1. The problem statement, all variables and given/known data What experimental evidence requires that the emission of energy by an atom be quantized? 3. The attempt at a solution I can completely confused on what the question is asking. What do they mean by "experimental evidence"? Do I talk about Bohr and how he proposed that to get spectral lines, the energy of the election in the atom must be quantized. Would that be "experimental evidence" that the question refers to? or do I just state that emission and absorption lines in atomic spectra are evidence of quantization? Any help would be appreciated since I am really stumped on this question. Thanks.