Chapter 10

Oxidation Numbers

Oxidation Number

• The oxidation number is the charge on a single atom.

Oxidation Number

• The oxidation number is the charge on a single atom.

• Examples:Na+ (oxidation number = +1)Clˉ (oxidation number = -1)Al3+ (oxidation number = +3)O2ˉ (oxidation number = -2)

We can use the periodic table to “predict” oxidation numbers

We can use the periodic table to “predict” oxidation numbers

• If you have the name of the ion as you did on the quiz that you just took. You should know the oxidation number of the ion and not have to make a prediction.

We can use the periodic table to “predict” oxidation numbers

• Metals tend to form positive ions.

• Positive ions other than groups 1 & 2, Ag, Cd, Zn, and Al can be predicted.

• Nonmetals tend to form negative ions.

• However other than fluoride (F-) which is always -1, all other nonmetals could have positive oxidation numbers as well.

Metals: Use the Roman numeral above each group up to 7+. If no Roman numeral or if Roman numeral is too high use 2+.Nonmetals: Count over from the closest Noble Gas (which has an oxidation number of zero) assigning a negative oxidation number.

• The periodic table can often be used to make a quick prediction as to possible oxidation numbers of cations

Remember that these oxidation numbers are only predictions. Most

oxidation numbers can vary.• We predict nitrogen to have an oxidation number of -3.• N can have positive oxidation numbers such as +5.• However other positive oxidation numbers are

possible as in nitrogen (I) oxide (laughing gas) N2O. Here nitrogen has an oxidation number of +1.

What common ions can Iron form?

Fe +3Fe +2

A good number of transition metal elements possess an ability to have more than one positive ion. (Remember +2 is the most common oxidation number for metal ions)

Fe +3Fe +2

What oxidation numbers might you predict for Manganese? (Mn)

The roman numeral is a common prediction if it is less than 8.

Mn +7

Remember +2 is the most common oxidation number for metal ions

Mn+2

Mn +7

Each electron is harder to remove than the one before so sometimes losing electrons can be a gradual process.

Mn +3Mn+2 Mn +4 Mn +5

Mn +7

Determining Oxidation Numbers

• The best way to determine oxidation numbers is to have the examine the elements within a compound.

Oxidation Number Rules• The oxidation number of all Group 1 metals (+1), Group 2

metals (+2), Ag+, Zn2+, Cd2+, and Al3+ within compounds is a set value.

• Hydrogen (H) has two possible oxidation numbers:– +1 when bonded to a nonmetal– -1 when bonded to a metal

• The oxidation number of fluorine (F) is always -1.• In ionic compounds the nonmetal closest to fluorine is

negative.• The sum of the oxidation numbers of all atoms (ions) in a

neutral compound = 0.• The sum of the oxidation numbers of all atoms (ions) in a

polyatomic ion = charge on the polyatomic ion.

ExampleNaCl

ExampleBaH2

ExamplePO3

3-

ExampleFePO3

ExampleFe3(PO3)2

Homework

Ions and Oxidation Numbers Worksheet