral molecule)
C will have the positive oxidation number since it is less EN than S
S will have a -2 charge since it is Gr # 6, (6 - 8 = -2)

C + 2 S = 0
(C?) + 2 (-2) = 0
(C?) + (-4) = 0
C? = +4

Oxidation number of C in CS₂ is +4
Oxidation number of each S in CS₂ is -2 (known)

Check ox. number to see if it falls within range:
+4 is the maximum value that C can have, (Gr#). So okay.

NH₄⁺: (Sum will equal +1 since it is an ion)
H will have a +1 ox. number since it is bonded to N, a nonmetal.

N + 4 H = +1
(N?) + 4(+1) = +1
N? = -3

Oxidation number of N in NH₄⁺ is -3
Oxidation number of each H in NH₄⁺ is +1 (known)

Check ox. number to see if it falls within range:
-3 is the minimum value that N can have, (Gr# - 8). So okay.

H₅IO₆: (Sum will equal 0 since it is neutral species.)
H will have a +1 ox. number since it is combined w/ nonmetals
Iodine will have a + charge since it is less EN than Oxygen

5 H + I + 6 O = 0
5(+1) + (I?) + 6(-2) = 0
(+5) + (I?) + (-12) = 0
(I?) + (-7) = 0
I? = +7

Oxidation number of I in H₅IO₆ is +7
Oxidation number of each H in H₅IO₆ is +1 (known)
Oxidation number of each O in H₅IO₆ is -2 (known)

Check ox. number to see if it falls within range:
+7 is the maximum value that I can have, (Gr#). So okay.

NaBH₄: (Sum will equal 0 since it is neutral species.)
H will have a -1 ox. number since it is combined w/ metals
Na will have a +1 charge (+ Gr# = +1)

Na + B + 4 H = 0
(+1) + (B?) + 4(-1) = 0
(+1) + (B?) + (-4) = 0
(B?) + (-3) = 0
B? = +3

Oxidation number of B in NaBH₄ is +3
Oxidation number of each Na in NaBH₄ is +1 (known)
Oxidation number of each H in NaBH₄ is -1 (known)

Check ox. number to see if it falls within range:
+3 is the maximum value that B can have, (Gr#). So okay.

H₂MnO₄: (Sum will equal 0 since it is neutral species.)
H is a +1 ox. number since it is combined w/ nonmetals (ignore metal)
Mn will have a + charge since it is less EN than Oxygen

2 H + Mn + 4 O = 0
2(+1) + (Mn?) + 4(-2) = 0
(+2) + (Mn?) + (-8) = 0
(Mn?) + (-6) = 0
Mn? = +6

Oxidation number of Mn in H₂MnO₄ is +6

Check ox. number to see if it falls within range:
+6 is less than the maximum value of Mn, +7, (Gr#). So okay.

** Note: If assumed -1 for H, then Mn would be a +10. This is greater than the maximum value allowed for Mn, +7, (Gr#). So +10 for Mn is not allowed as a possible oxidation number.