Oxidation Numbers
7-2
Beaker Breaker
• Name or write the formulas of the following
acids:
H3PO4 = ___________
Nitric acid = _________
Hydrochloric acid ________
HBr =____________
In NaCl, Na is 1+ and Cl is 1In H2O, what is the “value” (sign
and number) of H and O?
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H is a +1
O is a -2
But………are these really “charges” ????
Are H and O “ions” in water ?
Is H2O an ionically-bound compound?
NO, NO and NO !!! These are oxidation numbers
written as +1 and -2, rather than 1+ and 2-(charges)
Oxidation number
• also known as oxidation state
• number assigned to an atom in a molecular cmpd
that indicates that general distribution of electrons
among the bonded atoms
• gives an insight into the combining ability of an
atom – not “real” charges but are artificial
“bookkeeping” devices to keep track of overall edistribution
• Ionic cmpds: the charge on the ion IS the electron
distribution ∴ ox. # = charge on ion…physically
meaningful info
Summary of Ox # Rules (p.216)
1. Pure elements have an ox. # of ZERO
i.e. copper, Cu = ox # of zero
oxygen, O2 = ox # of zero
2. More electroneg. element has an ox. # = charge it
would have if it were an anion. Less electroneg.
element has an ox # = charge it would have if it
were a cation.
3. F: -1 oxidation #
4. O: -2 oxidation # except in peroxide
(H2O2) where it is –1, and with halogens
(OF2) where it is +2.
5. H: +1 except when it’s more electroneg.
than other atoms (metals….NaH) where it
is –1.
6. Algebraic sum of all ox. #s in cmpd = 0.
7. Algebraic sum of all ox. #s in polyatomic
ion = charge on ion.
8. IF ionic, ox. # = charge of the ion
Assign ox. #s to each element in
the following cmpds or ions:
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CCl4
NO2
LiH
sulfate ion
nitrate ion
H3PO4
P4O10
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CCl4
+4, -1
NO2
+4, -2
LiH
+1, -1
sulfate ion
SO42- +6, -2
nitrate ion
NO31- +5, -2
H3PO4
+1, +5, -2
P4O10
+5, -2
Using the newer Stock system,
write formulas for the following
molecular cmpds:
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nitrogen (II) oxide
phosphorus (III) chloride
sulfur (IV) oxide
bromine
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nitrogen (II) oxide
NO
phosphorus (III) chloride
PCl3
sulfur (IV) oxide
SO2
bromine
Br2
What would the previous molecular
cmpds be names using the prefix system?
•NO
•nitrogen monoxide
•PCl3
•phosphorus trichloride
•SO2
•sulfur dioxide
•Br2
•bromine
•Br2
Beaker Breaker
• Determine the oxidation numbers of the
following:
CaO =_________
CH4 = _________
H2SO4 =___________