The other day when I posted this I was pretty much ridiculed, downvoted and almost mocked at, so probably I did a very bad job at explaining what I meant.

So what I asked: "How chemical bonding happens at its most fundamental level."

So I´ll try again, hopefully doing a better job at explaining.

As far as I understand, there are 2 things governing the universe. 1) The energy of the universe is constant. 2) Every transfer of energy is mediated by one of the 4 fundamental forces. For every action (i.e. energy transfer) we need an exchange particle. And it seems, that pretty much ALL everyday phenomena not due to gravity are caused by electromagnetism. And the exchange particle of electromagnetism is a (virtual) photon. Of course all this is just a convenient visualization. Sometimes it is more useful to think in terms of force-fields instead of the force-carrying-particles (graviton, photon).

I know that human intuition and logic breaks down in the quantum realm, but still, all the contact forces (e.g. friction, heat (this is a big one), kinetic energy of an object, electricity, lifting an object against gravity, chemical bonding ect.) come ultimately down to electrons of the individual atoms repelling each other. And as far as I understand it, when an electron repels another electron, as they get pushed closer and closer, they exchange virtual photons, which ultimately are the repulsion, as the other electron absorbs it gaining energy.

So, when I press my finger on my table, as far as I understand it, the electron clouds in the molecules in my finger are repelled by the electron clouds in the molecules in the table and this repulsion (which is an exchange of energy) is mediated by (virtual) photons, I mean it HAS to since we need an exchange particle to transfer energy. To quote wikipedia: "The Coulomb force (static electric force) between electric charges is caused by the exchange of virtual photons."; "...the photon seems to be a point-like particle since it is absorbed or emitted as a whole by arbitrarily small systems, systems much smaller than its wavelength, such as an atomic nucleus (≈10−15 m across) or even the point-like electron."

Please correct me if I am wrong but it seems that all the contact-forces are due to the electromagnetic force. Everytime we touch something, we are exchanging photons with it as the electron clouds get closer -> photons increase the momentum of the electrons into opposite directions. The opposite happens when a free-moving-electron is attracted to the nucleus, photons are released in the opposite direction as the electron settles into lower and lower energy states. So basically, if one neglects gravity (which it seems is still not very well understood at the quantum level and the "graviton" is still very controversial), and the 2 nuclear forces, the "energy-currency" of the universe is basically the photon. (I know that I must not think of the photon as a "particle-like" particle, but still, "If you think you understand quantum mechanics, you don´t understand quantum mechanics.")

Back to my original question, how bonding happens at the MOST fundamental level. This is how I "visualize" it, being well aware that intuitive/logical visualizations are almost necessarily incorrect when thinking about quantum mechanics, since human intuition does not work there (e.g. virtual particles, wave-particle-duality, uncertainty, quantum randomness ect.)

How bonding works at its most fundamental level: As molecules with high speeds approach each other (i.e. collision theory), the repelling valence electrons of the 2 colliding molecules -all the other spinpaired electrons are attracted to the nucleus and shield the valence electrons- upon approaching each other are repelled by the valence electrons of the other molecule/atom --> their kinetic energy -which is essentially the result of past contact forces having acted upon them (via photons)- is used up as the electrons get closer and closer exchanging photons between each other (which is the repulsion as the photons are absorbed by the valence electron of the other molecule). After both valence elctrons have “bombarded” themselves with photons, they have now both been excited into higher energy levels. From there (as entropy causes everything to move towards lower energy), both valence electrons might now fall down in a lower energy molecular orbital, which they now share together -being attracted to both nuclei via the Coulomb force and in the process releasing the excess energy as photons. A chemical bond has now formed.

It does not matter were the photons came from -either from the kinetic energy of the colliding molecules or from outside heat/radiation sources. These photons are essentially needed to "kick" electrons into the necessary high energy orbitals (which are higher in energy than the molecular orbital that is about to form). This is the so called "activation energy" needed for every reaction to occur. When the electrons then drop down into a common lower-energy-molecular orbital, the energy difference is emitted via photons. If more net photons need to be supplied for a bond to form compared to the total amount of photons (i.e. energy) that are given off after the bond has formed, this we call an endothermic reaction and the excess energy is now stored within the chemical bond and can be released again upon breaking the bond as heat (i.e. photons), as the electrons go back to their initial lower-energy-state they had before the collision. If however more net photons are released upon formation of the bond (i.e. upon the dropping-down into the new molecular orbital), net energy (i.e. number/wavelength of photons) is released and we call this an exothermic reaction.

For both types of reactions the energy needed for the molecular orbital to form must be supplied (this we call the activation energy). Depending on the electron affinities involved, and where the electron pair probabilistically ends up being as it now interacts with nuclei, the bond is now perfectly nonpolar, polar or ionic, depending on which nucleus attracts the molecular orbital more.

Is this a very very wrong way to think about?


I do not think it is very wrong (except for the entropy part), but you are mixing mathematical frameworks, which can lead to confusion.

To start with quantum mechanics can be solved for individual potentials, the one between the electron and the proton, the hydrogen atom, analytically and accurately.

When one starts on many particle potentials and problems the plot thickens. One has to use models that can include many particles. That is where quantum field theory came in, to address the many particle problems in elementary particle scatterings which cannot have an analytic closed solution but can be described by perturbative expansions. The so called second quantization. This depends on the basic free particle solutions ( dirac, klein gordon,quantized maxwell) using an operator formalism working on the free particles which allows to describe many particle interactinons. The Feynman diagrams lead to solutions where the potentials are replaced by virtual particle exchanges (see page on photon) . For accuracy one should sum up higher orders of exchanges which have a diminishing contribution as happens to all series expansions.

Can you describe the energy levels of the hydrogen atom instead of a single analytic solution with a sum of Feynman diagrams? (those would be your virtual photons) . If you are mathematically ambitious you could, but it is like using a scalpel to dig a ditch.

So yes, in principle one could consider all electromagnetic interactions as describable by feynman diagrams with the exchange of virtual photons, but it is not very useful. It is more useful to find approximate analytic solutions, i.e. orbitals for the electrons and effective potentials that describe a bonding pair of hydrogen atoms to make H2, giving off some energy.That is , one finds a solution for the combined problem of the potentials of two protons with two electrons around them, one sees that the molecular energy is lower and thus a bond can form. This becomes more complicated as more atoms combine, but various models exist for bondings and are successful in describing what happens.

It is misleading to think of individual electrons interacting , it is the whole atoms when they join into molecules that lose the energy.


Your understanding is wrong. I'll correct the key errors:

Firstly, you should give up thinking in terms of virtual photons. They are really mathematical 'things' that arise in perturbative QFT. You could equally think in terms of non-perturbative QFT and be without virtual particles altogether.

After both valence elctrons have “bombarded” themselves with photons, they have now both been excited into higher energy levels.

No. The 'virtual photons' don't excite the electrons into higher energy orbitals in the way that, say, photons from a laser do. Rather, their effect is to perturb the potential energy surface and thus distort the wave-functions. The electrons, to a good approximation (see the Born-Oppenheimer approximation), instantaneously relax to the ground-state as the nuclei approach each other.

For this reason, the following points are also wrong:

both valence electrons might now fall down in a lower energy molecular orbital,

This is the so called "activation energy" needed for every reaction to occur.

And lastly,

From there (as entropy causes everything to move towards lower energy)

Entropy doesn't drive systems to lower energy, it typically does the reverse.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.