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When an electron absorbs a photon, it gets into a higher energy state and goes into the upper orbit/shell.

Does (rather should) this absorption of energy also have an impact on its mass (although incredibly small)?

Can we even measure the mass of an electron while it is it still bound to the nucleus?

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    $\begingroup$ Depends on which mass you are refering to..are you speaking on Gravitational Mass or Inertial Mass or Rest Mass? $\endgroup$
    – Aron
    Dec 1, 2014 at 17:09
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    $\begingroup$ @Aron That is a highly misleading statement. I'm even tempted to say that it is downright wrong since - as far as we know - inertial mass and gravitational mass are the same. Moreover, unless you are trying to distinguish them with some major subtelty (like the mass-energy density) rest mass is also equivalent to the other two terms. I'm not sure what you are trying to get at, but I think it is really confusing the issue at hand. $\endgroup$
    – Geoffrey
    Dec 1, 2014 at 23:57
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    $\begingroup$ @Aron No, you are very wrong. By the equivalence principle of GR inertial and gravitational masses are exactly the same. And they are equal to rest mass. If you show otherwise, it'll be a major discovery. $\endgroup$
    – Ruslan
    Dec 2, 2014 at 5:53
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    $\begingroup$ @Ruslan. I think you missed my point completely. When i said to a few ppm, that is that it has been experimentally confirmed to a few ppm. My major point is that there are different quantities called mass. Some of them scalar. $\endgroup$
    – Aron
    Dec 2, 2014 at 6:10
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    $\begingroup$ Just to add that, for electrons interacting with a lattice of atoms (most notably in semiconductors), there is also the concept of "effective mass" to be considered. It's just a device to summarize the effect of the interaction (more or less like 'relativistic mass'), but it comes in handy when dealing with crystals. $\endgroup$
    – Peltio
    Dec 2, 2014 at 8:13

6 Answers 6

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A fundamental particle's rest mass never changes. It's mass is a natural constant, and one of the numbers which uniquely identifies it (like its spin). On the other hand, the invariant mass of the atomic system does increase as the electron becomes excited, bringing the atom into a higher energy state. In that sense, the atom (not the electron) gets "heavier" because of the increased energy of the internal configuration of particles.

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  • $\begingroup$ So the atom on the whole gets heavier while the material of is composition remain at the same mass? By material I mean it's particles. So the increment in overall mass of a photon absorbing atom increases due to it's energy component instead of particles' mass increment? $\endgroup$
    – Youstay Igo
    Dec 2, 2014 at 20:04
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    $\begingroup$ Basically, yes. The conceptual explanation relies on the whole $E=mc^2$ idea. Roughly speaking, the increased energy of the atom translates into an increased mass of the atom through relativistic effects. I think that John's answer is an excellent explanation. $\endgroup$
    – Geoffrey
    Dec 2, 2014 at 20:11
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This is really an extended comment to Geoffrey's answer, so please upvote Geoffrey's answer rather than this.

The mass of a hydrogen atom is $1.67353270 \times 10^{-27}$ kg. If you add the masses of a proton and electron together then they come to $1.67353272 \times 10^{-27}$ kg. The difference is about 13.6eV, which is the ionisation energy of hydrogen (though note that the experimental error in the masses isn't much less than the difference so this is only approximate).

This shouldn't surprise you because you have to add energy (in the form of a 13.6eV photon) to dissociate a hydrogen atom into a free proton and electron, and this increases the mass in accordance with Einstein's famous equation $E = mc^2$. So this is a direct example of the sort of mass increase you describe.

However you can't say this is an increase of mass of the electron or the proton. It's an increase in mass of the combined system. The invariant masses of the electron and proton are constants and not affected by whether they're in atoms or roaming freely. The change in mass is coming from a change in the binding energy of the system.

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In the example you site, you are talking about a bound electron. In this case, the electron does not gain (any kind of) mass because the energy of the photon goes into changing the state of the electron (to a higher energy state). This energy is "given back" when the electron returns to its previous state, giving off an equivalent photon.

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No. The mass of an electron does not change with its increased energy (by absorbing photon energy).

A photon does not have rest mass but energy, and such photon energy does not convert into mass, in whatever form - as per the equation E=mc^2.

The photon energy rather converts into higher frequency, not mass, as can be seen in Max Planck's equation E=hv (eich nu) or, E=hf. Photon energy rather converts into frequency, as a result such energy excited election should increase its energy and frequency of its spin around the nucleous, not increasing mass of the electron or mass of the atom itself, but increament in it's energy frequency.

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  • $\begingroup$ The mass of the atom does increase, as explained by Geoffrey and John. $\endgroup$
    – PM 2Ring
    Mar 14, 2022 at 8:24
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Elegant really, the mass of the whole system increases, not the individual electron

"A fundamental particle's rest mass never changes. It's mass is a natural constant, and one of the numbers which uniquely identifies it (like its spin). On the other hand, the invariant mass of the atomic system does increase as the electron becomes excited, bringing the atom into a higher energy state. In that sense, the atom (not the electron) gets "heavier" because of the increased energy of the internal configuration of particles.

https://physics.stackexchange.com/questions/149744/does-the-mass-of-an-electron-change-with-its-energy-state#:~:text=It's%20mass%20is%20a%20natural,into%20a%20higher%20energy%20state.

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Even a free electron gets heavier under the influence of accelerating photons. Best example are colliders where some amount of photons energy stay on the electron and some amount the electron looses again. Then faster the electron then higher the loose. Physics lives from models and interpretations and your interpretation is nice. It brings closer together the terms energy and mass.

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    $\begingroup$ Only correct if you use a text from the Eisenhower administration (to quote a Physics SE regular). The invariant mass remains invariant. Nor is this answer helpful for a bound electron which does not have a well defined momentum. $\endgroup$
    – dmckee --- ex-moderator kitten
    Dec 1, 2014 at 22:59
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    $\begingroup$ I thought there is no such thing as "invariant mass" as all matter of our universe is in a state of constant motion. So all "rest masses" are a bit misleading if you keep the bigger picture of cosmos in view. No? $\endgroup$
    – Youstay Igo
    Dec 2, 2014 at 20:09
  • $\begingroup$ @YoustayIgo: Nice. $\endgroup$
    – HolgerFiedler
    Dec 2, 2014 at 20:12
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    $\begingroup$ @YoustayIgo This idea which you explain in you comment here is a common misconception caused by this thing people hear about moving particles becoming more massive. Under Special Relativity a fast-moving particle is generally harder to accelerate than Newton's Laws would predict which has often - and misleadingly - been described as an increase in the mass of the particle when, in reality, it is simply a fact of Relativistic Mechanics that it isn't Newtonian Mechanics. Relativity is a brave new world. Embrace it on its own terms. $\endgroup$
    – Geoffrey
    Dec 2, 2014 at 20:53
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    $\begingroup$ @YoustayIgo The invariant mass of a particle or system is defined as the length of the energy-momentum four vector. As such it is a Lorentz scalar and is measured to be the same in any frame of reference. All Lorentz scalars (including the proper-time) have that property, so people who are serious about relativity lean heavily on them because they simply all kinds of computations. A large fraction of relativists only talk about invariant mass, eschewing as unnecessary, obsolete and confusing the notion of "relativistic mass"; which is not to say that this concept can't be defined and used. $\endgroup$
    – dmckee --- ex-moderator kitten
    Dec 3, 2014 at 20:19

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