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In the above graph, I was confused at the point where the internuclear distance increases and potential energy become zero. Though internuclear distance is very small and potential energy has increased to zero.

Another question that though the internuclear distance at a particular point is constant yet potential energy keeps on increasing.

I think the point where potential energy become zero we can say that attractive forces between electron and nucleus of bonding hydrogen atoms just equals the repulsive force between bonding hydrogen electrons. It's just what I am assuming not sure either its true or not.


1 Answer 1


Normally, when the atoms are separated by a huge amount (infinity) we take this energy to be zero. Bringing the atoms closer to one another has an attractive potential and the bond has formed on reaching the lowest point on your curve. Moving the atoms closer together increases the potential energy because here we push electrons closer to one another and this energy increases rapidly with each small decrease in separation due to repulsion of similar charges. The slope of the curve gives the force on the atoms, so at small separation they experience a large force and repel one another. At large separation, where the slope is positive, there is a smaller attractive force.

  • $\begingroup$ Why there is no activation energy for this reaction? $\endgroup$
    – ado sar
    Nov 28, 2020 at 12:43
  • 2
    $\begingroup$ There does not have to be an activation barrier, it just depends on the reaction and how the energy varies with separation of the atoms. Some electron transfer reactions are activationless for example. Many other reactions are so fast because the activation barrier is tiny and the rate constant is limited by how quickly the reactants diffuse together. $\endgroup$
    – porphyrin
    Nov 28, 2020 at 13:38
  • $\begingroup$ @porphyrin How a potential diagram like this be also used for describing intermolecular interactions? I mean in the example of the OP it describes the formation of hydrogen molecule. But we also use Lennard-Jones potential for intermolecular forces. What I can't understand is how to interpret the minima in such cases. Is there a "bond formation" when talking about intermolecular potentials? $\endgroup$
    – Anton
    Apr 21, 2021 at 19:27
  • $\begingroup$ @Anton, there is no electron sharing in this case, polarisability and dipoles provide the interaction and this can have a minimum vs separation of species. Ask a question if you want a complete answer :) $\endgroup$
    – porphyrin
    Apr 22, 2021 at 7:50
  • 1
    $\begingroup$ @Shub If you want an accurate plot then you will have to calculate exactly for say H$^+$ but if you look for a Morse potential ora Lennard -Jones one these give a good approximation. $\endgroup$
    – porphyrin
    Sep 3, 2021 at 7:50

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