For standing waves with both ends fixed in a length L, the wavelength l is limited to (where n is an integer)
l = ; n = 1, 2, 3, ….
In quantum mechanics, electrons in atoms are treated as standing waves confined by the electric field due to the atomic nuclei. Thus, the electrons are represented by wave functions.
The wave, energy etc are called state of the electron, and with spin, each state accommodate 2 electrons. A state is called an orbital (not orbit)