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Opposites attract, as the saying goes. This attractive force is called an electrostatic force, which holds together ionic compounds by forming ionic bonds. What determines the strength of an ionic bond?
Electron Dot Diagrams
To illustrate how ionic bonds form, we can use electron dot diagrams, or Lewis structures. For example, the bonding between sodium and chloride in a 1:1 ratio can be demonstrated by the following:
Remember: make sure to include the + and - to indicate which is the cation and anion!
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Ionic Crystal Structure
While the dot diagrams show ionic compounds in discrete molecules, this is not actually the case. Ionic compounds create a crystal lattice structure with alternating anions and cations. How does the shape of a lattice structure maximize the system's potential energy?
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The coordination number is the number of ions that immediately surround an ion of the opposite charge within a crystalline lattice structure. What would a 1:1 ratio between ions (i.e. in NaCl) suggest about the coordination numbers? In a crystal of iron(III) chloride (FeCl3), how many more times chloride ions are there as iron(III) ions?
Remember: the shape of the crystal depends on the coordination numbers – not all crystals look the same!
The color of a compound can also be affected by the materials coordinated to a central ion.
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Physical Properties of Ionic Compounds
The melting points of ionic compounds tend to be high. Why would this be?
Ionic compounds are generally hard, but also brittle – they shatter easily.
Think about it: the ions are packed together in a neat lattice structure of alternating charges, but same charges repulse each other. If you shifted the structure so that the charges no longer alternate, what would happen?
Ionic compounds are also good at conducting electricity when melted or dissolved. Why would this be? How would an electric current run through the solution?
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