Chemistry Reference
In-Depth Information
four, or more atoms per formula unit, depending on the
compound.
All the ionic compounds considered in this topic are com-
posed of only one type of cation and one type of anion.
■
Learning and using a generalization is easier than memo-
rizing individual facts. For example, once we learn that
in
their compounds,
the metals of periodic groups IA, IIA,
and IIIB (1, 2, and 3) form ions with charges equal to their
group numbers ( and respectively), we do not
have to learn the charges on 16 separate metal ions.
Electron dot diagrams are most useful for main group ele-
ments, and the systematic procedure for drawing electron
dot diagrams works only for species in which all atoms
obey the octet rule.
■
■
1
, 2
,
3
,
Nonmetal atoms accept electrons from metal atoms if the
metal atoms are available, or else they share electrons; they
never donate electrons to form monatomic cations. The
largest charge on any monatomic cation is
■
An ion is a charged species. A single ion is just
part
of a
compound. The charges on ions are integral parts of the
formulas of the ions. We must always include the charges
when we write formulas for ions alone. For example, it
makes quite a difference whether we are referring to SO
3
(sulfur trioxide) or (sulfite ion). Writing the symbol
or formula for a single ion does not imply that it can exist
alone, but only that the ion of opposite charge is not of im-
mediate interest. We may write the charges on both ions in
an ionic compound while we are determining the com-
pound's formula, but we never write the charge on one ion
without writing the charge on the other. To finish the for-
mula, we rewrite it without the charges. For example, we
write NaCl for
■
4
,
and on any
monatomic anion, it is
3
.
The terms
single bond, double bond,
and
triple bond
refer
to covalent bonds only.
■
SO
3
2
Polyatomic ions are held together by covalent bonds and
are attracted to oppositely charged ions by ionic bonds.
■
After a little experience, we will recognize the monatomic
and polyatomic ions introduced in this chapter. For exam-
ple, every time the symbol for an alkali metal or an alkaline
earth metal appears
in a compound,
it represents the ion
with a charge equal to
■
1
or
2
,
respectively.
Na
Cl
.
Both
chloride ion
and
chloride
can be used to refer to the
ion. To refer to however, we must always include
the word
ion
because
sodium
can refer to the element,
the atom, or the ion.
■
All compounds, whether ionic or covalent, are electrically
neutral. The total positive charge on the cations of an ionic
compound must therefore be balanced by the total negative
charge on the anions.
■
Cl
Na
,
The alkali metals, the alkaline earth metals, the group IIIB
metals, aluminum, zinc, cadmium, and silver have ions
with charges equal to their classical group numbers, but
only in their compounds!
When the elements are uncom-
bined, they do not form ions, and the charge on each atom
is zero.
■
H
,
Except for monatomic anions have charges equal to
their classical group number minus 8. (Not all nonmetals
form monatomic ions.) Polyatomic anions containing oxy-
gen and one other element are not quite that easy to predict
charges for, but generally, the charge is
odd
if the periodic
group of the central element is odd and
even
if the periodic
group of the central element is even.
■
Answers to Snapshot Reviews
5.1 A. Nine hydrogen atoms per phosphorus atom
B. Chlorine is more electronegative; that is why it is writ-
ten after selenium.
C. O
2
5.2 A. (a) Two valence and two outermost electrons (the
same electrons) and (b) zero valence electrons (fourth
shell) and eight outermost electrons (third shell)
B. (a) Equal, (b) equal, (c) one more electron than proton
5.3 A.
5.4 A. La
2
S
3
B. (a)
V
2
and Cl
Pb
4
and O
2
(b)
Ti
3
and O
2
(c)
5.5 A. (a)
H H
(b)
H
H
H
H
C
C
O
H
C
C
C
H
H
H
B. (a)
N
C
O
(b)
H
H
C
O
P
Self-Tutorial Problems
5.1
(a) Distinguish between
valence shell
and
outermost
shell.
(b) Distinguish between
diatomic
and
binary.
5.3
Explain why hydrogen cannot form
(a) an
H
2
ion
H
2
(b) an
ion
5.2
Rules for writing diatomic molecules and for deducing
the charges on alkali metal ions were introduced in this
chapter. Which of these refers only to uncombined ele-
ments and which refers only to elements in compounds?
5.4
Which of the following have ionic bonds, and which
have covalent bonds?
(a) Cl
2
(b) MgCl
2
(c) SCl
2
