OXIDATION NUMBERSChapter 7.2

Objectives:1. List the rules for assigning oxidation

numbers2. Give the oxidation number for each

element in the formula of a chemical compound.

3. Name binary molecular compounds using oxidation numbers and the Stock system.

Oxidation Numbers

Assigned in order to indicate the general distribution of electrons among the bonded atoms in a molecular compound or polyatomic ion Only needed for molecules or polyatomic ions Ionic compound have charges

or oxidation state

Assigning Oxidation Numbers General rule

Shared electrons are assumed to belong to the more electronegative atom

1. Atoms in a pure element• Oxidation number equals zero

Ex: Na,O2, P4, S8 all zero 2. More-electronegative element in a binary

molecular compound• Oxidation number equals charge of what

the anion would be• Most electronegative always listed last

Ex: CCl4 SO2

-1

-2

3. Fluorine• Oxidation number equals -1

4. Oxygen• Oxidation number equals -2 almost all the time• Oxidation number equals -1 when it’s a peroxide• Oxidation number equals +2 when combined

with fluorine

Ex: CO2 H2O2 OF2

+2

-1

-2

5. Hydrogen• Oxidation number equals +1 when in

compounds with elements that are more-electronegative

• Oxidation number equals -1 when combined with metals

Ex: H2O NaH-1+

1

6. Algebraic sum of the oxidations numbers of all atoms in a neutral compound• Equal zero

Ex: CCl4 SO2

-1

-2

4 x -1 = -4so C is

going to be +4

+4

2 x -2 = -4so S is

going to be +4

+4

7. Algebraic sum of the oxidations numbers of all atoms in a polyatomic ion• Equal to the charge on the ion

Ex: NO3 -1

3 x -2 = -6

-2

+5

so N is going to be +5

8. Although rules 1 -7 apply to covalently bonded atoms• Oxidation numbers can also be assigned to

atoms in ionic compounds• Oxidation number is equal to the charge of ion

Practice

a) UF6

b) H2SO4

c) ClO3

-1

+6

+1

-2

+6

-1

-2

+5

Other practicea) HCl e) N2O5

b) HNO3

c) KHd) HClO3

Naming using prefix and stock systems

PCl3PCl5

N2O

NO

PbO2

Mo2O3

phosphorus trichloride phosphorus(III) chloridephosphorus

pentachloridephosphorus(V) chloride

dinitrogen monoxide nitrogen(I) oxide

nitrogen monoxide nitrogen(II) oxide

lead dioxide lead(IV) oxide

dimolybdenum trioxide

molybdenum(III) oxide