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Ionic Compounds

  1. Aug 7, 2012 #1
    Why are ionic compounds hard and brittle ?

    For instance, an alloy is a mixture of 2 or more elements and thus does not break easily relative to original elements. Shouldn't the ionic compounds also be like the alloys ?
  2. jcsd
  3. Aug 9, 2012 #2
    An alloy usually involves metallic elements and metallic bonding, very different from ionic bonding. I think ionic compounds are hard and brittle, because its component atoms are very strongly bound to each other and form a crystal structure, which means atoms don't have much leeway to move. In mettallic compounds, on the other hand, atoms are as if swimming in a sea of their (and other atom's) electrons, meaning they can slide and move morea easily.
  4. Aug 11, 2012 #3
    imagine a crystal of alternating Na+ and Cl- ions. You take a hammer and hit it. The Na+ and Cl- ions move around their equilibrium points, as you expect, and return to equilibrium after vibrating. However if you hit the crystal hard enough, you'll dislocate a section of it that happens to align Na+ on one face to Na+, and you get electrostatic repulsion that breaks the crystal apart very quickly. This is the rough reason for why cracks propagate extremely quickly in ionic crystals.
  5. Aug 13, 2012 #4
    Thanks to both of you :) I now understand how it works.

    Concerning the dislocation of the lattice structure, What did you meant by " align Na+ on one face to Na+ " ?

    Isn't the Na+ surrounded by 6 Cl- ions ?
  6. Aug 16, 2012 #5
    Maybe in the depths of the lattice, yes, but what about on the edges / faces?
  7. Aug 21, 2012 #6
    So I presume each lattice isn't connected to each other by ionic bonding, am I right ?

    One lattice has a layer on it ( inside which are the ions ) and which is could be cracked by hitting and then the whole object dislocating with electrostatic repulsion.

    That's what I could imagine.
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