1. Bohr’s Model of the Atom
Spectral Lines, Photons, and Quantum Mechanics!

2. Before Bohr’s Model Rutherford’s Model: The Model
Central, dense, positive nucleus
Electrons somewhere orbiting nucleus
Rutherford assumed that electrons orbited like planets -- he assumed they followed classical physics.
What’s wrong with this? Under classical mechanics, a charged particle traveling in a curved path emits energy and would therefore slow down.
The Classical
Physics Problem
Energy

3. Waves and Wavelength
The distance between any two corresponding points on one of these water waves, such as from crest to crest, is the wave’s wavelength, l .
We can measure the wave’s frequency, n , by observing how often the water level rises and falls at a given point, such as at the post.

4. Background on Light Wave nature of light: Particle nature of light
Light travels in a similar way to waves in water or Slinky
c = ln
c = speed of light
= 3.0 x 108 m/s
l = wavelength
n = frequency
Particle nature of light
Max Planck proposed that objects emit energy in small, specific amounts called quanta.
Einstein expanded on this, proposing that light is made of particles, called photons. These carry a quantum of energy, different for each wavelength. l

5. Photoelectric Effect
Einstein’s Nobel prize was for describing this effect:
Electromagnetic radiation strikes the surface of the metal, ejecting electrons from the metal and creating an electric current.
Only certain frequencies will cause this to happen.
ex. Red light, no matter how bright it is, does not have enough energy. Blue light, even if it’s dim, has enough.

6. Bohr’s Idea
Bohr said electrons were in set orbits, like rungs of a ladder.
You can only be on one orbit (rung) or the next, not in the middle -- they are quantized
Electrons can only move from one to the other by emitting or absorbing a particular wavelength of light (a photon with a certain energy), similar to the metal in the photoelectric effect.

7. Atomic Excitation
Picture by

8. Is there proof? Normal Spectrum of Light: Spectral lines:
White light, split up with a prism, looks like a continuous spectrum:
Spectral lines:
When an element is excited, there are lines rather than a spectrum of light.
To go from an excited state to the ground state, atoms must emit a certain frequency of light. These are constant for each element, as the electrons return from various outer “orbits.”

9. How is this used?
Astronomers use these unique patterns to identify the composition and heat of stars.
This is how we know that stars are 90% hydrogen.

10. Seeing these lines on Earth
Spectral lines can be observed by sending electric current through a type of light bulb which is filled with the gas form of a given element and using a prism to break up the light emitted.
ex. hydrogen, neon, mercury
Absorption lines can be observed when a colored, transparent substance is held in front of a white line.