Chemical Formulas and Compounds

1. Chemical Formulas and Compounds Oxidation Numbers

2. Oxidation Numbers • Objective • List the rules for assigning oxidation numbers • Give the oxidation number for each element in the formula of a chemical compound • Name binary molecular compounds using oxidation numbers and the Stock system

3. Oxidation Numbers • Oxidation Numbers • The charges on the ions in an ionic compound reflect the electron distribution of the compound • In order to indicate the general distribution of electrons among the bonded atoms in a molecular compound or a polyatomic ion, oxidation numbers are assigned to the atoms composing the compound or ion • Unlike ionic charges, oxidation numbers do not have an exact physical meaning: rather, they serve as useful “bookkeeping” devices to help keep track of electrons • In general when assigning oxidation numbers, shared electrons are assumed to “belong” to the more electronegative atom in each bond

4. Oxidation Numbers • Assigning Oxidation Numbers • The atoms in a pure element have an oxidation number of zero • Monatomic ions have an oxidation number equal to the charge of the ion • The more-electronegative element in a binary compound is assigned a negative number equal to the charge it would have as an anion. Likewise for the less-electronegative element • Fluorine has an oxidation number of –1 in all of its compounds because it is the most electronegative element • Oxygen usually has an oxidation number of –2 • In peroxides, such as H2O2, oxygen’s oxidation number is –1 • In compounds with fluorine, such as OF2, oxygen’s oxidation number is +2

5. Oxidation Numbers • Assigning Oxidation Numbers • Hydrogen has an oxidation number of +1 in all compounds containing elements that are more electronegative than it; it has an oxidation number of –1 with metals • The algebraic sum of the oxidation numbers of all atoms in an neutral compound is equal to zero • The algebraic sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion • Although rules 1 through 7 apply to covalently bonded atoms, oxidation numbers can also be applied to atoms in ionic compounds similarly

6. Oxidation Numbers • Oxidation Numbers • Sample Problem • Assign oxidation numbers to each atom in the following compounds or ions: • UF6 • H2SO4

7. Oxidation Numbers • Oxidation Numbers • Sample Problem Solution • Place known oxidation numbers above the appropriate elements • Multiply known oxidation numbers by the appropriate number of atoms and place the totals underneath the corresponding elements • The sum of the oxidation numbers must equal zero unless there is a charge associated with the compound +5 2 +5 6

8. Oxidation Numbers • Oxidation Numbers for Formulas and Names • Many nonmetals can have more than one oxidation number • These numbers can sometimes be used in the same manner as ionic charges to determine formulas • What is the formula of a binary compound formed between sulfur and oxygen? • From the common +4 and +6 oxidation states of sulfur, you could predict that sulfur might form SO2 or SO3 • Both are known compounds

9. Oxidation Numbers • Common Oxidation States of Nonmetals • Pg. 235

10. Oxidation Numbers • Oxidation Numbers for Formulas and Names • Using oxidation numbers, the Stock system, introduced in the previous section for naming ionic compounds, can be used as an alternative to the prefix system for naming binary molecular compounds