Ionic Compounds

  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. 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. 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. 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. Maybe in the depths of the lattice, yes, but what about on the edges / faces?
     
  7. 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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