Ionic – Bonding and Crystal Structure. Valence and Lewis Bond Theory metals and non-metals exchange electrons eg. Na 2 O O [He]    2s 2 2p 4 Na [Ne]

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Presentation transcript:

Ionic – Bonding and Crystal Structure

Valence and Lewis Bond Theory metals and non-metals exchange electrons eg. Na 2 O O [He]    2s 2 2p 4 Na [Ne]  3s 1 Na [Ne]  3s 1 2 Na 1+ [Ne] 3s 0 O 2- [He]  2s 2 2p 6

Crystal Structures Ions 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. CNr - /r + Hole/packing 81.0 – 1.37body centered packing – 2.44octahedral holes – 4.55tetrahedral 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 Crystals 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 Holes The coordination number of these ions is 6. # n ions = n holes.

Tetrahedral Holes The coordination number of these ions is 4 # n ions = 2n tetrahedral holes.

Ionic Compounds Since anions are often larger than cations, ionic structures are often viewed as a close-packed array of anions with cations added, and sometimes distorting the close-packed arrangement. 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 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 Sphalerite structure- Anions (S 2- ) ions are in the frame with cations (Zn 2+ ) in half of the tetrahedral holes.

4. The Fluorite (CaF 2 ) and Antifluorite structures A frame of Ca 2+ ions with F - ions in all of the tetrahedral holes.