The History of AtomicTheory

• Try to place yourself in the time-frame of these scientists…

• Remember, our current technologies were not yet available

• Pay attention to the chronology of these discoveries (i.e. the pace of discovery)

• “Excuse me... how can you discover a particle so small that nobody has ever seen one?”--J.J. Thomson

Ancient Greece, 400 B.C.

• Democritus• Through philosophical

thought process, he rationalized that there must be some smallest particle upon which nature was founded

• Called this particle Atomos

• Other philosophers such as Plato, Sophocles, and Aristotle developed ideas of their own regarding the nature of matter, however, Democritus is generally credited

England, 1800 A.D.

• John Dalton• Performing exacting

experiments with gases and how they combine, Dalton developed Four Postulates describing the results of his work.

Picture courtesy of http://atoms.hypermart.net/dalton/

Dalton’s Four Postulates

1. Elements are composed of atoms (sub-microscopic, indivisible particles).

2. Atoms of an element are similar to each other, but different than other element atoms.

3. Elements can mix together, or chemically combine to form compounds, but only in whole number ratios.

4. Atoms can rearrange, but an element cannot be changed into another element as a result of a chemical reaction.

Dalton’s Model (solid sphere)

Germany, 1886

• Eugen Goldstein

• Expanding on the work of Crookes and his vacuum tubes, Goldstein combines cunning logic and a “simple” experiment to discover the proton

• Goldstein reasoned that if a neutral particle gives off a negatively charge particle in the presence of an electric field, then surely it must also produce a positively charged particle as well.

Goldstein’s Apparatus

Picture courtesy of http://dbhs.wvusd.k12.ca.us

England, 1897• Sir Joseph John Thomson

• Thomson expanded upon the work of other scientists such as:• Thomas Edison who had

already constructed a complete electric circuit (circa 1883)

• William Crookes who developed the original gas vacuum tube, called a Crookes Tube (circa 1870)

Picture courtesy of www.aip.org

Thomson’s Experiment• Thomson utilized a

sealed gas in a chamber/tube with two electrodes, called a Cathode Ray Tube.

• The electrodes were connected to a high voltage source, at which point a glowing beam of light formed.

• Thomson discovered that regardless of the gas present in the tube, or the type of metal used for the electrodes, the eerie beam of light was always present.

• He concluded that a small particle, which he called a “corpuscle” and was later known as an electron, must be a part of the atoms of all elements.

http://micro.magnet.fsu.edu/electromag/java/crookestube/index.html

Thomson’s Apparatus

Animation & diagram courtesy of www.aip.org

Plum Pudding Model

Diagram courtesy of Addison Wesley Publishers

America, 1916• Robert A. Millikan

• Calculated the charge on an electron in his very famous “Oil Drop” experiment

• He was also able to calculate the mass of an electron from the “Oil Drop” experiment

• http://www.bun.falkenberg.se/gymnasium/amnen/fysik/millikaneng.html

Picture courtesy of http://www.nobel.se

England, 1932• Sir James Chadwick• The existence of an electrically

neutral particle had already been theorized, but as yet no one had documented proof of its existence.

• Expanding on his own work with radioactive Beryllium (and other elements) and y-radiation, Chadwick reasoned that the energy emitted from Beryllium could not be solely y-radiation.

• Chadwick then hypothesized the presence of another particle, the neutron, and included this particle in his calculations.

• With the addition of the neutron into the equation, Chadwick’s observations of the emitted energy agreed well his own calculations.

The History of AtomicStructure

• Now that we are familiar with the history behind protons, neutrons and electrons, we can ask ourselves the question:

• How did we come to our current model of the structure of the atom?

Manchester, England 1911

• Ernest Rutherford

• Rutherford poses the following questions:• How are the neutrons and

protons arranged in the atom?

• How are electrons so easily stripped from an atom to form ions?

Picture courtesy of www.aip.org

Gold Foil Experiment

• Rutherford decided to test the “Plum Pudding” model of atomic theory…

• He devised the following apparatus utilizing radioactive alpha particles…

Diagram courtesy of Addison Wesley Publishers

 

http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.htmlsetup explanation  http://www.micro.magnet.fsu.edu/electromag/java/rutherford/gold foil scattering  http://www.phys.virginia.edu/classes/109N/more_stuff/Applets/rutherford/rutherford2.htmlThomson scattering http://www.phys.virginia.edu/classes/109N/more_stuff/Applets/rutherford/rutherford.htmlRutherford scattering 

Rutherford con’t

The Nucleus

• Rutherford’s new proposal for the structure of an atom…

• If the nucleus were the size of a penny, then the entire atom would be the size of the metrodome!

Diagram courtesy of Addison Wesley Publishers Diagram courtesy of www.britannica.com

The story of the atom is to be continued next semster…

Coming up:

Quantum Theory of the early 1900’s!

Planck

Einstein

Bohr

Heisenberg

Schroedinger

England, 1900

• Max Planck• Studying radiation

emission from hot objects (think of a hot stove burner) relative to temperature, intensity and wavelength

• The current belief was that a continuous spectrum of energy emission was possible

Picture courtesy of www.nobel.se

Quite the Conundrum• The prevailing law of

physics predicted continuous light spectrums…

• Experimental results, however, showed a different picture…

Diagram courtesy of http://csep10.phys.utk.edu

Diagram courtesy of http://spacesciencegroup.nsula.edu

Climbing the Ladder

• Planck made a revolutionary statement:• Energy can be released or

absorbed by atoms only in fixed “chunks” or quantum…

• Energy of quantum is proportional to frequency

Diagrams courtesy of http://csep10.phys.utk.edu

Germany to America, 1905

• Albert Einstein

• Expanding on Plank’s work, Einstein studied the unusual phenomenon known as the photoelectric effect…

Picture courtesy of www.chembio.uoguelph.ca

Photoelectric Effect

• When light shines on a clean metal surface, sometimes the surface emits electrons…

• For each metal, there is a minimum frequency of light below which no electrons are emitted…

What a Genius…

• Einstein assumed that energy (light) striking the metal surface must behave like a stream of tiny energy packets, called photons…

• Each photon MUST have a quantized energy proportional to the frequency of the light…

• Simplified Photoelectric Effect Animation

• http://faculty.ucc.edu/chemistry-pankuch/Photoelectric/PE3.html

Denmark, 1913

• Niels Bohr• Bohr posed the question:

• If the electron is negatively charged, and the proton is positively charged…why don’t orbiting electrons crash into the nucleus and collapse the atom?

Picture courtesy of www-groups.dcs.st-and.ac.uk

Look to the Heavens• Bohr looked at our

solar system for inspiration in creating his atomic model…

• He proposed that electrons orbit the nucleus in fixed orbits corresponding to quantized energy

Diagram courtesy of www.howstuffworks.com

Germany, 1920’s• Werner Heisenberg• Knowing that matter

behaves both as a particle and as a wave, Heisenberg extrapolated the Uncertainty Principle…• It is inherently impossible

to simultaneously know both the exact momentum and location in space

Picture courtesy of www. scienceworld.wolfram.com

Austria, 1926

• Erwin Schrodinger

• Combined Heisenberg’s Uncertainty Principle with Bohr’s atomic model (and a whole lot of Super-duper complex mathematics) to create…

Picture courtesy of www.nobel.se/physics/laureates/

Atomic Orbitals & Quantum Mechanics

Pictures courtesy of www.orbitals.com