Video Transcript
Which of the following is not a limitation of the Bohr model of the atom? (A) Electrons move around the nucleus in circular, planar orbits. (B) Electrons are only considered as particles and not as waves. (C) It is possible to precisely determine the position and momentum of an electron simultaneously. (D) Electrons within atoms can only occupy quantized energy levels. Or (E) it explains the line emission spectrum of the hydrogen atom only.
Niels Bohr and Ernest Rutherford proposed the Rutherford–Bohr model, often just called the Bohr model, where electrons occupy orbits around the nucleus like planets around the sun. The Bohr model of the atom was at one point the best description of an atom. But later discoveries in physics and the formulation of quantum mechanics highlighted inadequacies in the model. So the Bohr model has limitations. We need to find out which of the options (A) to (E) is not a limitation of the Bohr model.
Option (A) is, “Electrons move around the nucleus in circular, planar orbits.” As we have already seen. The Bohr model did suggest this to be true. The Bohr model treats electrons as particles, but de Broglie demonstrated that electrons have wave particle duality. Wave particle duality is when physical objects can exhibit wave-like properties and particle-like properties. The degree to which they exhibit this behavior depends on what they are and how fast they’re moving.
Additionally, Schrödinger’s wave mechanical model described electrons as having a range of possible positions. At any point in time, an electron may be closer to the nucleus or further away. As the Bohr model only considers electrons to be particles and not waves and it suggests that they move around the nucleus in circular planar orbits, the model does not account for these currently accepted observations. Therefore, option (A) is a limitation of the Bohr model. Thus, it’s not the answer to this question.
Option (B) is, “Electrons are only considered as particles and not as waves.” We have already mentioned that the Bohr model only considers electrons as particles. But in fact, they exhibit wave particle duality so can exhibit wave-like properties. For this reason, option (B) is a limitation of the Bohr model and is not the answer to this question either.
Option (C) states, “It is possible to precisely determine the position and momentum of an electron simultaneously.” The Bohr model treats electrons purely as particles that orbit in a predictable way, which suggests that you would be able to determine the position and momentum of an electron simultaneously.
However, Heisenberg’s uncertainty principle states that the more accurately you determine a particle’s position in space, the more uncertain your measurement of its momentum becomes, and vice versa. So the more accurately you determine a particle’s momentum, the more uncertain your measurement of its position will be. As the Bohr model does not account for Heisenberg’s uncertainty principle, option (C) is a limitation of the Bohr model, so it’s not the answer to this question.
Option (D) is, “Electrons within atoms can only occupy quantized energy levels.” In classical physics, all types of energy are seen to be continuous. Energies can be any value. But quantum theory introduced the idea that certain types of energy come in discrete packets called quanta. We say that this energy is quantized.
The Bohr model of the atom introduced the idea of electrons occupying orbits of fixed energies. So the Bohr model suggests that electrons within atoms can only occupy quantized energy levels. This is in line with what is currently accepted within the quantum model. The notion of electrons occupying quantized energy levels is still part of the modern atomic theory. So, option (D) is not a limitation of the Bohr model and is likely to be the answer to this question.
But to confirm, let’s look at option (E). It explains the line emission spectrum of the hydrogen atom only. The Bohr model accounted for the fact that when we excite atoms or ions, an electron is promoted to a higher orbit. Then when the electron drops down again, light is emitted. The light produced has specific wavelengths and creates a pattern of lines in a line emission spectrum. Each atom has a distinct line emission spectrum. For atomic hydrogen, the Bohr model was very accurate. But for atoms with more than one electron, the Bohr model did not reproduce experimental results.
The fact that the Bohr model only explains the line emission spectrum of the hydrogen atom is a limitation of the Bohr model. So option (E) is not the answer to this question.
Therefore, the answer to the question, “Which of the following is not a limitation of the Bohr model of the atom?” is (D). Electrons within atoms can only occupy quantized energy levels.
