Bohr Model of the Atom

Bohr atomic model diagram

Bohr Model – Core Concepts:

In this tutorial, you will learn what the Bohr Model is, how it improved upon previous models of the atom, and what problems the Bohr model fails to solve.

What is the Bohr Model?

The Bohr Model is a model of an atom. The model was proposed by physicist Niels Bohr in 1913. In this model, the electrons travel around the nucleus of an atom in distinct circular orbits, or shells. The model is also referred to as the planetary model of an atom.

The electrons orbit around the nucleus similar to how planets orbit around the sun. The planets are held in orbit by gravitational force and the electrons are held in their orbit by the electrostatic or Coulomb force between the electrons and protons. In this atomic model, all electrons must exist in a discrete shell and can’t be between shells.

Lithium Bohr model of the atom
The lithium Bohr model. Electrons (blue) orbit in circular shells around the nucleus (red).

Each shell has a specific energy level, and electrons cannot exist outside of these orbits. The closer the shell is to the nucleus, the smaller the energy of that shell. For an electron to move to another shell it must absorb or release energy. The amount of energy absorbed or emitted is dependent on the difference in energy between the shells.

Bohr model of an atom. Diagram shows how energy is released in this atomic model
When an electron moves to a smaller shell, it releases energy which we observe as light.

Energy Release

When the electron moves from a larger higher-energy shell to a smaller lower-energy one it releases energy. The energy is released in the form of light. These discrete energy steps are what cause atomic line spectra, like the one seen for hydrogen below.

Only light of specific energy (or color) is released, shown by the sharp lines seen in the spectra, not all colors of light. For example, the red line would be caused by the electron moving from shell 2 to shell 1. And the blue line would be caused by an electron moving from shell 3 to shell 2. Each orbit change has a unique energy difference.

Atmoic line spectra of hydrogen that is explained by the Bohr atomic model

Atomic line spectra of hydrogen. (From Wikipedia Commons)

Improvements From Previous Models

The Bohr model replaced earlier models such as the plum-pudding model (by J.J. Thomson in 1904), the Saturnian model (by Hantaro Nagaoka in 1904), and the Rutherford model (by Ernest Rutherford in 1911).

Bohr’s model is different from the preceding model (the Rutherford model) because electrons can only orbit at certain radii or energy. It is the first atom model that accounts for quantized or discrete energy steps. No other model had done this before and was a big step towards the development of quantum mechanics.

Using Bohr’s model of the atom the previously observed atomic line spectrum for hydrogen could be explained. Previous models had not been able to explain the spectra.

Hydrogen Bohr Model

The Bohr model of hydrogen is the only one that accurately predicts all the electron energies. When there is more than one electron the model does not accurately predict the energies. Other ions that also have one electron can also be explained accurately (for example, He+). When there is more than one electron interactions between the nucleus and electrons become too complicated for the Bohr model.  

bohr model atomic model
Depiction of the Bohr model of hydrogen. Hydrogen and other 1 electron systems are the only ones accurately explained by the Bohr Model

Problems with Bohr’s Model

  1. The Bohr atomic model could not accurately describe larger atoms. The emission spectrum of atoms with more than one electron could not be explained.
  2. The atomic model could not explain the different line intensities in emission spectra.
  3. The Bohr model did not describe the changes seen in emission spectra when a magnetic field was present (known as the Zeeman effect).
  4. Does not match what scientists would later learn that an electron can be both a wave and a particle.
  5. Electrons in circular orbits should emit energy, eventually resulting in their collision with the nucleus.

Bohr Atomic Model Fun Facts

  1. Niels Bohr was awarded the Nobel Prize in physics in 1922 for his work investigating the structure of an atom
  2. In his later years, Niels Bohr advocated for openness between nations in atomic weapons development.
  3. Niels Bohr helped rescue and provide jobs for scientists escaping Germany during the Nazi regime by giving them positions at the theoretical physics institute he ran and helping them get visas to other countries.
  4. The Bohr model is the one most often depicted when drawing an atom.

Further Reading