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Atomic Theory Timeline

time-line of the history and development of the atomic theory
by

Mallory S

on 9 February 2011

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Transcript of Atomic Theory Timeline

Atomic
Theory
Timeline Events Leading to the
Development of the Atomic Theory The first theory of the atom was proposed by Democritus around 430 B.C. However it was John Dalton who proposed the first Atomic Theory in 1811. His theory had four parts. First, all matter is made up of tiny indivisible and indestructible atoms. Second, atoms of the same element are the same. Third, atoms of different elements are different. And finally, atoms combine in whole number ratios to form compounds. Dalton based his theory on the work of Lavoisier (Law of Conservation of Mass) and Proust (Law of Definite Proportions). His theory was supported by Gay-Lussac (Law of Combining Volumes) and Amedeo Avogardo (Avogardo’s Hypothesis).
The discovery of the three atomic particles (proton, electron, and neutron) between 1897 and 1932 led to the first revision of Dalton’s atomic theory. It was revised for a second time when isotopes were discovered, and it was determined that atoms of the same element are not identical, and may have different numbers of electrons.
Wilhelm Roentgen discovered X-rays in 1895 and Henry Mosley experimented with them in 1913 to discover the atomic number of an element. Ernest Rutherford conducted the gold-foil experiment in 1911, which determined that the atom is mostly empty space, with a very small, positive nucleus. Niels Bohr improved upon the atomic model by trying to explain why electrons are not pulled into the nucleus. Bohr based his model of the atom on a theory developed by Max Planck, called Quantum Theory. This theory suggests that energy is not continuously emitted, but is sent out in small packets called quanta. The foundation for Modern Atomic Theory was based on DeBroglie’s Hypothesis (1923), Heisenberg’s Uncertainty Principle (1927), and Schrodinger’s Wave Equation (1926). Importance Of These Discoveries These ideas and discoveries are important for later scientific endeavors because we can refer back to them and learn from our mistakes. We can use them to see what we do and don’t know which can give us ideas on how to find new information. They could also be used as research for current experiments. These ideas are also the basis of all our knowledge of atoms and subatomic particles. 430 BC
Democritus proposed first idea of the atom:
substances made up of tiny indestructible, invisible particles called "atomos." 1810-1811
John Dalton Law of Multiple Proportions:
same two elements can combine in different compounds; ratio of first element is fixed, while the second element varies

proposed first Atomic Theory
1- all matter is made up of tiny indivisible atoms
2- atoms of the same element are the same
3- atoms of different elements are different
4- atoms combine in whole number ratios to form compounds

"Dalton's Model" LaVoisier
1788 Law of Conservation of Mass:
matter cannot be created or destroyed, only rearranged 1794
Proust Law of Definite Proportions:
a pure substance will always contain the same elements, combined in the same proportion by mass Gay-Lussac
1804 Law of Combining Volumes:
at constant temperature and pressure, volumes of reacting gases and gaseous products are in the ratio of small whole numbers Amedeo Avogardo
1811 Avogardo's Hypothesis:
equal volumes of gases, under the same conditions have the same number of particles (6.02x10^23) 1895
Wilhelm Roentgen discovered X-rays Henri Bequerel
1896 discovered that uranium ores emit radiation resembling X-Rays and that it exposed film 1897
JJ Thompson used cathode rays to discover the electron
measured the bending of the rays to determne the charge to mass ratio of the electron

modified cathode ray tubes were used to discover the positively charged proton

Plum Pudding Model Max Planck
1900 Quantum Theory:
energy is not emmited constantly, but in small packets called quantum 1905
Albert Einstein Photoelectric Effect:
when light of a certain frequency shines on a metal, electrons are emitted Ernest Rutherford
1911 studied substances, especially alpha particles

conducted gold foil experiment: He bombarded gold foil with alpha particles. Most particles passed through the foil but some were deflected. He concluded that the atom is mostly empty space, with a very small positive nucleus core

Nuclear Model 1911
Robert Millikan performed oil drop experiment, which determined the charge of electrons as negative Henry Mosley
1913 used X-Ray experiments to discover the atomic number of an element 1913
Niels Bohr improved upon the atomic model by trying to answer the question: why aren't electrons pulled into the nucleus?

electrons travel in definite energy levels without radiating energy
electrons in each orbit have a certain amount of energy
energy increases as distance from nucleus increases
electrons lose energy by dropping energy levels

Bohr's Model 1923 DeBroglie's Hypothesis:
if waves can behave like particles, then particles can behave like waves 1926 Schrodinger's Wave Equation
-treat the electron like a wave
-developed an equation used to determine the probability of finding the electron in any given place around the nucleus
-solutions to the equation are quantum numbers

Quantum Model 1927 Heinsenberg's Uncertainty Principle:
it's impossible to know both position and velocity of an electron at the same time 1932
James Chadwick discovered the neutron New Discoveries According to an article published in The Seattle Times in July of 2010, European physicists made a discovery that could change everything about atomic structure. Researchers discovered new measurements, which show the radius of the proton is actually four percent smaller than what it was previously thought to be. If this is true, then the standard model of the atom would be deemed incorrect. This would be a huge problem, and would mean we know a whole lot less about atoms than we thought. The consequences of such a drastic change could be revolutionary. The Seattle Times
Article-"Discovery Shakes Up Standard View Of Atomic Structure"
http://seattletimes.nwsource.com/html/nationworld/2012302516_proton08.html Societal Issue One recent societal issue about the atom is whether or not to use the atomic bomb to seal off the oil spill in the Gulf of Mexico. I can see both the pros and cons to this idea. It would be the perfect solution because the extreme heat generated by the explosion might seal off the oil well. But then there is the overwhelming chance that it wouldn't work. The only tests on this theory that have been done have all been on land and none of them involved oil. Therefore I do not agree with this idea, because it is not likely to be successful, and could end up causing much worse problems. Had scientists tried to go through with this idea, then it would definitely have impacted science and society, because future oil spills would not be as much of a problem if we had a definite solution. However the idea was deemed too dangerous, and wasn't tested, and therefore did not impact science or society. Oil Spill The New York Times
"Nuclear Option on Gulf Oil Spill? No Way, U.S. Says"
June 2, 2010
http://www.nytimes.com/2010/06/03/us/03nuke.html
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