2Oxidation Numbers“Imagined charges” assigned to atoms in covalent moleculesIn fact, they can be quite arbitrary.
3They Are UsefulIn naming compounds, in writing formulas, in balancing chemical equations, and in studying certain types of chemical reactions (Ch. 19).
4Assigning Oxidation Numbers: General Rule Shared electrons are assumed to belong to the more electronegative atom in each bond.
5Specific Rules For Assigning Oxidation Numbers Atoms of a pure element have an oxidation number of zero.Ex:NaO2S8
6II. ElectronegativityThe 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).
7III. Fluorine Assigned a value of -1 in all compounds because it is the most electronegative element.
8IV. OxygenAssigned 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)
9V. Hydrogen+1 in all compounds with elements that are more electronegative than it is.It is -1 when it is combined with metals.
10Algebraic SumsIn 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.
11Because of Rules I-VIII It is often possible to assign oxidation numbers when they are not known.
12Using 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
13Both Used Interchangeably For Simple Compounds Phosphorous trichloride PCl3 Phosphorous (III) chlorideDinitrogen monoxide N2O Nitrogen (I) oxide
14Assignment7.2 Worksheet7.2 Textbook Problems (Warning!!! I’s and l’s look a lot alike, think about the formula, does it make sense?)Due Wednesday BOP