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The Bohr model of the atom, developed in the early twentieth century, was an attempt to explain observations about the way atoms and electrons absorb, retain and release energy. The model assumed that the atom has a structure similar to that of the solar system with the atomic nucleus at the center of the atom and the atom’s electrons move in circular orbits similar to the way planets orbit the sun. The Bohr model represented a great advancement in the understanding of atomic structure and contributed to the development of quantum mechanics.
Above is a Bohr Atom. Click on the grey rings to move the electron from orbital to orbital. Change the number of excitation states the electron has with the slider at the lower left and click on the hidden, visible and comment buttons to toggle information about the atom on and off.
The Bohr atom is popular as a teaching tool because it helps visualize the relationship between energy, electron position and the emission of electromagnetic energy. However, it is important to understand that the planet-like imagery is just a representation. The planetary model is not consistent with modern observations of the relationship between the nucleus of an atom and the electrons associated with that atom.
Exploration of the Bohr model of the atom contributed to the development of a framework for understanding how electrons absorb, and release discrete amounts (quanta) of energy by suggesting that the electrons associated with an atom do not have free range to be anywhere around that atom. Rather, electrons maintain discrete positions around the nucleus. In the Bohr Atom
- Electrons travel in circular paths around the nucleus of an atom
- Electrons can exist only in a finite number of orbitals.
- Each orbital is at a different distances from the nucleus.
- Electrons in each orbital contain a set quantity of energy.
- As long as an electron remains in the same orbital, the energy content of that electron remains constant.
- Electrons can move between orbits by releasing or absorbing energy.
The lowest energy level an electron can occupy is called the ground state. The higher orbitals represent higher excitation states. The higher the excitation state, the more energy the electron contains.
When an electron absorbs energy, it jumps to a higher orbital. An electron in an excited state can release energy and ‘fall’ to a lower state. When it does, the electron releases a photon of electromagnetic energy. The energy contained in that photon corresponds to the difference between the two states the electron moves between. When the electron returns to the ground state, it can no longer release energy, but can absorb quanta of energy and move up to excitation states (higher orbitals).
The number of movements an electron can make depends on the number of excitation states available. In the case of one ground state plus one excitation state, there is only one possible state change. The electron can absorb one quantum of energy and jump up to the excitation state. From that excitation state, the electron can then drop back down, releasing a photon with a predictable amount of energy in the process.
The addition of a second excitation state increases the number of moves possible from one to three. One associated with movement between the ground state and the lower excitation state, and two associated with movement between the ground state and the second excitation state.
The number of possible moves increases as an arithmetic series as the number of excitation states increase. With four excitation state, the number of state changes is 10, which is 4 plus 3 plus 2 plus 1. The Bohr representation of the atom is also makes it possible to visualize movements of electrons from particular states.
In a Bohr atom with six excitation states, an electron can jump from the ground state up to any one of those six states. An electron in the fifth excitation state, can absorb energy and jump up to the highest excitation state, or fall to any one of the five lower energy states, releasing a photon in the process.
It is important to remember that the Bohr atom is not an accurate representation of atomic structure. However, this model helps illustration some basic concepts of energy absorption and release by atoms and their electrons.