As with any other quantum particle, an electron exhibits wavelike and particlelike characteristics. Which aspect predominates in a solid depends on how an electron interacts with its neighbors. According to the Bloch theorem, for instance, an electron placed in a periodic lattice behaves like an extended plane wave. However, when the number of free electrons in a solid becomes comparable to the number of the constituent atoms and the mutual electron–electron interaction becomes strong, electrons may lose their mobility.