The Bohr Model and Atomic Spectra - SS2 Physics Lesson Note

The Bohr model, proposed by Niels Bohr in 1913, is a simplified model of the atom that successfully explains the discrete emission and absorption spectra observed in atomic systems.

Bohr Model of the Atom:

The Bohr model describes the structure of atoms based on the following key postulates:

-       Energy Levels: Electrons in an atom occupy specific energy levels or orbits around the nucleus. Each energy level corresponds to a fixed amount of energy.

-       Stationary Orbits: Electrons can only exist in certain stable, non-radiating orbits called stationary states. In these orbits, electrons do not emit or absorb energy.

-       Quantized Energy: Electrons can change energy levels by absorbing or emitting energy in discrete packets called quanta or photons. The energy of a photon is directly proportional to the frequency of light emitted or absorbed.

-       Stability: The stable orbits are those for which the angular momentum of the electron is an integer multiple of Planck's constant divided by 2π.

Atomic Spectra:

One of the major successes of the Bohr model is its ability to explain the observed emission and absorption spectra of atoms. When atoms are excited or de-excited, they emit or absorb electromagnetic radiation, respectively, resulting in distinct spectral lines.

-       Emission Spectra: When an electron transitions from a higher energy level to a lower energy level, it emits energy in the form of photons. These photons have discrete wavelengths, which correspond to specific spectral lines observed in the emission spectrum.

-       Absorption Spectra: When atoms absorb photons of specific energies, electrons are excited to higher energy levels. The absorbed energy matches the energy difference between the initial and final energy levels, resulting in the absence of specific wavelengths in the absorption spectrum.

Explanation of Atomic Spectra:

The Bohr model provides a simple explanation for the discrete spectra observed in atoms:

-       Quantized Energy Levels: The discrete energy levels in the Bohr model result from the quantization of electron orbits. Only certain orbits are allowed, with specific energies corresponding to specific energy levels.

-       Transitions between Energy Levels: When an electron transitions between energy levels, energy is emitted or absorbed as a photon. The energy of the photon is determined by the energy difference between the initial and final levels, resulting in discrete spectral lines.

-       Balmer Series: The Bohr model successfully predicts the wavelengths of the spectral lines in the hydrogen atom's visible emission spectrum. The Balmer series, for example, corresponds to electron transitions from higher energy levels to the second energy level.

Limitations and Advances:

While the Bohr model successfully explains the atomic spectra of hydrogen and some other simple atoms, it has limitations when applied to more complex atoms. It does not account for electron-electron interactions or fully incorporate quantum mechanics.

However, the Bohr model laid the foundation for the development of quantum mechanics and inspired further research into the behaviour of electrons in atoms. It provided valuable insights into the quantized nature of energy levels and played a crucial role in the understanding of atomic structure and spectroscopy.