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My book says,

A molecule as compared to the atoms from which it is formed is more stable because it possesses energy lower than the energy of the uncombined atoms. This difference in energy is due to the fact that when atoms combine to form molecule, the attractive forces are created which result in release of energy.

And, I found out a theory for explanation which says that all that happen because the bonds between these atoms reduce their kinetic energy as they are not free to move like they move in unbonded state and therefore the energy is released in form of heat energy. So, maybe in this way the attractive forces/bonds release energy. No?

Then what is the actual reason a molecule has less energy than its individual constituents?

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And, I found out a theory for explanation which says that all that happen because the bonds between these atoms reduce their kinetic energy as they are not free to move like they move in unbonded state and therefore the energy is released in form of heat energy. So, maybe in this way the attractive forces/bonds release energy. No?

When we are talking of atoms and molecules we are in the quantum mechanical regime. In quantum mechanics the atoms of the molecule are trapped in a effective potential well made up by the in total attractive forces from the spill over electric and magnetic fields of the individual atoms . In the same way that an electron is trapped at a lower energy level in the hydrogen potential well, the molecules are trapped in a lower energy level for the agglomerate . The extra energy leaves as a photon, as heat has no meaning at an individual molecular state. Heat is a thermodynamic manifestation of an ensemble of molecules. In this sense a photon could interact with the ensemble of molecules and be part of the heat energy of the total ensemble, but not a molecule by molecule solution.

With this as background let us examine:

And, I found out a theory for explanation which says that all that happen because the bonds between these atoms reduce their kinetic energy as they are not free to move like they move in unbonded state and therefore the energy is released in form of heat energy. So, maybe in this way the attractive forces/bonds release energy. No?

Yes, the binding of two or more atoms in a molecule releases energy to the ensemble of molecules in the form of radiation which contributes to heat. Within the molecule the kinetic and potential energy of the atoms have no one-to- one meaning because the atoms move in probability loci, not classical orbits. The general concept of conservation of energy though holds.

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  • $\begingroup$ I don't see how van der Waals forces are related to the topic ) $\endgroup$
    – freude
    Mar 15 '16 at 15:35
  • $\begingroup$ @freude as a generic term, of spill over electric and magnetic fields. May be I will replace it with a description, thanks. $\endgroup$
    – anna v
    Mar 15 '16 at 16:23
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There are many forces contributing the potential energy curve (dependence of the molecule energy on the configurational coordinates) for multiatomic molecules. Some of them reduce the energy, while others enhance it. The Coulomb repulsion between positively charged nuclei gives positive contribution as well as repulsion between electrons. The Coulomb attraction between electrons and nuclei together with electron exchange interaction (which is purely quantum effect) reduce the energy. Mainly because of the electron exchange interaction the resulted energy is lower in most of stable molecules with covalent bonds comparing to their dissociated counterparts.

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  • $\begingroup$ Can you please point out as well what is wrong in my theory? $\endgroup$
    – user104909
    Mar 15 '16 at 14:47
  • $\begingroup$ Even being bonded the atoms are moving by vibrations. Also, if it would be true, how it would explain the fact that there are unstable molecules that are not able to form bonds at normal conditions - not all atoms are forming molecules $\endgroup$
    – freude
    Mar 15 '16 at 14:58
  • $\begingroup$ If they are not able to form bonds then its not my concern nor of my theory. Is it? $\endgroup$
    – user104909
    Mar 15 '16 at 15:02
  • $\begingroup$ Yeah but in a way it may look paralise. Because, atoms may keep vibrating with same speed which essentially means their kinetic energy may remain same. But what's next? Your answer doesn't tell me why bonds reduce energy. please explain. $\endgroup$
    – user104909
    Mar 15 '16 at 15:06
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    $\begingroup$ By the way, you did not provide any theory, although you call it theory, because you made a statement, but did not try to prove it using any experimental or logical (mathematical) evidences. $\endgroup$
    – freude
    Mar 15 '16 at 15:41

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