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If i had an ionic molecule which needed 4 eV to break the ionic bond, a 7 eV photon is shot at it. It is absorbed by the molecule and breaks, where do the rest of the energy goes?

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I'm not confident enough to post as an answer, but I'm guessing as heat. Or maybe some electrons end up in an excited state, resulting in light emission when they go back. We'll see when someone more knowledgable comes along :) –  Brian Feb 11 '13 at 18:46
    
The question as posed is confusing. An ionic bond is really just electrostatic attraction between a cation and its neighboring anion. You can speak of how much energy it would take to separate the ions to infinite distance given a starting distance. But 'breaking' the bond as is done photochemically with covalent bonds is not meaningful. –  Rich Apodaca Feb 24 '13 at 0:44

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Breaking an (single) ionic bond is a nonsensical concept. Ionic bonding it the aggregation combination of many electrostatic interactions with a bulk solid. To liberate an ion from an ionic solid you have to break many bonds, not just one.

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Aside of that, a ionic crystal like common salt will vaporize as a mixture of sodium and chlorine atoms, when heated to about 1000 °C. having ions in vacuum needs still higher temperatures, like in electric arcs or in plasma generally. –  Georg Feb 27 '13 at 19:55
    
@Lighthart I think "ionic" in this context is some sort of "non-covalent" bond rather than a crystal. It may be a red herring in terms of the answer to the question. –  Eric Brown Jun 10 '13 at 1:41

The energy can go to several different places:

1) The electrons may get some of the energy. In this case the dissociated atoms will be in excited states.

2) A photon might be emitted.

3) The atoms may have some kinetic energy left over after they are separated.

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