Why do atoms emit a definite line of spectrum of only certain energies instead of a continuous spectrum of all

Answer 1

In a nutshell, there are certain distances away from the nucleus of an atom where electrons go to when they are given excess energy (from heat, strong light sources, etc etc). When the electrons receive this energy, they jump up from the 'ground state' (where all electrons are very close to the nucleus), to their 'excited state' (far away from the nucleus). The excited state is less stable, so the electrons give off a burst of energy in the form of light, which drops them back down into the ground state. All atoms have excited states which are at different distances from the nucleus, and thus the electrons absorb various amounts of energy to get to the excited state, and hence give off different wavelengths of light to return to the ground state.

BF
4th yr Chem Major
U of O (Canada)

Answer 2

Max Planck received a Nobel Prize for this concept.

Just as we know that matter has very small discrete pieces, the atom, which cannot be subdivided and still be that element.

Energy has very small discrete "pieces" as well. They are called quanta. Each element has a different size of energy quanta. When an electron gains or loses energy, it can only do so in quanta units. So an electron gives off discrete frequencies of energy and not a continuous stream.

Answer 3

Because the energy between the different levels electrons can occupy are discrete. Therefore it can only accept or re-emit certain energies that corresponds to the difference of these levels.