Ionic bonding
Many compounds can be thought of as a
collection of ions (Mn+, Xn-) held together
electrostatically
This idea arose out of experiments by
Arrhenius looking at the conductivity of
solutions prepared by dissolving “ionic
compounds” in water
– Not believed at first, but got the 1903 Nobel
prize
Characteristics of ionic compounds
Most simple ionic compounds tend to form hard
and brittle crystals
They usually have high melting points
– several hundred or thousand Kelvin
» however, salts that are liquid at room temperature have been
prepared using organic cations
When molten they conduct electricity
Most dissolve in high polarity solvents to form
conducting solutions
1
The ionic to covalent continuum
In practice, no compound is truly ionic
Compounds containing elements with very
different electronegativities tend to be more
ionic
Ionic size
Cations are always smaller than the parent atom
and anions are always larger than their parent
atoms
– outermost electrons in a cations experience a higher
effective charge than the outer electron in the neutral
atom would
» Na 186 pm but Na+ 116 pm
– outermost electrons in a anions experience a lower
effective charge than the outer electron in the neutral
atom would
2
Determining ionic radii
Many different ways to do this. Each gives
slightly different answers. Be consistent with the
source of you data when doing calculations
– A good way involves measuring electron density in
crystals. Minimum in density between ions is the
boundary between ions
Electron density map for NaCl
Effect of ion charge
Isolectronic ions get smaller as the nuclear charge
goes up
Ion
Radius / pm
Ion
Radius / pm
Νa+
116
Ν3-
132
Mg2+
86
O2-
124
Al3+
68
F-
117
3
Periodic trends in size
Similar to those found for atoms
Increase down a group decrease across a
period assuming the ion has the same
charge
Ion
Radius / pm
F-
117
Cl-
167
Br-
182
I-
206
Trends in physical properties
Decreasing ion size and increasing ion charge
favor better binding of the solid (higher lattice
energy)
– this tends to give increased melting and boiling points
Compound
Melting point / ºC
Compound
Melting point / ºC
ΚF
857
NaF
988
KCl
772
MgF2
1266
KBr
735
AlF3
1291 sublimes
KI
685
4
Polarization and covalency
“Ionic” compounds tend have a
considerable covalent contribution to their
bonding when they contain polarizing
cations
– polarizing cations are cations capable of
distorting the anion’s electron cloud towards
the cation
Fajan’s rules
Small highly charged cations are more
polarizing
Large highly charged anions are more
polarizable
Polarization is favored for cations that do
not have a noble gas electron configuration
– Ag+, Cu+, Zn2+, Cd2+, Hg2+, Tl+ etc.
5
Physical effect of covalency
“Ionic” solids with a significant covalent
contribution to bonding show “anomalous”
physical properties
– may not be water soluble AgCl, CuI etc.
– AlF3 MP 1290 oC, AlI3 MP 190 oC
Hydration of ions
Ionic solids are usually soluble in water because
the dipole on water interacts with the ion charges
– negative end of dipole coordinates to cation
– strength of interaction increases with decreasing cation
size and increasing charge
Strong coordination may lead to the formation of
hydrates
– [Al(OH2)6]3+3Cl-
6
Ion hydration on disolution
Structures of ionic compounds
It is often convenient to think about the
cations lying in holes (interstices) between
arrays of anions
Typically, assume ions are hard spheres
Usually, a compound will adopt a structure
that maximizes the number of anions
around each cation without causing the
anions to touch
7
Radius ratio rules
It is possible to predict the type of ion
coordination that you will get if you know
the ratio of the cation to anion size
r+/r- values
> 0.732
Preffered coordination
number
8 – cubic coordination
0.414 – 0.732
6 – octahedral coordination
0.225 – 0.414
4 – tetrahedral coordination
How the limiting values were
calculated
8
Structures with simple cubic packing
A simple cubic array contains holes that are
eight coordinate
– structures include CsCl and CaF2
Structures with octahedral cation
coordination
Close packed arrays of anions have both
octahedral and tetrahedral interstices
– filling octahedral holes in a cubic close packed
array gives the NaCl structure
– filling octahedral holes in a hexagonal close
packed array gives the NiAs structure
9
Holes in close packed arrays
There are one octahedral and two
tetrahedral holes for every atom in a close
packed array
x Marks octahedral holes
x Marks tetrahedral holes
The NaCl structure
10
Tetrahedral coordination
Structures based on filling tetrahedral holes
in close packed anion arrays are commonly
found
– fill all tetrahedral sites in cubic close packed
array - ZnS zinc blende
– fill all tetrahedral sites in a hexagonal close
packed array - ZnS Wurtzite
ZnS structures
11
Violations of the radius ratio rules
Radius ratio rules only work for ~2/3 known
compounds
– ions are not really hard spheres
– covalent contribution to bonding can mess things up
– ionic radius varies with coordination number
There are empirical methods that can be used to
reliably predict structure
– structure maps
Structure maps
Structure map
for AB compounds
The structure of a compound can be predicted based on the difference
in electronegativity between the elements and the average principle
quantum number of the valence orbitals
12
The bond triangle
There is a continuum of different bonding
types
13