An atom emits a photon when one of its electrons:

collides with another of its electrons
exchanges quantum states with another of its electrons
undergoes a transition to a quantum state of lower energy
undergoes a transition to a quantum state of higher energy

Answer:

undergoes a transition to a quantum state of lower energy

Explanation:

When electrons in an atom move to another quantum state, they emit/absorb a photon according to the following:

- If the electron is moving to a higher energy state, it absorbs a photon (because it needs energy to move to a higher energy level, so it must absorb the energy of the photon)

- if the electron is moving to a lower energy state, it emits a photon (because it releases the excess energy)

In particular, the energy of the absorbed/emitted photon is exactly equal to the difference in energy between the two levels of the electron transition:

[tex]E=|E_1 - E_2|[/tex]

An atom emits a photon when one of its electrons undergoes a transition to a quantum state of lower energy. Hence, the correct option is 3.

This happens when an electron in an excited energy state returns to a lower energy state or the ground state. The energy difference between these states is released as a photon, which corresponds to the energy of the gap between the two levels.

To summarize, the emission of a photon occurs due to:

In contrast, if an electron moves from a lower energy level to a higher one, it must absorb a photon, not emit it.