Ionic –Ionic – Bonding and Bonding and

Crystal StructureCrystal Structure

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

Octahedral HolesOctahedral Holes

The coordination number of these ions is 6.

# n ions = n holes.

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.

4. The Fluorite (CaF4. The Fluorite (CaF22) ) and Antifluorite and Antifluorite

structuresstructuresA frame of Ca2+ ions with F- ions in all of the tetrahedral holes.