Question

Describe the two shortcomings of Bohr model of atom.

Hint:

When asked for Bohr's failures, the easiest to remember are: its failure for atoms "beyond hydrogen" and its inability to explain the "splitting of spectral lines" (Zeeman/Stark effects).

Answer 1

Step 1: Understanding the Concept:
The Bohr model was a significant step forward from the Rutherford model, successfully explaining the spectrum of the hydrogen atom. However, it was a semi-classical model and had several limitations when applied to more complex systems or when observed with high-resolution instruments.
Step 2: Detailed Explanation:
Here are two key shortcomings of the Bohr model:
1. Inability to Explain Multi-Electron Spectra:
The model's calculations and postulates were based on a single electron orbiting a nucleus (like hydrogen, He\(^+\), Li\(^{2+}\)). It failed to predict the spectral lines of neutral multi-electron atoms (e.g., Helium) because it did not account for the complex electrostatic interactions between multiple electrons.
2. Inability to Explain the Fine Structure and Splitting of Spectral Lines:
When the spectral lines of hydrogen were observed with high-precision spectrometers, they were found to consist of several closely packed finer lines (fine structure). Bohr's model could not explain this. Furthermore, it could not explain the splitting of a single spectral line into multiple lines when the atom was placed in an external magnetic field (Zeeman effect) or an external electric field (Stark effect). This indicated that the energy levels themselves were more complex than the simple circular orbits Bohr proposed.
Other shortcomings include the violation of the Heisenberg Uncertainty Principle (as it defines both the position and momentum of an electron simultaneously) and its inability to explain the relative intensities of spectral lines.
Step 3: Final Answer:
The Bohr model is limited as it only works for single-electron systems and fails to account for the Zeeman and Stark effects.