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The History of the Atom Early Greeks to Present. Democritus (460 B.C.-370 B.C.) Greek Philosopher  Among the first to suggest the existence of atoms.

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Presentation on theme: "The History of the Atom Early Greeks to Present. Democritus (460 B.C.-370 B.C.) Greek Philosopher  Among the first to suggest the existence of atoms."— Presentation transcript:

1 The History of the Atom Early Greeks to Present

2 Democritus (460 B.C.-370 B.C.) Greek Philosopher  Among the first to suggest the existence of atoms.  Believed that atoms were indivisible and indestructible.  No experimental support.  Ideas not based on scientific method.

3 Aristotle (384 B.C.-322 B.C.) Greek Philosopher  Believed all matter was made up of atoms.  Gave matter the name hyle.  No experimental data.

4 John Dalton ( ) English Chemist and School Teacher  Used experimental methods.  Transformed Democritus’s ideas on atoms into a scientific theory.  Responsible for Dalton’s Atomic Theory.

5 Dalton’s Atomic Theory  All elements are composed of tiny indivisible particles called atoms.  Atoms of the same element are identical. The atoms of one element are different from those of any other element.  Atoms of different elements can physically combine in simple whole-number ratios to form compounds.  Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed into atoms of another element.

6 J.J. Thomson( ) Excuse me... how can you discover a particle so small that nobody has ever seen one?  1897-English Physicist  Discovered the electron.  Used cathode ray tube experiment to explain his theory.

7 Cathode-Ray Tube

8 Cathode Ray-Tube (Crooke’s Tube)

9 Cathode-Ray Tube Experiment  Thomson performed experiments that involved passing electric current thru gases at low pressure.  He sealed the gases in glass tubes fitted at both ends with metal disks called electrodes.  Electrodes were connected to a source of electricity.  Two ends of the tube 1. cathode- negatively charged electrode. 2. anode- positively charged electrode.  Cathode Ray- glowing beam that travels from the cathode to the anode.

10 Cathode Ray Tube Experiment Continued…  The beam traveled across the tube from the cathode to the anode.  The beam was deflected by negative charges, but was attracted to positive charges.  He concluded that the particles that made up the beam must be negatively charged. He called them electrons.

11 Plum Pudding Model  J.J. Thomson proposed this model of the atom.  Electrons were stuck into a lump of positive charge, similar to raisins stuck in dough.

12 Robert A. Millikan ( ) U.S. Physicist  Calculated the mass of electron.  Values for electron charge and mass reported in 1916 are very similar to those accepted today.  Electrons carry exactly one unit of negative charge.  Electrons have a mass of 1/1840 the mass of a hydrogen atom.

13 Eugen Goldstein ( ) German Physicist  Observed rays traveling in the direction opposite to that of the cathode rays.  He called these rays canal rays and concluded they were positively charged.  Now called protons.  Mass of a proton is 1840 times that of an electron.

14 James Chadwick ( ) English Physicist  1932-Confirmed the existence of another particle with no charge- neutron.

15 Ernest Rutherford ( ) Student of Thomson  Tested the theory of atomic structure using the gold-foil experiment.  Suggested a new theory base on his results.

16 Rutherford’s Gold Foil Experiment

17 Summary of Gold-Foil Experiment  A narrow beam of alpha particles was directed at a very thin sheet of gold foil (1 atom thickness).  According to the previous theory, the particles should have passed thru.  Most particles passed through and some of them bounced back or were deflected.

18 Rutherford’s Conclusions  Atoms are made up of mostly empty space.  Atoms have a positive center called a nucleus.  Most of the mass of the atom is located in it’s nucleus.  Improvement over the Plum-Pudding Model, but still incomplete. Rutherford’s New Atom Plum Pudding Model

19 Niels Bohr ( ) Danish Physicist Bohr debating Quantum Theory with Albert Einstein

20 Bohr Model of the Atom  Planetary Model was correct.  Electrons were in orbits around the positively charged nucleus.  Based on Quantum Physics.

21 Electron Cloud Model  Electrons are not in perfect orbits around the nucleus.  Imagine, as the electron moves it leaves a trace of where it was.  This collection of traces quickly begins to resemble a cloud.  This collection of traces quickly begins to resemble a cloud.

22 Thanks to the following Websites:  hys05/catomodel/cloud.htm hys05/catomodel/cloud.htm hys05/catomodel/cloud.htm   terest/Harrison/BohrModel/BohrModel.ht ml terest/Harrison/BohrModel/BohrModel.ht ml terest/Harrison/BohrModel/BohrModel.ht ml  911.html 911.html 911.html


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