1. The problem statement, all variables and given/known data An atom initially at rest emits a photon with frequency f. Explain why the mass of the atom decreases, and by an amount greater than hf/c^2. 2. Relevant equations E= mc^2 E = hf 3. The attempt at a solution So I know that the atom must lose mass when the photon is emitted because in order for it to emit energy it must lose mass (by E = mc^2), but I don't understand why it should be more than hf/c^2, since from these 2 equations: E = mc^2 = hf therefore m = hf/c^2 Why is this wrong? Thanks.