Bohr’s atomic model shows the atom as a small positive nucleus surrounded by electrons in circular layers around the nucleus .
With the help of Plank’s quantum theory, the light spectra of the elements, and Rutherford’s nuclear theory, Bohr managed in 1913 to establish a new atomic model where the electrons described circles around the nucleus.
This model served to explain the light spectra emitted by the elements and the regularities of the periodic table. Furthermore, with the Bohr model the quantum era began.
Bohr’s Atomic Model Postulates
Electrons move a certain distance from the nucleus
For Bohr, the electron’s circular motion had a specific radius of rotation, so there could be no electrons between two layers.
The electrons move stably, that is, without releasing energy, in the steady state . When an external amount of energy is applied to them, that is, the electrons are excited, they can jump to a higher energy level. This is the least stable excited state of the electron.
Electrons follow a circular path
Steady state electrons move in “energy levels” or “layers.” Each energy level is designated by letters, with the lowest level being the letter K, followed by L, M, N, and so on. The layers can be imagined as onion rings wrapping around the core.
In this sense, the layers closest to the nucleus have less energy. Each layer could also contain more than one electron, for example: the K layer can have up to two electrons, the L layer eight, and so on.
Electrons emit light when they change levels
All elements when heated emit light of specific colors or frequencies, known as the electromagnetic spectrum.
Bohr was able to explain this phenomenon in the following way:
When an electron jumps from a high energy level to a lower energy level, the energy difference is released in the form of electromagnetic radiation or light. Thus, energy is related to the frequency or color of light f by the Max Planck ratio h : Energy = hf .
Characteristics of the Bohr model
- The electrons describe circular orbits around the nucleus.
- Electrons travel at predetermined levels of energy.
- Electrons can jump from a lower energy level to a higher one if you supply them with energy.
- When the electrons return to their stationary energy level, they release light.
How did Bohr arrive at the atomic model?
Niels Bohr was born in Copenhagen (Denmark) in 1885. In 1912 he entered Ernest Rutherford’s laboratory with the task of discovering how the nuclear structure of the atom was stable.
Rutherford’s atomic model was based on a central nucleus surrounded by electrons, just like in the solar system. This very simple model had a problem: an electron rotating around the nucleus had to emit electromagnetic radiation, with the consequent loss of energy. This would cause the electron to fall into the nucleus, causing the atomic collapse.
Bohr proposed that the electrons were distant by a certain distance from the nucleus, rotating in circles. Each circle or orbit was provided with a predetermined amount of energy. If an electron were to jump into a lower energy orbit, it would emit electromagnetic radiation.
The simplest model was hydrogen, which consists of an electron and a proton. By calculating the energy that hydrogen emits when it receives a discharge, the experimental values approximated the values calculated by Bohr.
Failures of the Bohr atomic model
Bohr’s model of the atom is valid only for the hydrogen atom.
At first Bohr’s model looked promising. It was a perfect fit for the hydrogen atom. But when it came to applying this model to other atoms, it didn’t work.
Electrons do not move in circular orbits
Some tried to fit the model to an elliptical orbit, without success. Today it is known that the electron behaves like a wave and a particle, which is why it is better spoken of a wave function, a space around the nucleus where there is a probability of finding the electron.