Free electron model
（重定向自Electron gas）
![In three dimensions, the density of states of a gas of fermions is proportional to the square root of the kinetic energy of the particles.](/Images/godic/202501/11/Free-electron_DOS.svg1800.png")
![Plane wave traveling in the x-direction](/Images/godic/202501/11/Plane_Wave_3D_Animation_300x216_255Colors1800.gif")
In solid-state physics, the free electron model is a simple model for the behaviour of valence electrons in a crystal structure of a metallic solid. It was developed principally by Arnold Sommerfeld who combined the classical Drude model with quantum mechanical Fermi–Dirac statistics and hence it is also known as the Drude–Sommerfeld model.