Why do electrons transition between energy levels within the atom, and how do we detect these transitions?

See explanation

Explanation:

Electrons transition between energy levels in an atom due to gain or loss of energy. An electron may gain energy and move from its ground state to one of the accessible excited states. The electron quickly returns to ground state, emitting the energy previously absorbed as a photon of light. The wavelength of light emitted is measured using powerful spectrometers.

Atoms can be excited thermally or by irradiation with light of appropriate frequency.

nuclear fission produces elements that are heavier than helium.

the elements that are used in nuclear fission and their products are much heavier than helium.