ATOMS

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ATOMS

• A Brief History

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Democritus 400 B.C.

• A philosopher
• Good ideas but no scientific proof
• No one believed him because Aristotle was more
  popular (Earth, Air, Fire, Water)
• First to suggest the existence of atoms (atomos)
• Small indestructible pieces that make up
  everything
• Nothing is smaller than an atom

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Dalton 1800 A.D.

• A teacher and chemist
• Father of the Modern Atomic Theory
• Transformed Democritus ideas into a scientific
  theory
• Used experimental methods
• Studied how elements combine in chemical reactions

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Dalton (contd)

• Observations from experiments led him to the
  following explanations
• Everything is made of atoms. Nothing is smaller
  than an atom.
• Elements are a collection of same atoms. Atoms
  of an element are different from atoms of another
  element.
• Atoms can combine to form compounds in whole
  ratios.
• Chemical reaction atoms being rearranged
• Atoms can join together or groups of atoms can
  separate
• An atom cannot change into another atom

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Thomson 1897

• A physicist who was studying how matter could
  carry electricity.
• Observation
• When electricity passes through a few gas
  particles in a tube, a glowing ray appears.

http//www.makingthemodernworld.org.uk/icons_of_in
vention/science/1880-1939/IC.026/
http//www.aip.org/history/electron/jjcrooke.htm
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Thomson contd

• Hypothesis
• The ray is a stream of negative particles because
  it seems to originate from the negative electrode
  (cathode) of the tube.
• Experiment
• Pass electricity through different kinds of gas
  particles.
• Subject the rays to a magnetic field and to an
  electric field to see if anything changes.

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Thomson (contd)

• Results
• All rays are deflected by both magnetic and
  electric fields.
• All rays are deflected away from the negative
  part of the field.
• There is a constant ratio relating the mass and
  charge of the particles that made up the ray.

http//dev.physicslab.org/Document.aspx?doctype3
filenameMagnetism_CathodeRays.xml
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Thomson (contd)

• Conclusion
• Negative particles are a part of all matter
• Negative particles are the same in all matter
• What it means
• All atoms contain ELECTRONS!
• Follow-up
• Another scientist used CRT to discover the
  PROTONS in a similar way!

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Rutherford 1911

• A chemist
• Gold-Foil Experiment
• Thinnest possible layer of gold atoms
• Theory Large positive particles fired at the
  foil should be only slightly deflected when
  reaching the detector on the other side.

http//www.shsu.edu/chm_tgc/sounds/sound.html
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Rutherford contd
http//www.sparknotes.com/testprep/books/sat2/phys
ics/chapter19section2.rhtml
http//www.dlt.ncssm.edu/TIGER/chem1.htm

• Results Most went straight through! Some
  actually bounced back!
• Conclusion Atom is mostly empty space. The
  positive charge of the atom is concentrated in a
  tiny nucleus which contains most of the mass.

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Bohr 1913

• A physicist and student of Rutherford
• Studied changes in atoms when interacting with
  light
• Observation atoms change in specific ways when
  interacting with light.
• Explanation the electrons are actually organized
  around the nucleus and exist in set circular
  paths like planetary orbits.

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Quantum Model

• Newer theories now exist
• Nucleus still has protons and neutrons and
  contains most of the mass.
• Electrons are in highly complicated regions
  outside the atom. All of these together make it
  look like a simple electron cloud.
• Details will follow later this year!