Question

Describe two major shortcomings of Bohr’s model of the atom.

Short answer

Bohr’s model assumes fixed electron orbits and fails to explain spectral lines of complex atoms.

Step-by-step solution

Identify Bohr's Model

Bohr’s model of the atom describes electrons orbiting the nucleus in fixed paths or 'shells' with quantized energy levels.

Problem with Electron Orbits

The first major shortcoming of Bohr’s model is that it assumes electrons move in fixed, defined orbits. This idea contradicts the principles of quantum mechanics, which state that we cannot know an electron's exact position and momentum simultaneously. This is better described by the concept of atomic orbitals in the quantum mechanical model.

Explanation of the Issue with Spectral Lines

Bohr’s model accurately described the spectral lines of hydrogen, but it failed to explain the spectral lines of more complex atoms. More advanced quantum mechanical models, such as Schrödinger's wave equation, provide a better explanation for the emission spectra of atoms beyond hydrogen.

Key concepts

quantum mechanics

Bohr’s model assumes that electrons move in fixed, defined orbits around the nucleus. However, this view was later overturned by the principles of quantum mechanics. This branch of physics deals with the behavior of particles at the atomic and subatomic levels. According to quantum mechanics, electrons do not travel in fixed orbits. Instead, they exist in regions of space known as 'clouds of probability.' This means we can only predict the probability of finding an electron in a specific area, but not its exact position and momentum at the same time.
This fundamental aspect of quantum mechanics is illustrated by Heisenberg's Uncertainty Principle. The principle states that the more precisely we know an electron’s position, the less precisely we can know its momentum, and vice versa. This probabilistic nature was a key advancement beyond Bohr’s model.

atomic orbitals

In Bohr's model, electrons orbit the nucleus in fixed paths called shells. This simplistic view was replaced by the concept of atomic orbitals to better match observations. Atomic orbitals are regions around the nucleus where an electron is likely to be found. They come in various shapes and sizes and are described by quantum numbers.
The quantum mechanical model introduces four quantum numbers to describe atomic orbitals:

• The principal quantum number ()
• The angular momentum quantum number ()
• The magnetic quantum number ()
• The spin quantum number ()

These quantum numbers define the size, shape, and orientation of orbitals. For example, =1 refers to the smallest orbital closest to the nucleus, while higher values of indicate orbitals further away.
Unlike fixed paths in Bohr’s model, atomic orbitals provide a much more accurate and nuanced understanding of electron positioning and behavior in atoms.

emission spectra

Bohr’s model of the atom accurately explained the emission spectra of hydrogen, which appear as discrete lines of color in a spectrometer. However, it couldn't adequately describe the emission spectra of more complex atoms. Emission spectra result from electrons moving between energy levels or orbitals and emitting photons of specific energies.
Quantum mechanics provides a more comprehensive explanation for these phenomena. When an electron transitions between orbitals, the energy difference between the initial and final states is emitted as a photon of light. This photon carries the energy away in the form of electromagnetic radiation, producing the lines observed in emission spectra.
Each element has a unique emission spectrum, as the energy levels and transitions vary. More complex atoms have more electrons and more possible transitions, leading to more complex spectra that Bohr’s model couldn't account for. Advanced models, such as Schrödinger's wave equation, help explain these spectra with great precision.