Chapter 7: Chemical Formulas and Chemical Compounds Section 2: Oxidation Numbers

Overview We will list the rules for assigning oxidation numbers. We will be able to give the oxidation number for all the elements in a compound. We will name binary molecular compounds using oxidation numbers and the stock system.

Oxidation Numbers A way to indicate the distribution of electrons among bonded atoms in a molecular compound or a polyatomic ion. Also known as an atom’s oxidation state. This is not the same as an atom’s charge! Oxidation numbers are helpful in some cases in naming a compound. Later we will use them in balancing equations.

How to Assign Oxidation Numbers Atoms by themselves or bonded to themselves have an oxidation state of 0. Hydrogen has an oxidation number of +1 and when bonded to a metal -1. Fluorine is always a -1. Oxygen is -2 unless in H 2 O 2 (-1) or OF 2 (+2). The most electronegative atom has an oxidation number equal to its charge as an ion. (Closest to F) The charge of an individual ion is its oxidation number. The sum of the oxidation numbers must equal the charge of the compound.

Binary Molecule Examples HF Both have fixed values H = +1F = -1 Cl 2 Bonded to SelfCl = 0 CS 2 Sulfur is more electronegative S = -2 There are two sulfur atoms so their oxidation totals to -4. The carbon atom must equal this charge to make the molecule’s total oxidation 0.C = +4

Polyatomic Ion Examples SO 4 -2 Oxygen is more electronegative & fixed value. O = -2 There are four oxygen atoms, so the total negative charge due to oxygen is -8. The total charge needs to equal -2 so sulfur must account for that. S = +6 SO 3 -2 Oxygen is more electronegative & fixed value. O = -2 There are three oxygen atoms, so the total negative charge due to oxygen is -6. The total charge needs to equal -2 so sulfur must account for that. S = +4

Molecular Compounds w/ Polyatomic Ions H 2 CO 3 CO 3 has a -2 charge so that is its oxidation number. Hydrogen is always +1.H = +1 Oxygen is the most electronegative atom & fixed value.O = -2 Total oxidation number of the three oxygen atoms is -6 So the oxidation number of carbon and -6 need to equal -2.C = +4

Using Oxidation States to Name We already do this in the stock system for ionic compounds. The prefix system for molecular compounds can be modified using the oxidation numbers to the stock system. NCl 3 Nitrogen trichloride Nitrogen (III) chloride