I have done reading on the momentum versus mass of light. However there is one issue which I'm still wondering about: When we use a magnesium lamp which is enclosed by a glas-bulb and ignite it, it will emit light during the chemical reaction. We know that two atoms have a higher mass when they are separate than when they form a molecule. Now we can conclude that the magnesium lamp has a lower mass after than before by Δm. The only thing that left the lamp was light.(?) Is it right to say that the mass difference of the lamp equals the energy of the photons that were emitted by n*hf=Δm*c^2? Is it at that size? From what I read this is probably nonsense(?) So how is the mass difference accounted for? Thank you in advance.