Why nλ= 2∏r? this is the explaination from website , Now, one complete orbit has the same length as the circumference of the circle that the orbit traces out, so is given by 2πr. This distance must be equal to an integral number of wavelengths. (If it were not, the wave traced out by the particle on its first orbit would be out of phase with those traced out on all subsequent orbits.) Over an infinite number of orbits, the out of phase waves would cancel each other out to zero amplitude, which implies that the particle cannot exist under such circumstances. Only if the waves traced out on all orbits overlap exactly, i.e. if the orbit is an integral number of wavelengths, is the situation a satisfactory one under which the particle can exist. My question is , 1)electron is moving like a wave when it is orbiting the nucleus, correct? 2) why out of phase will cancel out each other? What is being cancelled? Waves of electron? In the orbit, they are not radiating , they only orbiting around the nucleus, what is going to be cancel? Thank you.