In 1913, Danish physicist Niels Bohr offered the world a model of the atom. The field of quantum mechanics was just beginning to be formed when Bohr proposed his ideas. Bohr had noticed and studied the atomic emissions spectrum of the hydrogen atom. Bohr noticed that when energy was added to the hydrogen atom, light was given off, but unlike white light, the colors given off were not continuous. When white light (the light from a typical light bulb) is diffracted with say a prism, all the colors of the visible spectrum can be seen. Each color corresponds to a specific amount of energy. However when the light given off by the hydrogen atom was passed through a prism, only certain colors of light could be seen instead of all the colors. This led Bohr to theorize that electrons can only have certain energies in an atom; they had to be in energy levels. Bohr said that the electrons have certain energies in the atom and can only change that energy by absorbing or emitting a quanta of energy or a photon. Bohr found the energy of the colors of light that the hydrogen atom released. He used these energies to find the energies that the single electron in the hydrogen atom could have. Bohr said that the electron had to release energy to change its energy so the differences between the energies of light seen in the atomic spectrum should correspond to the differences in energies of the energy levels.
Based on the information Bohr gathered from the atomic emission spectrum of hydrogen, he created a model that had the electrons orbiting the nucleus like the planets orbit the sun.
Bohr proposed that each orbit was defined by a certain energy so he said that the electron was in an energy level. In the stable atom, the electron is in the lowest energy level but when energy is added to the atom, then the electron jumps to a higher energy level because it now has more energy. When the electron returns to its groundstate (or most stable state, the lowest energy), it has to give off energy, and it does so in the form of light.
Using the information he gathered, Bohr was able to give exact energies for each energy level of his model and his model worked great for the hydrogen atom. It predicted all of the phenomena seen from the hydrogen atom; the problem came when he tried to apply it to the another atom like the lithium atom. Bohr's model and the equations for calculating the energy levels in an atom did not work for any atom but hydrogen. The energy levels predicted by Bohr's model for the lithium atom and other atoms did not match the atomic emissions spectra which scientist could see. Bohr's model had failed for other atoms so scientists were in need of a new model. The need for a new model was filled by the Quantum Mechanics and its model of the atom. This model succeeded where Bohr's had failed.
Two things came out of Bohr's model that were important to the next model of the atom. First, the idea that electrons must have specific energies. The quantum mechanical model of the atom also states that this must be true. Along with this idea goes the idea that electrons can only inhabit certain orbitals around the nucleus. Bohr's definition of an orbital turned out to be too defined: the quantum mechanical versions of an orbital in much different: but the idea that only certain orbitals exist still holds true in the quantum mechanical model. Bohr's model was one of the final stepping stones to our modern model of the atom. Without it, it may have taken scientists much longer to come up with the newest model.