"The mass of the two H atoms is unchanged" -> No, if an electron on an H atom is excited to a higher energy state, the mass of the atom increases (although with

"The mass of the two H atoms is unchanged" -> No, if an electron on an H atom is excited to a higher energy state, the mass of the atom increases (although with the relatively low energies involved, that mass increase is extremely tiny). The mass of nucleus isn't equal to the mass of neutrons and protons, the binding energy matters, and the mass of an atom isn't equal to the mass of the nucleus plus the mass of the electrons, the energy states matter. Not an authoritative reference, but perhaps a sufficient explanation is here - https://physics.stackexchange.com/questions/149744/does-the-mass-of-an-electron-change-with-its-energy-state https://physics.stackexchange.com/questions/149744/does-the-...