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Ionic Crystal Structure

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Level 1 - Ionic & Covalent Crystal Structures
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Credit: Adrian Pingstone (Wikimeda: Arpingstone)
Source: http://commons.wikimedia.org/wiki/File:Ruby_cristal.jpg
License: CC BY-NC 3.0

Why are crystals appealing?

Crystals are found everywhere chemical deposits are located.  The ruby crystal shown above is extremely valuable, both because of its beauty and its utility in equipment such as lasers.  For some people, crystals are said to have magical qualities.  For others, the “magic” is in the regular structure of the crystal as the cations and anions line up in a regular order.

Ionic Crystal Structure

Electron dot diagrams show the nature of the electron transfer that takes place between metal and nonmetal atoms.  However, ionic compounds do not exist as discrete molecules, as the dot diagrams may suggest.  In order to minimize the potential energy of the system, ionic compounds take on the form of an extended three-dimensional array of alternating cations and anions.  This maximizes the attractive forces between the oppositely charges ions.  The figure below shows two different ways of representing the ionic crystal lattice.  A ball and stick model makes it easier to see how individual ions are oriented with respect to one another.  A space filling diagram is a more accurate representation of how the ions pack together in the crystal .

Credit: (A) Eloy; (B) Ben Mills (Wikimedia: Benjah-bmm27)
Source: (A) http://commons.wikimedia.org/wiki/File:NaCl-estructura_cristalina.svg; (B) http://commons.wikimedia.org/wiki/File:Sodium-chloride-3D-ionic.png
License: CC BY-NC 3.0

Two models of a sodium chloride crystal are shown. The purple spheres represent the Na + ions, while the green spheres represent the Cl ions. (A) In an expanded view, the distances between ions are exaggerated, more easily showing the coordination numbers of each ion. (B) In a space filling model, the electron clouds of the ions are in contact with each other. [Figure2]

Naturally occurring sodium chloride (halite) does not look at first glance like the neat diagrams shown above.  It is only when we use modern techniques to analyze the crystal structure at the atomic level that we can see the true regularity of the organized ions.

Credit: Ingo Wölbern (Wikimedia: Iwoelbern)
Source: http://commons.wikimedia.org/wiki/File:Halit_crystal,_Pedra_Lume,_Cape_Verde.jpg
License: CC BY-NC 3.0

Halite crystals. [Figure3]

Covalent bonding - giant covalent networks

excerpted from BBC Bitesize

Giant covalent networks contain many non-metal atoms joined to adjacent atoms by  covalent bonds . Atoms are usually arranged into giant regular  lattices  that make these structures extremely strong because of the many bonds involved. The graphic below shows the molecular structure of diamond and graphite, two  allotropes  of carbon, and the molecular structure of silica (silicon dioxide; SiO 2 ), on the far right.

  Properties of giant covalent structures

  • Very high melting points  - Substances with giant covalent structures have very high melting points, because a lot of strong covalent bonds must be broken. Graphite, for example, has a melting point of more than 3,600ºC.
  • Variable conductivity  - Diamond does not  conduct  electricity. Graphite contains free  electrons , so it does conduct electricity. Silicon is semi-conductive - that is, midway between non-conductive and conductive.
Summary
  • Ionic compounds take on the form of extended three-dimensional arrays of cations and anions.
  • The arrangement maximizes the attractive force between oppositely-charged ions.
  • Crystalline structures can be created by giant covalent networks.
Practice

Use the link below to answer the following questions:

http://www.chemguide.co.uk/atoms/structures/ionicstruct.html

  1. How many chloride ions are touched by a sodium ion?
  2. How many sodium ions are touched by a chloride ion?
  3. What contributes to the stability of an ionic compound?
  4. Why do CsCl and NaCl have different structures?
Review
  1. Do ionic compounds exist as discrete molecules?
  2. What does this three-dimensional array do?
  3. What gives the most accurate rendition of how the ions arrange themselves?

Image Attributions

  1. [1]^ Credit: Adrian Pingstone (Wikimeda: Arpingstone); Source: http://commons.wikimedia.org/wiki/File:Ruby_cristal.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: (A) Eloy; (B) Ben Mills (Wikimedia: Benjah-bmm27); Source: (A) http://commons.wikimedia.org/wiki/File:NaCl-estructura_cristalina.svg; (B) http://commons.wikimedia.org/wiki/File:Sodium-chloride-3D-ionic.png; License: CC BY-NC 3.0
  3. [3]^ Credit: Ingo Wölbern (Wikimedia: Iwoelbern); Source: http://commons.wikimedia.org/wiki/File:Halit_crystal,_Pedra_Lume,_Cape_Verde.jpg; License: CC BY-NC 3.0

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