When were the Bohr model and the quantum-mechanical model for the atom developed? What purpose do these models serve?
Bohr model and the and the quantum-mechanical model for the atom were developed in early 1900.
Both the model were used to describe the arrangements of the electrons within the atomic structure.
Neil Bohr attempted to develop a model for the atom that explained atomic spectra. According to Bohr model , electrons travel around the nucleus in circular orbits .The orbits exist only at specific, fixed distances from the nucleus. The energy of each Bohr orbit is also fixed, or quantized .These orbits are known as stationary states.