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Section 7-2
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Oxidation Numbers
• “Imagined charges” assigned to atoms in covalent molecules
• In fact, they can be quite arbitrary.
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They Are Useful
• In naming compounds, in writing formulas, in balancing chemical equations, and in studying certain types of chemical reactions (Ch. 19).
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Assigning Oxidation Numbers:General Rule
• Shared electrons are assumed to belong to the more electronegative atom in each bond.
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Specific Rules For Assigning Oxidation Numbers
I. Atoms of a pure element have an oxidation number of zero.
Ex:
Na
O2
S8
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II. Electronegativity
• The more electronegative element in a binary compound is assigned the number equal to the negative charge it would have as an anion, the less electronegative one is positive (as if it were a cation).
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III. Fluorine
• Assigned a value of
-1 in all compounds because it is the most electronegative element.
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IV. Oxygen• Assigned a number of -2
in almost all compounds. Exceptions:
There are exceptions, like in H2O2, when you would have to find the oxidation number by looking at the molecule as a whole (i.e. charge of H)
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V. Hydrogen
• +1 in all compounds with elements that are more electronegative than it is.
• It is -1 when it is combined with metals.
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Algebraic Sums
VI. In a neutral compound all oxidation numbers add up to zero.
VII. In a polyatomic ion the sum is equal to the charge of the ion.
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Because of Rules I-VIII
• It is often possible to assign oxidation numbers when they are not known.
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Using Oxidation Numbers For Formulas and Names
• Both metals and nonmetals can have more than one oxidation number.
Fe = +2 or +3
SO2; S = +4
SO3; S = +6
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Both Used Interchangeably For Simple Compounds
Phosphorous trichloride PCl3 Phosphorous (III) chloride
Dinitrogen monoxideN2O Nitrogen (I) oxide
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Assignment
• 7.2 Worksheet• 7.2 Textbook Problems
(Warning!!! I’s and l’s look a lot alike, think about the formula, does it make sense?)
Due Wednesday BOP