# Mass and energy of radiation

We know that mass can be  converted to energy and vice versa. So does the mass of molecules or atoms change when they absorb a radiation and go to the higher energy state?

• I'm suspecting yes, though it may not be practically measurable – electronpusher Sep 17 '19 at 19:10
• – Rodney Dunning Sep 17 '19 at 19:48
• Not a duplicate of physics.stackexchange.com/q/149744, because that question asks about the mass of the electron itself, but the answers to that question also address the present question. – Chiral Anomaly Sep 20 '19 at 13:29

Yes. For example, when a hydrogen atom absorbs a photon and transitions from the $$1s$$ ground state to the $$2p$$ excited state, with 10.2 electron-volts more energy, its mass increases by $$1.8\times 10^{-35}$$ kilograms, or about one part in 100 million. This is just $$\Delta m=\Delta E/c^2$$.