Bohr's Model of the Atom

Bohr's Atomic Model

Niels Bohr proposed a model of the atom that improved on earlier ideas by introducing the concept of electrons existing in fixed orbits around the nucleus. These orbits are also called energy levels.

• Electrons travel in specific circular paths or orbits around the nucleus.
• Each orbit corresponds to a fixed energy level; electrons in these orbits have quantised (set) energies.
• Electrons in a stable orbit do not lose energy by radiation, so they do not spiral into the nucleus.

This idea explained why atoms are stable and why electrons do not simply collapse into the nucleus, which was a problem with earlier atomic models.

For example, in a hydrogen atom, the electron orbits the nucleus at certain allowed distances, each with a specific energy. The lowest energy orbit is called the ground state.

For instance, if an electron moves from an orbit with energy $-3.4 \times 10^{-19}$ J to one with energy $-1.5 \times 10^{-19}$ J, the energy difference is $1.9 \times 10^{-19}$ J, which corresponds to the energy of the emitted photon.

Energy and Electron Transitions

Electrons can move between these fixed orbits by absorbing or emitting energy:

• When an electron absorbs energy, it jumps to a higher energy orbit (excited state).
• When it falls back to a lower energy orbit, it emits energy as a photon (a particle of light).
• The energy of the photon emitted or absorbed equals the difference in energy between the two orbits.
• The frequency $f$ of the photon is related to the energy difference $E$ by the equation:

$$E = hf$$

where $h$ is Planck6s constant ($6.63 \times 10^{-34}$ Js).

This explains why atoms produce specific lines in their emission or absorption spectra, as only photons with certain energies (and so frequencies) are emitted or absorbed.

For instance, if an electron in a hydrogen atom moves from the second orbit to the first orbit, it emits a photon with energy equal to the difference between those two energy levels.

Historical Context

Bohr6s model was an important improvement on the earlier Rutherford model, which described the atom as a nucleus surrounded by electrons but could not explain why electrons did not spiral into the nucleus.

Bohr introduced the idea of quantised energy levels, which explained:

• Why atoms are stable.
• Why atoms emit or absorb light at specific frequencies, producing line spectra.

However, the Bohr model has limitations:

• It works well only for hydrogen-like atoms with one electron.
• It cannot fully explain the behaviour of atoms with many electrons or more complex spectra.
• Later models, including quantum mechanics, provide a more accurate description of atomic structure.