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 Atoms of a pure element have an oxidation number of zero. Ex: Na O2 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 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 +3 SO2; S = +4 SO3; S = +6

Both Used Interchangeably For Simple Compounds Phosphorous trichloride PCl3 Phosphorous (III) chloride Dinitrogen monoxide N2O 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