Bohr made a hypothesis-he proposed that there are certain special state of motion called stationary states, in which the electron may exist without radiating electromagnetic energy. In these states, according to Bohr, the angular momentum of electrons takes values that are integer multiples of ℏ. In stationary states, the angular momentum of the electron may have magnitude ℏ,2ℏ,3ℏ ....., but never such as 2.5ℏ or 3.1ℏ. This is called the quantization of angular momentum.