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Electron affinity of chlorine = -349 kJ per mole. And, first ionization energy of sodium = 496 kJ per mole while energy required for combining= 147 kJ per mole.

Isn't it a contradiction against genereal physics and G.N. Lewis' "electronic theory of valency"?

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closed as off topic by Waffle's Crazy Peanut, Emilio Pisanty, twistor59, Chris White, Brandon Enright Jun 1 '13 at 1:34

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I think this would be better on chemistry.stackexchange.com –  Colin McFaul May 9 '13 at 13:19
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Definitely a chemistry question –  twistor59 May 19 '13 at 11:35
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2 Answers

$$Na+Cl\longrightarrow [Na^+Cl^-]\ \ \ \ ...\Delta H$$ $$\text{Includes:}$$ $$Na\longrightarrow Na^+ + e^- \ \ \ \ \ \ ....\Delta H_1$$ $$Cl+e^-\longrightarrow Cl^-\ \ \ \ \ \ \ \ ....\Delta H_2$$

$$\text{Where:}\Delta H=\Delta H_1+ \Delta H_2$$

After some addition I can't see a problem.

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You are quite correct that the enthalpy change to form an isolated Na$^+$ and an isolated Cl$^-$ ion from gaseous Na and Cl atoms is positive. However you need to include the very large and negative enthalpy change when the ions come together to form solid NaCl. A quick Google found this energy diagram:

NaCl

To calculate the enthalpy of formation of NaCl from solid sodium and gaseous chlorine you need to sum the energies for all the steps. If you do this you find the overall enthalpy change is negtive and that's why NaCl is formed.

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