Valence and Lewis Bond Valence and Lewis Bond TheoryTheory metals and non-metals exchange electrons
eg. Naeg. Na22OO
O [He] 2s2 2p4
Na [Ne] 3s1
Na [Ne] 3s1
2 Na1+ [Ne] 3s0
O2- [He] 2s2 2p6
Crystal StructuresIons are viewed as hard spheres. The packing pattern adopted
provides the greatest energy stability.
The packing pattern is: the larger ion creates the overall
frame
the smaller ion is placed in the “holes” created by the frame
the type of frame depends on the ratio of the radii of the two ions
Predicting Crystal Structures
General “rules” have been developed, based on unit cell geometry, to predict crystal structures using ionic radii.
Radius ratios, usually expressed as the (radius of the anion)/(radius of the cation) are used. CN r-/r+ Hole/packing 8 1.0 – 1.37 body centered packing 6 1.37 – 2.44 octahedral holes
4 2.44 – 4.55 tetrahedral holes
Body Centered Ionic crystal
Occurs when the ion sizes are very similar.
The smaller ion fits in the center of a cube formed by 8 opposite ions.
Holes in Close Packed Holes in Close Packed CrystalsCrystals
Octahedral holes lie within two staggered triangular planes of ions
There are 2 types of holes created
Tetrahedral holes lie within a triangular plane with another ion on top
Tetrahedral HolesTetrahedral Holes
The coordination number of these ions is 4
# n ions = 2n tetrahedral holes.
Ionic CompoundsIonic Compounds
Since anions are often larger Since anions are often larger than cations, ionic structures than cations, ionic structures are often viewed as a close-are often viewed as a close-packed array of anions with packed array of anions with cations added, and sometimes cations added, and sometimes distorting the close-packed distorting the close-packed arrangement.arrangement.
Common examples:Common examples:
1. Rock Salt (NaCl) Is viewed as an array
of the anions, with the cations in all of the octahedral holes, or
The other way around – depending on which is larger
The coordination number is 6 for both ions.
2. The CsCl structure2. The CsCl structure
Chloride ions occupy the corners of a cube, with a cesium ion in the center or vice versa.
Both ions have a coordination number of 8, with the two ions fairly similar in size.
3. The Zinc-blende or 3. The Zinc-blende or Sphalerite structure-Sphalerite structure-Anions (S2-) ions are in the frame with cations (Zn2+) in half of the tetrahedral holes.
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