A specific, discrete amount of energy that an electron can have within a material.An energy level is a discrete, quantized amount of energy that an electron (or other quantum system) can occupy within an atom, molecule, or solid. In atoms, electrons exist in specific shells and subshells, and transitions between these levels produce absorption or emission of photons at characteristic frequencies. In solids, energy levels broaden into bands, and the arrangement of these bands determines whether a material is a conductor, semiconductor, or insulator.
The concept emerged from Niels Bohr's 1913 model of the hydrogen atom, which introduced quantized orbits. It was generalized by Arnold Sommerfeld and later formalized in quantum mechanics by Erwin Schrödinger and Werner Heisenberg in the mid-1920s.
Quantum mechanics dictates that bound electrons cannot have arbitrary energies—only certain values allowed by the system's wavefunction and boundary conditions. The Pauli exclusion principle ensures each level holds a limited number of electrons. When an electron absorbs a photon with energy exactly matching the gap between two levels, it jumps to a higher level; when it falls back, it emits a photon of that same energy.
Energy levels underpin lasers, LEDs, solar cells, and semiconductor electronics; spectroscopy uses them to identify elements in stars and materials.
The exact energy levels of hydrogen are what allowed astronomers to deduce the composition of distant stars and even measure the expansion of the universe through redshifted spectral lines.