Chemistry - A Molecular Science

Example 4.10 a) Predict the formulas of selenate ion, hydrogen selenate ion, selenite ion, and

hydrogen selenite ion. Selenium is a 6A nonmetal, so an ending of -

ate

tells us that the selenium atom is in its

highest oxidation state, +6. Oxygen is -2

. Three oxygens would make the compound

neutral, so a fourth oxygen is needed to ma

ke an anion. One selenium atom, at +6,

combined with four oxygens, at -2 each, results in a -2 ion. Thus, selenate ion is SeO

2- 4

,

which is to be expected because sulfate ion is SO

2- 4

, and sulfur and selenium are in the

same family. Addition of an H

1+ to selenate yields the hydrogen selenate ion, HSeO

1- 4

(^).
Removal of an oxygen atom results in SeO
2- 3
, the selenite ion, and adding an H
1+ results
in the hydrogen selenite ion, HSeO
1- 3
.
b) Predict the formulas of the chromate and dichromate ions.
Chromium is a 6B metal and is in the +6 oxidation state in the chromate ion. A minimum of four oxygen atoms, at -2 each, is requir
ed to produce the anion. Thus, the chromate ion
is CrO
2- 4

. Note the similarity of the chromate an

d sulfate ions. In each case, the central

atom is a Group 6 element; one is a 6B while the other is a 6A. The

di-

prefix in dichromate indicates that there

are two chromium atoms in this ion, while

the

-ate

suffix indicates that each is in a +6 oxidation state. A minimum of seven oxygen

atoms is required to produce the anion. The

charge on the anion is 2(+6) + 7(-2) = -2. The

formula of the dich

romate ion is Cr

O 2

2- 7

.

4.7

STRUCTURE OF IONIC COMPOUNDS

Figure 4.6 NaCl structure NaCl is a network of alternating Na

1+ (blue) and Cl

1- ions

(yellow) with no clearly discernable NaCl molecules. Thus, NaCl forms an extended solid and is not molecular.

Monatomic anions and cations are spherical ch

arges that pack around one another in their

compounds. The points of contact can be anywhere along the surface of the spheres. In other words,

ionic bonds are not directional

. In addition, several spheres of one charge can

pack around one sphere of opposite charge, so

there are no units that are identifiable as

molecules. That is,

ionic compounds are not molecular

.

For example, consider the structure of a crys

tal of NaCl shown in Figure 4.6. Note that

there is no unique NaCl pair that can be id

entified as an NaCl molecule. Instead, each Cl

1-^

ion is surrounded by six Na

1+ ions and each Na

1+ ion is surrounded by six Cl

1- ions.

Consequently, a single crystal of table salt cons

ists of an enormous

number of sodium and

chloride ions, which are all bound together in

a solid network. Thus, NaCl is a compound,

not a molecule. Molecular substances involve on

ly nonmetals and are the topic of Chapters

5 and 6. We revisit solid networks in Chapter 8.

Chapter 4 The Ionic Bond

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