5 Steps to a 5 AP Chemistry

The electron affinity (EA)is the energy change that results from adding an electron to

an atom or ion. The trends in electron affinity are not quite as regular as size or ionization

energy. In general, electron affinity increases from left to right within a period (owing to

the increased effective nuclear charge), and decreases from top to bottom within a group

owing to increased atomic or ionic size. Noble gases are an exception—they have no EA.

Do not forget that the trends mentioned in this section may help you on the multiple

choice portion of the AP exam. However, it is the underlying reasons that you need for the

free-response portion.

Oxidation Numbers

Oxidation numbersare bookkeeping numbers that allow chemists to do things like bal-

ance redox equations. Don’t confuse oxidation numbers with the charge on an ion.

Oxidation numbers are assigned to elements in their natural state or in compounds using

the following rules:

 The oxidation number of an element in its elemental form (i.e., H 2 , Au, Ag, N 2 ) is zero.

 The oxidation number of a monoatomic ion is equal to the charge on the ion. The oxi-

dation number of Mg^2 +is +2. Note that the charge is written with number first, then

sign; for oxidation numbers it is sign, then number.

 The sum of all the oxidation numbers of all the elements in a neutral molecule is zero. The

sum of all the oxidation numbers in a polyatomic ion is equal to the charge on the ion.

 The alkali metal ions have an oxidation number of +1 in all their compounds.

 The alkaline earth metals have an oxidation number of +2 in all their compounds.

 The oxidation number of hydrogen in compounds is +1, except it is −1 when combined

with metals or boron in binary compounds.

 The oxidation number of halogens in their compounds is −1 except when combined

with another halogen above them on the periodic table, or with oxygen.

 The oxidation number of oxygen is −2 in compounds, except for peroxides, in which it is −1.

Determine the oxidation number of sulfur in sulfuric acid, H 2 SO 4. The sum of all the oxidation

numbers must equal zero, since this is a neutral compound. The oxidation numbers of hydrogen

(+1) and oxygen (−2) are known, so the oxidation number of sulfur can be determined:

The oxidation number of sulfur in this compound must be +6.

Nomenclature Overview

This overview covers some of the rules for naming simple inorganic compounds. There are

additional rules, and some exceptions to these rules. The first part of this overview discusses

the rules for deriving a name from a chemical formula. In many cases, the formula may be

determined from the name by reversing this process. The second part examines situations

in which additional information is needed to generate a formula from the name of a com-

pound. The transition metals present some additional problems; therefore, there is a section

covering transition metal nomenclature and coordination compounds.

21 +++ − =42 0()? ( )

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KEY IDEA