# Does electron mass decrease when it changes its orbit?

I have studied this in my chapter atomic structure that when an electron changes its orbit from lower energy to higher energy state , it does not state in my book that it moves there but that it disappears.It just kind of reaches that orbit without crossing the path.We don’t know whether it moves there or not.

So , I also read about the fundamental properties of electron in which it said that when an electron moves or is in motion , it’s mass changes.

If we see , We know that electron only absorbs certain wavelength of light. If we can find that when it changes its orbit , if it’s mass decreased or not during that interval.Is it that we can find that whether electron moves and changes orbit or it just disappears kind of.

Yes , I am only talking electron in an hydrogen atom here.

Please do give your answer in reference to other atoms if it is helpful.

• What is called an "electron orbit" is not something like a planetary orbit but a quantum state. Quantum states mostly escape human intuition. – Gyro Gearloose Dec 31 '20 at 13:06
• Ohk.what I did write about is neils Bohr theory of atomic structure.We also know that his theory did came right without much explanation but with experiments. – srijan Sri Dec 31 '20 at 14:05
• Note that an electron need not to change its location to change its quantum state. The wave functions aka orbitals with different quantum numbers spatially overlap. – Poutnik Dec 31 '20 at 14:39
• – Andrew Dec 31 '20 at 17:20

## 3 Answers

I'd like to copy the answer by John Rennie to a similar question at Physics.SE, since it's much better than current answers here IMHO. Although it mostly speaks about binding energy, the same principle applies to the quanta of excitation energy.

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.

• Yes, this is an important point, the masses of the particles don't really change. If they did, then fundamental particles like electrons would cease to be indistinguishable (you could distinguish them by their different masses!), and this would wreak incredible havoc in their quantum mechanical description. – Nicolau Saker Neto Dec 31 '20 at 23:31
• Wanted to post the same link as Ruslan. Comment is to stress that this are confusing/difficult concept and this answer picked up a nice one that I recommend to OP – Alchimista Jan 1 at 9:01

Does electron mass decrease when it changes its orbit?

Essentially yes. If you add the mass of a free proton and a free electron you'll get a greater mass than that of a hydrogen atom. The mass difference will be equivalent to 13.6 eV which is the ionization energy of hydrogen.

Now for any "practical" chemistry experiment the assumption is that masses don't change. The reason is that the mass change is so small that it is undetectable with ordinary chemical laboratory procedures.

There is also another weird effect here. For the very heavy elements the binding energy, and the orbital so small that the particle model of the electron has the electron traveling at relativistic velocities. This increases the apparent mass of the electron.

• I disagree on purely formal aspects. The system as such surely changes energy, but I don't see any reason why or how to attribute it to the mass of the electron. Nitpicking, but has to been said. – Gyro Gearloose Dec 31 '20 at 20:22
• It is a change in kinetic energy of the electron, too, so we actually know that the mass is changing if we consider relativistic effects. – Greg Jan 1 at 2:37
• @Greg your comment use the relativistic mass idea which is confusing. Better to stick to system atom (nucleus and electrons) and say energy input - - > increase in rest mass, as it is for a bottle of gas. – Alchimista Jan 1 at 8:51
• In spite of my comment @Greg, essentially the energy goes to the electron because its mass is small. However the answer is nice as it is about proton and electron. It says "essentially"... – Alchimista Jan 1 at 8:59
• @Alchimista There is also a tiny difference between invariant and rest mass, as the former is not necessarily the latter. – Poutnik Jan 1 at 10:43

The Bohr Model tried, quite successfully for its time, to model the energy states of an electron.

This model has turned out inadequate, as it cannot answer question like yours.

There have been more refined models (where your question isn't possible), but they all have their drawbacks.

Not an answer to what you were asking, but some kind matching your question.

• In my younger days I was fooled be thinking of hydrogen atoms as of planets orbited by a moon. This would have given them a disk-like 2-dimensional shape with really strange properties. – Gyro Gearloose Dec 31 '20 at 14:10
• We have quantum and Newtonian mechanics.Is it that my question is of point of view from Newtonian and not quantum mechanics.Conclusion : Bohr model is actually wrong and so answer does not exactly become valid since it is mostly on the basis of only Neils Bohr theory – srijan Sri Dec 31 '20 at 14:11
• @srijannahar The question as in the title still makes sense in context of the Special Relativity and relativistic wave equations. – Poutnik Dec 31 '20 at 14:21
• @Poutnik yes, but with very different aspects. – Gyro Gearloose Dec 31 '20 at 14:26
• @GyroGearloose Sure. In context of non-relativistic quantum mechanics, Schroedinger wave equation and classical Newtonian mechanics, an electron mass is constant. – Poutnik Dec 31 '20 at 14:28