# Bonding energy

This at first may sound like a question based in chemistry, but I feel it is more of a particle physics question.

When a chemical reaction occurs, it can be exothermic or endothermic. It is my understanding that if bonds are formed, energy is released, and when bonds are broken, energy is used. The explanation that I heard was that when two atoms are bonded, they have less mass than if they were separate, and the extra mass is released as energy, according to E=mc^2.

But why are do they have less mass when they are bonded? Bonds are essentially just the sharing of electrons, right? (I know there can be ionic, metallic and covalent, but they are all based around the electrons) So what I'm having trouble getting my head around is, if you have a couple of atoms flying around, and then they happen to bond, why does that release energy? How can the transition from an electron moving to an electron staying 'still' release energy? And why do two atoms have less mass when they are joined than when they are separate? Heat is just radiation, right? So where is the radiation coming from when the electrons have stuck?

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## Answers and Replies

mathman
Science Advisor
It sounds like you are confusing nuclear reactions, where these is mass to energy conversion, with chemical reactions. I don't know very much about the latter, but I believe the energy comes from changes in the energy levels of the electrons.

QuantumPion
Science Advisor
Gold Member
It sounds like you are confusing nuclear reactions, where these is mass to energy conversion, with chemical reactions. I don't know very much about the latter, but I believe the energy comes from changes in the energy levels of the electrons.

There is no fundamental difference between the change in mass due to chemical bonds and the change in mass due to nuclear bonds, only the amount of energy involved.

There is no fundamental difference between the change in mass due to chemical bonds and the change in mass due to nuclear bonds, only the amount of energy involved.

So what is the theory behind this? Why do things have less over all mass when they are bonded? The idea of having less mass and so releasing energy makes sense to me (well as much sense at it can, at this stage), but I've yet to come across any reason why the over all mass is less.

So what is the theory behind this? Why do things have less over all mass when they are bonded? The idea of having less mass and so releasing energy makes sense to me (well as much sense at it can, at this stage), but I've yet to come across any reason why the over all mass is less.
All matter wants to degenerate to a state where all mass is converted to pure energy (heat), subject to constraints. One constraint is conservation of baryon number. So a neutron (baryon number 1) will decay to a proton (baryon number 1) because the proton is lighter, and the process stops there, because the proton is the lightest baryon.

Bob S

All matter wants to degenerate to a state where all mass is converted to pure energy (heat), subject to constraints. One constraint is conservation of baryon number. So a neutron (baryon number 1) will decay to a proton (baryon number 1) because the proton is lighter, and the process stops there, because the proton is the lightest baryon.

Bob S

This is the kind of thing I'm looking for. So how does this apply to chemical bonding, where the bond is the electromagnetic force between protons and electrons? As far as I'm aware the proton number in the individual atoms of the molecules doesn't change, so where is the heat coming from when a bond is formed in this case?

This is the kind of thing I'm looking for. So how does this apply to chemical bonding, where the bond is the electromagnetic force between protons and electrons? As far as I'm aware the proton number in the individual atoms of the molecules doesn't change, so where is the heat coming from when a bond is formed in this case?
It's still the same thing. The mass of a hydrogen molecule is about 4.5 eV less than the sum of masses of two hydrogen atoms. See

http://hyperphysics.phy-astr.gsu.edu/hbase/molecule/hmol.html

So the natural form of hydrogen gas is a diatomic molecule. The molecular bonding energy is probably released as a ~2800-Angstrom photon.

Two hydrogen atoms have a total mass of ~ 2(938.3 + 0.511 - 13.6·10-6) MeV.

Bob S