Section 7-2

Oxidation Numbers

• “Imagined charges” assigned to atoms in covalent molecules

• In fact, they can be quite arbitrary.

They Are Useful

• In naming compounds, in writing formulas, in balancing chemical equations, and in studying certain types of chemical reactions (Ch. 19).

Assigning Oxidation Numbers:General Rule

• Shared electrons are assumed to belong to the more electronegative atom in each bond.

Specific Rules For Assigning Oxidation Numbers

I. Atoms of a pure element have an oxidation number of zero.

Ex:NaO2

S8

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).

III. Fluorine

• Assigned a value of -1 in all compounds

because it is the most electronegative element.

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)

V. Hydrogen

• +1 in all compounds with elements that are more electronegative than it is.

• It is -1 when it is combined with metals.

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.

Because of Rules I-VIII

• It is often possible to assign oxidation numbers when they are not known.

Using Oxidation Numbers For Formulas and Names

• Both metals and nonmetals can have more than one oxidation number.

Fe = +2 or +3SO2; S = +4SO3; S = +6

Both Used Interchangeably For Simple Compounds

Phosphorous trichloride PCl3 Phosphorous (III) chlorideDinitrogen monoxideN2O Nitrogen (I) oxide

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