Atomic Structure & Electromagnetic Radiation Chapter 5

The atom is very small. About a million atoms stacked on top of each other = the thickness of a sheet of paper

The Atom- can we see it?? 1981- Gerd Binnig and Heinrich Rohrer (Switzerland) developed a scanning tunneling microscope (STM)

Scanning tunneling microscope: Works by moving extremely sharp electrode over a surface and measuring the tunneling current that results (the electrical current that flows between the tip of the electrode and the surface, even though the two are not in physical contact) The tunneling current is extremely sensitive to distance, making it possible to a precise separation between the tip and surface by moving the tip up or down to compensate for changes in current. As the tip goes over an atom, it moves up away from the surface to maintain a constant current. As it goes over a gap between atoms, it moves down toward the surface to maintain a constant current The microscope creates an image that shows the location of individual atoms on that surface.

Scanning tunneling images of atoms

Scanning tunneling image of Kanji characters for “atom” written with iron atoms (red) on a copper surface (blue). Later work by other scientists showed that STM can be used to pick up and move individual atoms or molecules, allowing structures and patterns to be made one atom at a time If all the words in the books in the Library of Congress- 35 million books occupying 840 miles of shelves- were written in letters the size of these Kanji characters, they would fit into an area of about 5 square millimeters!

1808- Dalton’s Atomic Theory 1) Each element composed of tiny, indestructible particles called atoms 2) All atoms of given element have the same mass and other properties that distinguish them from atoms of other elements 3) Atoms combine in simple, whole-number ratios to form compounds 4) Atoms of one element cannot change into atoms of another element. In a chemical reaction, atoms only change the way they are bound together with other atoms.

Atomic Structure… major experiments JJ Thomson- Cathode ray tube (electrons) Robert Millikan- oil drop experiment (magnitude of charge of electron) Ernest Rutherford- gold foil experiment (nuclear theory) James Chadwick- neutrons

Ernest Rutherford’s Nuclear Theory 1) Most of atoms mass and all of its positive charge is contained in a small core called the nucleus 2) Most of the volume is empty space, throughout which tiny, negatively charged electrons are disperse. 3) There are as many negatively charged electrons outside the nucleus as there are positive protons within the nucleus. Therefore, the atom is electrically neutral.

If the atom is empty space, why does it appear solid If the atom is empty space, why does it appear solid? If the nucleus was the size of a grape, its nearest electron would be 1 mile away! Imagine scaffolding 100 stories high and the size of a football field. The volume is mostly empty space. But, from an airplane it would appear solid.