1. Review: Common Metal Structures
hcp
ccp (fcc)
bcc
ABCABC
not close-packed
ABABAB
Features
Filled outer shells  spherical atom cores, isotropic bonding
Maximize number of bonds  high coordination number
High density

2. Ionic Bonding & Structures
Isotropic bonding; alternate anions and cations
Which is more stable?  + – + – – +
Just barely stable

3. central atom drawn smaller than available space for clarity

4. Radius Ratio Rules CN (cation) Geometry min rc/RA 2 none (linear) 3
0.155
(trigonal planar) 4
0.225
(tetrahedral)
Consider: CN = 6, 8 12

5. Octahedral Coordination: CN = 6
rc + RA
2RA
rc + RA a
2RA

6. Cubic Coordination: CN = 8
2(rc + RA) a
2RA

7. Cuboctahedral: CN = 12
2RA
rc + RA
rc + RA = 2RA
rc = RA  rc/RA = 1

8. CN
Geometry
min rc/RA 6
0.414
(octahedral) 8
0.732
(cubic) 12 1
(cuboctahedral)

9. Radius Ratio Rules CN (cation) Geometry min rc/RA (f) 2 linear none 3
trigonal planar
0.155 4
tetrahedral
0.225 6
octahedral
0.414 8
cubic
0.732 12
cubo-octahedral 1
if rc is smaller than fRA, then the space is too big and the structure is unstable