Chemistry Reference
In-Depth Information
Solution
2
O
O
Al
3
Al
2
O
£
O
Al
3
Al
2
O
O
The 6 valence electrons of the two aluminum atoms are transferred to three oxy-
gen atoms, yielding two
Al
3
O
2
ions and three
ions. The reaction can be writ-
ten more simply as follows:
2 Al
3
2
2 Al
3
O
£
3
O
or
2 Al
3 O £ Al
2
O
3
Practice Problem 5.6
Use electron dot diagrams to picture the com-
bination of magnesium and nitrogen atoms.
1s
2
2s
2
2p
6
3s
2
3p
1
Al atom:
Al
3
1s
2
2s
2
2p
6
ion:
We can predict the charges on the ions of some elements but not others. In
their compounds, the metals of periodic groups IA, IIA, and IIIB (1, 2, and 3),
as well as zinc, cadmium, aluminum, and silver, always form ions with posi-
tive charges equal to the element's classical periodic group number. These ele-
ments are indicated with a blue background in Figure 5.11. The charge on every
monatomic anion except is equal to the classical group number of the ele-
ment minus 8. These elements are presented with a red background in Fig-
ure 5.11. The number of added electrons is the absolute value of that difference.
(Not all nonmetals form monatomic anions, however.) Hydrogen can react with
very active metals to form the hydride ion, which has the 2-electron con-
figuration of helium. The maximum positive charge on a monatomic cation is
; the maximum negative charge on a monatomic anion is In addition to
the generalities just presented, note that each of the elements of the first transi-
tion series except scandium forms an ion with a charge, and the eight mid-
dle ones also form an ion having another charge. (The transition metals form
ions having different charges by donating varying numbers—0, 1, or 2—of their
inner electrons to nonmetals.)
Because the overall charge on any ionic compound is zero, we can deter-
mine the formula of an ionic compound by balancing the charges on the cations
and anions. That is,
In compounds, the metals of
periodic groups IA, IIA, and IIIB
(1, 2, and 3), as well as zinc,
cadmium, aluminum, and silver,
always form ions with positive
charges equal to the element's
classical periodic group number.
H
H
,
The charge on every monatomic
anion (except H
) is equal to the
classical group number of the
element minus 8. The number of
added electrons is the absolute
value of that difference.
4
3
.
2
Number of positive charges
Number of negative charges
Pb
4
O
2
The compound of and is thus PbO
2
because there are four negative
charges on two ions to balance the four positive charges on one ion.
We can even write formulas for compounds whose ions are totally
unfamiliar to us, as long as we know their charges. For example, the com-
pound of
O
2
Pb
4
AB
4
2
XY
3
2
AB
4
2
XZ
2
3
and
is AB
4
XY
3
and that for
and
is
(AB
4
)
3
(XZ
2
)
2
.
