We will learn in Chapter 27, that a hydrogen atom (1 proton + 1 electron) can exist only in certain discrete energy states corresponding to different discrete electron orbits. The lowest energy state (called the ground state) has an energy of Eground=-13.6eV. The next highest energy state (the first excited) state has an energy of -3.4eV. A hydrogen atom in the first excited state can drop down to its ground state energy by emitting a photon.
When a hydrogen atom undergoes a transition from the 1st excited state to the ground state, the mass of the hydrogen atom...
Pink: remains unchanged.
Yellow: decreases by about 1 part in 107
Green: decreases by about 1 part in 108
Blue: decreases by about 1 part in 109
Purple: decrease by about 1 part in 1010
Answer: decreases by about 1 part in 108. The total energy of the hydrogen atom is mostly the rest mass energy of the proton (the electron mass is small compared to the proton mass). The rest mass energy of a proton = mprotonc2 =939MeV; or roughly 109eV. The change in energy is about 10eV. So we have
. But the fractional change in
energy is the same as the fractional change in mass 
.