Question

What changes in the atomic structure cause the absorption and emission of photons (light)?

i) Absorption

ii) Emission

Answer 1

1) In the atom we have positively charged protons (and neutral neutrons ) in the nucleus of an atom, surrounded by orbiting negatively charged electrons. The electrons can only go in very special orbits at discrete distances from the nucleus.

Electrons can go up into a higher energy state (called an excited state) by giving it some energy. Later, the electron will fall back down to the ground state, and give back the energy it absorbed in going up to the excited state. It will therefore emit a photon.

The light emitted by the electron in falling down to the ground state can only have a very particular wavelength, since the distance (energy) separation between the ground state and excited state is discrete. The electrons will become excited and go up to a higher energy state. They will then fall down and emit light at wavelengths corresponding to the energy jumps they took in falling back down. If multiple levels are involved, the electron can have a choice: it can jump from top to bottom all at once, or it can jump into and out of intermediate levels. An observer will see a series of emission lines, corresponding to the differences between orbital energies. This is an emission spectrum.

2) Another way for electrons to become excited is to absorb light. Suppose you shine light of all wavelengths at a collection of hydrogen atoms in the ground state. Those photons whose energies (wavelengths) are exactly equal to the distance between orbits will be absorbed. An observer will see the continuous spectrum minus the absorption lines of hydrogen. This is called an absorption spectrum.

This means that the if the amount of light absorbed is high enough the electron could be ionized which implies also changes in the atomic structure