Bohr's postulates

#### Introduction

- 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.**

- Note that

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.**

- If this coincided with Bohr's
- 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.

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