7.5
The Bohr Model
Following the work of Ernest Rutherford and his colleagues in the early twentieth century, the picture of atoms consisting of tiny dense nuclei surrou…
The planetary model describes the hydrogen atom as a tiny solar system, with the electron orbiting a positively-charged nucleus.
Classical physics suggests that because the electron is a charged particle moving in a circular orbit, it should continuously radiate energy while spiraling into the positively-charged nucleus. Eventually, the atom would collapse.
However, this isn’t observed. Atoms are stable.
Niels Bohr proposed that there are orbits at certain, fixed distances from the nucleus. These orbits are numbered integers labeled by the principal quantum number, n. The orbit closest to the nucleus is n = 1. n can be any positive integer, but never zero.
As n increases, so does the radius of the orbital. An electron that is farther away from the nucleus experiences a weaker electrostatic force and is less tightly attracted to the proton.
Each orbit corresponds to a particular energy level or state. The levels are quantized, meaning no energies in between are possible.
An electron at its lowest, most stable energy state, at n = 1, is said to be at its ground state.
The higher energy states, where n > 1, are called the excited states.
An electron can only move to a different energy state either when it absorbs energy — and jumps to a higher level — or when it transitions back down to a lower energy state and releases excess energy in the form of a photon.
The energy absorbed — or emitted — is related to the difference in energy between the final and initial energy levels.
Electrons relaxing from a higher energy level emit shorter-wavelength light than electrons relaxing from a lower energy level.
Although Bohr’s model is suitable only for hydrogen or single electron ions, it set the groundwork for more accurate atomic models.
View the full transcript and gain access to JoVE Core videos
Q1: Why does the planetary model predict that atoms should collapse?
According to classical physics, an electron orbiting the nucleus is a charged particle moving in a circular path. This acceleration should cause it to continuously radiate electromagnetic energy. As the electron loses energy, its orbit shrinks until it spirals into the nucleus, making atoms inherently unstable. However, atoms are observed to be stable, revealing a fundamental flaw in the classical model.
Q2: What are quantized energy levels in the Bohr model?
The Bohr model proposes that electrons occupy orbits at fixed distances from the nucleus, each corresponding to a specific energy level labeled by the principal quantum number n. These energy levels are quantized, meaning only certain discrete energies are allowed—no intermediate energies exist. As n increases, the orbital radius and energy increase, with n=1 representing the lowest, most stable ground state.
Q3: How does an electron transition between energy levels in the Bohr model?
An electron transitions between energy levels by absorbing or emitting energy. When absorbing energy, it jumps to a higher energy level (excited state). When transitioning back to a lower level, it releases excess energy as a photon. The energy of the emitted or absorbed photon equals the difference between the final and initial energy levels, allowing prediction of spectral lines through emission spectra hydrogen emission.
Q4: What is the ground state and why is it significant?
The ground state is the lowest energy state of an atom, where the electron occupies the n=1 orbit closest to the nucleus. This is the most stable configuration because matter naturally seeks the lowest possible energy. An electron in the ground state requires energy input to move to higher energy levels. Most electrons exist in the ground state unless external energy excites them to excited states.
Q5: Why does the Bohr model work for hydrogen but not for multielectron atoms?
The Bohr model assumes a single electron orbiting the nucleus with a simple electrostatic attraction. In multielectron atoms, electrons repel each other, creating complex interactions that the model cannot account for. The model also cannot explain electron configuration of multielectron atoms or predict their spectral properties accurately. More sophisticated quantum mechanical models are needed for atoms with multiple electrons.
Q6: What relationship exists between orbital radius and energy in the Bohr model?
In the Bohr model, as the principal quantum number n increases, both the orbital radius and the energy of the electron increase. An electron farther from the nucleus experiences weaker electrostatic attraction and is less tightly bound. This means higher energy states correspond to larger orbits, with electrons in outer orbits requiring less energy to remove from the atom compared to those in inner orbits.
Q7: Can the Bohr model be applied to ions other than hydrogen?
Yes, the Bohr model applies to hydrogen-like atoms and ions with a single electron, such as He+, Li2+, and Be3+. These species differ from hydrogen only in their nuclear charge but maintain the same single-electron structure. The model's predictions for energy levels and spectral lines remain valid for these one-electron systems, though the specific energy values change based on the nuclear charge.