In many textbooks, energy changes of the hydrogen atom are attributed to the electron transitioning between energy levels. However, the energy itself is that of the whole system (proton+electron) so how can we attribute its changes to the electron? what's preventing us from attributing these changes to the proton??

  • $\begingroup$ I don't know what text book you are using but when we solve the Schrodinger equation for hydrogen atom we solve for the whole atom as a two body problem. $\endgroup$ Apr 23, 2021 at 17:49

2 Answers 2


The energy is indeed that of the whole system, but the electron has a much smaller mass (1/1836) than the proton, so the latter does not contribute much here. But for accurate results you have to consider the proton as well. When you solve the corresponding two-body problem for two masses $m_1$ and $m_2$, you can reduce it to a one-body problem by using the reduced mass

$$\mu=\frac{m_1 m_2}{m_1+m_2}$$

If $m_2$ is much larger than $m_1$ then this equals approximately $m_1$, so $m_2$ doesn't contribute much to the energy and momentum of the whole system.


THe protons are reside inside the nucleus . Hence they arent the ones who can transitions between different orbits . The thought of protons leaving the nucleus is just a vague thought . That would just mean that we can make ions by loss and gain of protons too which we all know is not possible


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