CHEM1612 - Pharmacy
Week 7: Oxidation Numbers
Dr. Siegbert Schmid
School of Chemistry, Rm 223
Phone: 9351 4196
E-mail: [email protected]
Unless otherwise stated, all images in this file have been reproduced from:
Blackman, Bottle, Schmid, Mocerino and Wille,
Chemistry, John Wiley & Sons Australia, Ltd. 2008
ISBN: 9 78047081 0866
Oxidation numbers (states)
Textbook: Blackman, Bottle, Schmid, Mocerino & Wille, “Chemistry”, John Wiley
& Sons Australia, Ltd., 2008.
Today’s lecture is covered in Sections 4.6, 4.8, 12.1, 13.1 and 13.2
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USE OF OXIDATION NUMBERS
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O.N. is written as
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Properties of compounds
Identifying redox reactions
Naming compounds
a roman numeral (I, II, III, etc.)
a number preceded by the sign (+2)
Ionic charge has the sign after the number (2+)
Lecture 21 -3
Oxidation numbers: definition
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Each atom in a molecule is assigned an OXIDATION NUMBER (O.N.).
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The oxidation number is the charge the atom would have if the
electrons in a bond were not shared but transferred completely to the
more electronegative atom.
Electrons shared equally as both Cl
atoms in Cl2 have the same
electronegativity.
Oxidation number = 0.
Unequal sharing of electrons, F has
higher electronegativity than H.
Therefore oxidation number of H will be
positive (+I), and F will be negative (-I).
Lecture 21 -4
Electronegativity
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Definition: Ability of a bonded atom to attract the shared electrons.
(Different from electron affinity, which refers to the ability of an
isolated atom in the gas phase to gain an electron
and form a gaseous anion).
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Electronegativity is inversely related to atomic size.
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Atomic size: increases down group (electrons in outer shells)
decreases across period (electrons in same shell)
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Electronegativity is directly related to ionization energy (energy
required to remove an electron from atom).
Lecture 21 -5
Electronegativity and the
Periodic Table
Linus Pauling defined
electronegativity in
arbitrary units 0.7 to 4.0
•  smallest at lower left
of PT - caesium (Cs)
•  greatest at upper right
of PT – fluorine (F)
•  Group 18 not included
since few covalent
compounds formed
Lecture 21 -6
Rules for assigning O.N.
1.  The oxidation number for any free element (eg. K, Al, O in O2) is
zero.
2.  The oxidation number for a simple, monatomic ion is equal to the
charge on that ion (e.g. Na+ has oxidation number +I)
3.  The sum of all the oxidation numbers of the atoms in a neutral
compound must equal zero (e.g. NaCl). The sum of all the oxidation
numbers of all the atoms in a polyatomic ion must equal the charge
on that ion (e.g. SO42-).
4.  In all its compounds fluorine has oxidation number –I.
5.  In most of its compounds hydrogen has oxidation number +I.
6.  In most of its compounds oxygen has oxidation number -II.
Blackman pg. 464
Lecture 21 -7
Oxidation numbers
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Molecules and polyatomic ions: shared electrons are assigned to the
more electronegative atom.
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Examples:
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1. 
2. 
3. 
HF
CO2
CH4
NO3-
H F
F-I H+I
O-II C+IV
O=C=O
+I
-IV
H C
-1 charge on anion
= 3 x O-II + N+V
Determining an atom’s oxidation number:
H
H-C-H
H
The more electronegative atom in a bond is assigned all the shared
electrons; the less electronegative atom is assigned none.
Each atom in a bond is assigned all of its unshared electrons.
The oxidation number is given by:
O.N. = no. of valence e- - (no. of shared e- + no. of unshared e-)
For F in HF, O.N. = 7 – (2 + 6) = -1
Lecture 21 -8
Application
What is the oxidation number of Cr in the following?
CrO3
x + 3(-2) = 0,
x = +6, Cr(VI)
Cr2O3
2(x) + 3(-2) = 0,
x = +3, Cr(III)
[Cr2O7]2-
2(x) + 7(-2) = -2,
x = +6, Cr(VI)
Lecture 21 -9
Transition Metals
Multiple oxidation numbers – ns and (n-1)d electrons are used for bonds.
Lecture 21 -10
Transition Metals
Multiple oxidation numbers – ns and (n-1)d electrons are used for bonds.
Lecture 21 -11
Filling of Atomic Orbitals (Aufbau)
In general,
the (n-1)d orbitals are
filled between the ns
and np orbitals.
Lecture 21 -12
Demo: Changing oxidation states
Consider the Period 4 Transition Metals: as the d orbitals fill, the 3d orbital
becomes more stable than the 4s.
∴the 4s electrons are lost before the 3d electrons to form the Period 4
transition metal ions.
Lecture 21 -13
3d electrons
Common
O.N.
+III +IV +V +VI +VII +III
+IV +III +IV +II
+II +II
+III +II
+II
+II
+II
Lecture 21 -14
Colour and Transition Metals
Aqueous oxoanions of transition elements.
One of the most characteristic
chemical properties of transition
elements is the occurrence of
multiple oxidation states.
Mn(II)
Mn(VI)
Both the colour as well as
another characteristic property
of transition elements –
paramagnetism - are due to
unpaired d-electrons.
Mn(VII)
V(V)
Cr(VI)
Mn(VII)
Lecture 21 -15
Pop Quiz
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What are the Oxidation Numbers for the following examples?
I2
O.N.=0 (elemental form)
Zn in ZnCl2
O.N.=+2 (Cl=-1, sum of O.N.s =0)
Al3+
O.N.=+3 (ON of monatomic ion=charge)
N in HNO3
O.N.=+5 (O=-2, H=+1, sum of ONs=0)
S in SO42-
O.N.=+6 (O=-2, sum of O.N.s=charge on ion)
N in NH3
O.N.= -3 (H=+1, sum of O.N.s = 0)
N in NH4+
O.N.= -3 (H=+1, sum of O.N.s =charge on ion)
Lecture 21 -16
Influence of Oxidation State
Hexavalent Chromium
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Cr(VI) is classified as “carcinogenic to humans”
Cr(VI) compounds are soluble in water & may have a harmful effect
on the environment.
Cr(VI) is readily reduced by Fe2+ and dissolved sulfides.
Trivalent Chromium
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Cr(III) is considered an essential nutrient.
Most naturally occuring Cr(III) compounds are insoluble and it is
generally believed that Cr(III) does not constitute a danger to health.
Cr(III) is rapidly oxidised by excess MnO2, or slowly by O2 in alkaline
solutions.
Lecture 21 -17
Properties of N-compounds
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Some non-metals like sulphur or nitrogen or chlorine also have a
very wide range of oxidation states in their compounds.
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N-compounds have a very wide range of properties.
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N has an intermediate
electronegativity and has
an odd number (5) of
valence electrons. N has
one of the widest ranges
of common oxidation
states of any element.
Lecture 21 -18
Oxidation states of N
NV
HNO3 / NO3-
Strong acid
NIV
NO2, N2O4
Smog
NIII
HNO2 / NO2-
Weak acid / weak base
NII
NO
Smog + biology
NI
N 2O
Greenhouse gas + laughing gas
N0
N2
Stable
N-I
NH2OH
Hydroxylamine
N-II
N 2H 4
Hydrazine, rocket fuel
N-III
NH3 / NH4+
Weak base / weak acid
Lecture 21 -19
Properties of N-compounds
O
N
HIGHLY VARIED!
Incredibly stable:
CH3
O
N
O
O
N2
N
O
Extremely explosive:
O
trinitrotoluene (TNT)
Strong acid:
HNO3
Weak base:
NH3
Photochemical smog:
NO2
O
Biologically important:
NO + amino acids
O
O
O
O N
O
O N
O
N O
nitroglycerine
Lecture 21 -20
Air pollution
Picture from www.consumercide.com
Picture from http://pdphoto.org
Sydney
The brown haze is largely NO2
ß
Los Angeles
Lecture 21 -21
Summary
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Rules for assigning oxidation numbers
Trends in electronegativity
Electron configuration of elements and ions
Aufbau – rule for filling atomic orbitals
Electron configuration of transitions metals
Lecture 21 -22