Chemistry Reference
In-Depth Information
10.5.3 Combination of Ions (“Left and Right in Periodic Table”)
The nondirectional bonding forces of ions lead to the formation of ionic lattices,
when ions of two or three different kinds are bonding. The structure of ionic lattices
basically depends on two factors (1) ionic charge, (2) radius ratio of the ions.
Ionic charge
: According to attraction and repulsion forces an ionic structure is in
equilibrium: equal numbers of negative charges are grouping around every posi-
tively charged ion (cation), and equal numbers of positive charges are around every
negatively charged ion (anion). If the charge on cations and anions is equal, an ionic
lattice with the ratio 1:1 results (see 1 in Fig.
10.18
). If the cations are charged 2+
and the anions are charged 1
, the ion ratio has to be 1:2 (see 2 in Fig.
10.18
).
If three kinds of ions form a lattice, the ion ratios have to be such that the ionic
compound is electrically neutral (see Table
10.3
).
Size ratio of ions
: Usually ionic lattices can be described with close packing of
the big negatively charged ions (anions), where the small, positively charged ions
(cations) occupy the interstitial sites between the anions. According to the radius
ratio rule different interstitial sites are occupied, depending on the size difference
(see Fig.
10.19
). If the cation is relatively big, the anions form a simple cubic
packing in which the cations occupy the
hexahedral sites
: cesium chloride is one
example for this kind of ionic structure.
Fig. 10.18 Mental models for charge balanced arrangements of ions (ratio 1:1 and 1:2)
Table 10.3 Examples for ion ratios in ionic structures and corresponding formulae
Charge of
Ratio of numbers
Cations
Anions
Cations:anions
Examples for ion lattices
1+
1
1:1
NaCl, CsCI, NaNO
3
, NaH
2
PO
4
2+
2
1:1
CaO, ZnS, FeS, BaSO
4
2+
1
1:2
CaF
2
, MgCl
2
, Fe(OH)
2
4+
2
1:2
TiO
2
, PbO
2
1+
2
2:1
Li
2
O, Na
2
S, Na
2
SO
4
3+
2
2:3
Al
2
O
3
,Fe
2
O
3
,Fe
2
(SO4)
3
1+, 2+
1
1:1:3
KMgCl
3
2+, 2+
2
1:1:2
CuFeS
2
2+, 3+
2
1:2:4
MgAl
2
O
4
2+, 4+
2
1:1:3
CaTiO
3
