Topic 12: Ionic Bonding Flashcards
(30 cards)
5 common AX structure types examples
NaCl (Na^+ and Cl^-)
Zinc Blende/ZnS (Zn^2+ and S^2-)
NiAs (Ni^3+ and As^3-)
wurtzite/ZnS (Zn^2+ and S^2-)
CsCl (Cs^+ and Cl^-)
Many simple ionic solids can be described by …
Anions are usually … than cations, therefore consider close-packed … arrays with … in the holes
Remember the close-packed spheres only take up … of the available space
Starting from the packing of spheres
larger
anion
cations
74%
… occur between layers where
the centres of triangles from adjacent layers overlap
Tetrahedral holes
… occur between layers where
the centres of triangles from adjacent layers overlap
Octahedral holes
Tetrahedral holes occur where a … from one layer is over the … above or below
sphere
centre of a triangle in the layer
Octahedral holes occur between layers where the …
centres of triangles from adjacent layers overlap
HCP Spheres, Octahedral holes tetrahedral holes
2
2
4
CCP Spheres, Octahedral holes tetrahedral holes
4
4
8
If the cation becomes larger so can its …
It will push the anions apart until the cation radius gets large enough to … site where they again just touch
coordination number
force the anions to rearrange and form a higher coordination number
Radius ratio =
radius of smaller ion/radius of larger ion
Coordination number 3 shape and radius ratio
trigonal planar
< 0.255
Coordination number 4 shape and radius ratio
tetrahedral
0.225 - 0.413
Coordination number 6 shape and radius ratio
octahedral
0.414 - 0.731
Coordination number 8 shape and radius ratio
cubic
0.732 - 0.999
Coordination number 12 shape and radius ratio
close-packed
1
NaCl/rock salt properties
CCP anions
Octahedral holes filled
Zinc blende/ZnS
CCP anions
1/2 Tetrahedral holes filled to minimise cation-cation repulsions
4 coordination cation
NiAs
HCP anions
Octahedral holes filled
Metal-Metal bonding
Most NiAs structures distort to reduce the M–M distance
NiAs structure only found with transition metal ions that can form M-M bonds by direct overlap of d-orbitals
6 coordination cation
wurtzite/ZnS
HCP anions
1/2 Tetrahedral holes filled to minimise cation-cation repulsions
4 coordination cation
CsCl
Primitive Cubic array
8 coordination cation
Lattice Enthalpy (U)
The amount of energy required to break up a solid and form gaseous ions
AX(s) → A+(g) + X–(g)
High = stable
Hess’s Law
The energy change of a process depends only upon the energies of the initial and final states and not the route taken
z^+/z^- =
charges on the ions
e =
charge of an electron (1.602 × 10^–19 C)
