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Back to chemistry after 2 months and got stuck in the beginning! That's why I don't like it. Anyways, come to the question please.

Following are the words from my textbook (Don't ask the name ;-)

$$\ce {Na -> Na^+ + e^-}\quad\Delta{H}=+495 \frac{KJ}{mol}$$

$$\ce {Cl + e^- -> Cl^-}\quad\Delta{H}=-348 \frac{KJ}{mol}$$

$$\ce {Na^+ + Cl^- -> NaCl}\quad\Delta{H}=-786 \frac{KJ}{mol}$$

We conclude that it is essential for the formation of ions and ionic bond that the sum of energies released in second and third reactions must be larger than what is required in the first step.

The great attractive forces (bonds) reduce the energy of the system greatly as it emits 786 KJ of energy.

a. Why should it happen? i.e. sum must be greater et cetera.

b. How does attractive forces tend to reduce the energy?

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  • $\begingroup$ For second question I think the answer is that that bonds reduce the momentum of ions by not letting them move freely and therefore heat energy is reduced and released. But for question a I have absolutely no idea. $\endgroup$ – Sufyan Naeem Mar 14 '16 at 9:45
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Ok your question is a little ambiguous but I'll try.

a. Why should it happen? - If you look at Na, it has a lone electron in its outer shell and that essentially unbalances the atom, so it prefers to lose it and become an ion, this is almost spontaneous and hence requires less energy. When $\ce{Na+}$ joins up with $\ce{Cl-}$ they neutralise the electrostatic charge introduced by them becoming ions, releasing more energy. More energy because they have gone from an energy state A (medium energy) to B (higher energy) to become an ion. Then when they come together they both, each atom, falls to a lower energy state C than state A even and hence the change (NB) from B to C is greater than the change from A to B. In your book the change from my A to B is the change of an electron ( first two equations in your book) and then my change from B to C is the Na + Cl part.

b. How do attractive forces tend to reduce energy? - This is an important one to note the point of the text book, which is pretty badly written if you ask me, is referring to the energy of the system and not the energy of attractive forces. When it says system it means the newly bonded together $\ce{NaCl}$. The energy is reduced because of the neutralisation mentioned above. Positive and negative prefer to be neutral and hence, are attracted to one another. But before that, you must realise that being charged or an 'ion' is less stable and therefore in a state of higher energy than a neutral system. So it is a matter of stability as well.

If you want quantum explanations please say but I get the feeling you'd prefer the simpler option.

I hope this helps, even if only a bit.

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  • $\begingroup$ Why do ions have greater energy as compared to their neutral state? Why would $Na$ have less energy than its ion $Na^+$. And in fact ions have their octet complete so they must be stable which implies they should have less energy. No? $\endgroup$ – Sufyan Naeem Mar 15 '16 at 10:57

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