Ionic bonding Characteristics of ionic compounds
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
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
