# What do you know about Atomic Physics ?

## Atomic Physics

### My summary

Rutherford shot alpha particles at a thin film of gold foil

Most of the alpha particles went right through

Some had high angles of deflection

He was shocked when a small number of the alpha particles came zooming back out in the direction they went in

He concluded that the atom must have a nucleus that is

1. a) very small compared to the total size of the atom.  That’s why most of the alpha particles go right through

1. b) very dense, with most of the mass of the atom concentrated there.  That’s why some alpha particles can bounce right back

1. c) positively charged.  That’s why the positively charged alpha particles’ paths are deflected

Narrated animation of the experiment:  http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

## Discrete emission spectra

When we heat or pass electricity through a gas, we see different colors of light depending on what element the gas is.

When we pass that light through a prism, we discover that the color is really a blend of a few discrete colors emitted by the gas

Each gas has a unique pattern of emissions

#### Art meets physics

The artist Georges Seurat used discrete dots or “quanta” to make paintings that look continuous.  This was in the late 1800’s, around the same time Max Planck was quantizing his formulas to try to explain the light emitted by heated metals

http://en.wikipedia.org/wiki/Georges_Seurat#mediaviewer/File:A_Sunday_on_La_Grande_Jatte,_Georges_Seurat,_1884.png

#### Bohr Model

The key idea of the Bohr model is that the electrons orbit around the nucleus in orbits with certain “special” radii

Transitions from one orbit to a lower orbit cause the emission of a photon

The distance of the jump determines the energy of the photon, and the energy determines the frequency

So Bohr has explained why we see discrete emission spectra

But he hasn’t explained why special radii exist

And his model has other shortcomings:

1. a) It only works for hydrogen

1. b) It doesn’t explain why some spectral lines are very bright and others are not

1. c) It doesn’t explain the fine detail of the hydrogen atom’s emissions

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