Final answer:
A photon of electromagnetic radiation is emitted when an electron in an atom transitions from a higher to a lower energy state, moving from an excited state back to the ground state.
Step-by-step explanation:
When an electron jumps from a higher energy state to a lower energy state, it emits a photon of electromagnetic radiation. This emission occurs because when an atom's electrons are excited to a higher energy level, they are in an excited state, and this state is unstable. Eventually, the electron will return to a lower energy level, referred to as the ground state, releasing the energy difference between the higher and lower energy levels in the form of a photon. This process is governed by the law of conservation of energy which states that energy cannot be created or destroyed, so the energy emitted as a photon when the electron returns to the ground state is equal to the energy that was absorbed to elevate it to the excited state.