Energy Levels and Photon Emission

Atoms do not absorb or emit energy continuously. Instead, the electrons in an atom can only occupy specific, discrete energy levels. Transitions between these energy levels produce photons of precise wavelengths, giving rise to the characteristic line spectra that allow us to identify elements and understand the structure of matter.

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Discrete Energy Levels

In the early 20th century, experiments revealed that electrons in atoms can only exist at certain specific energies. These allowed energies are called energy levels (or energy states).

The ground state is the lowest energy level — the most stable state of the atom. All higher energy levels are called excited states.

Energy levels are usually represented on an energy level diagram, with the ground state at the bottom and higher levels above. The energies are typically given in electronvolts (eV), and they are negative because energy must be supplied to remove the electron from the atom (the zero of energy is defined as the energy of a free electron at rest, far from the nucleus).

A typical energy level diagram for hydrogen: