Is there a force between a particle of ionic compound?

  1. Is there a force between a particle of ionic compound? (such as salt) (image that I uploaded), I know that there are intermolecular force between atoms, molecules, ions but how about ionic compound? Thank you.

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  2. jcsd
  3. Borek

    Staff: Mentor

    Technically there is no such thing as a "particle of an ionic compound", unless you are talking about some exotic situations (say, NaCl in gas phase, or ion pairs in low polarity solvents).
  4. Borek is wrong, to the point of ridiculousosity. (yeah, I made that word up) Of course there are particles of ionic, as well as covalent compounds. One term for it is "dust". Anyone living by the ocean is aware that salt spray can deposit fine particles (crystals) of salt. (Which is another, more common, term for them).(Not saying they're pure or anything.)
    The crystals or particles themselves will be almost electrically neutral...I say "almost" because friction and particle impacts can dislodge electrons (temporarily) and create charge (so can ionizing radiation, if you want a stretch). But lets ignore that effect.
    So, what we have is a material composed of positive charge centers and negative charge centers (anions and cations). Any charge separation creates what we call an electric dipole, which is an electric field. Cut to the chase and the charge separation will on average create an attractive force between particles, BUT the strength of this force is miniscule for any particle that you can see. That is, its only important for distances on the order of the size of small molecules. So, yeah you can have attractive forces, but unless the two surfaces are very close together, the + and - charges interior to them 'average out' and result in virtually no net force. IOW yes but only if the two particles are very very close to one another. Look up Intermolecular Forces, London Forces, or Dispersion Forces on wikipedia. (but note that those explanations tend to ignore the effect between ionic crystals, you have to read in between the lines.)
    There's lots that's been writen with regard to the formation of the Solar System and the tendency of small particles to form large particles, the dipole forces are one of the considerations in building models of how the Solar System developed. It has lots of other applications, too.
    Last edited: Mar 18, 2014
  5. DrDu

    DrDu 4,105
    Science Advisor

    If you ever did a titration of, say, chloride with silver nitrate solution, you know that the silver chloride crystalites remain in suspension until the equivalent point is reached when suddenly all AgCl falls out. The reason is that superfluous chloride ions remain absorbed to the crystals charging them up negatively so that they repell each other.
    This is also how colloids are stabilized.
    However, if the crystals are neutral in sum, there are only weak forces between the crystals, as most of the multipole moments of an arrangement of many positive and negative charges vanish which leads to a rapid decrease (with a high negative power of distance) of forces between crystals.
  6. Borek

    Staff: Mentor

    Sorry, my mistake. "Particle" translates to Polish "cząsteczka" which means both "particle" and "molecule". Somehow I read the question as referring to "molecules".
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