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# Bohr Model of the Atom

## Also known as the planetary model, electrons are organized into energy levels with the nucleus at the center.

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Scanning Tunneling Microscopy

### Scanning Tunneling Microscopy

Credit: Taner Yildirim (The National Institute of Standards and Technology - NIST)
Source: http://en.wikipedia.org/wiki/File:Chiraltube.gif

A scanning tunneling microscope is used to see surfaces at the atomic level. Seen above is a chiral nanotube seen in STM experiments.

#### Amazing But True

• One of the methods currently available to scientists to image individual atoms is done so by a scanning tunneling microscope. The STM works on the concept of quantum tunneling. To understand quantum tunneling, you need to first understand what happens classically with electrons in metals. Classically, the loosest electrons in metals are always held in the metal unless given enough energy which is defined by the work function. If the electrons are not given energy greater than the work function, they are not knocked loss.
• When looked at quantum mechanically though, it is possible for the loosest bound electrons to tunnel through the barrier in less energy than that required to leave the metal. By placing two pieces of metal together, a finite well is created where the most weakly bound electrons have a probability of tunneling through the energy barrier given by \begin{align*}e ^{-\alpha a}\end{align*}.
• By using a very small electrically charged probe, you can monitor the electrical current and plot where atoms are located on the surface of a metal. This is how a scanning tunneling microscope works.
• Credit: O. Usher (UCL MAPS)
Source: http://www.flickr.com/photos/uclmaps/11796325765/

Scanning Tunneling Microscopes attain atomic scale images of metal surfaces [Figure2]

#### Explore More

Using the information provided above, answer the following questions.

1. Why don't we see objects tunnel through walls in our everyday life?
2. Why can't someone use a regular microscope to view atoms?

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