Bohr's postulates


  • As shown previously and later, Bohr had seen Balmer's equation which made "everything clear" for him.
  • He then was able to apply quantum theory to form his own model of the atom, which attempted to fix the limitations of Rutherford's model.

Bohr's postulates

1. Electrons in an atom exist in STATIONARY STATES.

  • Bohr stated that electrons orbit the nucleus WITHOUT emitting EM radiation.
  • Any permanent change in their motion must be accompanied by a complete transition from one stationary state to another.
    • Note that Bohr could NOT explain this.

2. Transmission between stationary states produces/absorbs EM Radiation.

  • When an electron moves between stationary states, it is accompanied by the emission or absorption of a photon.
    • This photon's energy is given by $\Delta E = hf$
  • Thus, EM radiation is produced by the movement of electrons between energy states which account for Planck's "atomic oscillators."

3. The ANGULAR MOMENTUM of a stationary electron is QUANTISED

  • To explain, recall from Rutherford's model of the atom, in which the electrons orbited in circular orbits at ANY RADII.
    • If this coincided with Bohr's 2nd postulate, it would mean that every element can emit a FULL SPECTRUM as any transition would be possible.
    • As this is not the case, it would imply that electrons orbited at FIXED RADII.
  • Thus, this 3rd postulate allowed Bohr to explain the distinct spectra lines.
  • This can be represented mathematically by:
\begin{align} mvr = \frac{nh}{2\pi} \end{align}
  • Note that you will never be asked to do equations with this, just remember it.