The History of the Atom

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The History of the Atom
A Timeline of Thousands of Years of Thinking
about Matter
http//www.youtube.com/watch?vDYW50F42ss8
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Leucippus and Democritus
Democrituss universe theorized that the two
fundamental and oppositely characterized
constituents of the natural world are indivisible
bodiesatomsand void or space atoms move about
in the void (empty space), collide, attach to
others to form compounds. He believed atoms
contained
NO OPEN SPACE
Atom as solid matter
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History of the Atom

• 430 B.C.
• Leucippus of Miletus and Democritus of Abdera
• They debated whether a substance could be split
  indefinitely (continuous matter), or if
  eventually there would be a point when you could
  no longer break it down (discontinuous matter).
• They theorized that the universe was made up of
  void (space) and small indivisible ATOMS.
• This is the start of ATOMISM.
• 400 B.C.
• Aristotle and Plato
• Rejected Atomism believed in Fire, Earth, Air,
  and Water as the 4 elements
• This belief dominated science for over 2000
  years!

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History of the Atom

• 1770s
• Lavoisier proposed that in ordinary chemical
  reactions, matter cannot be created or destroyed
  (law of conservation of mass). He also changed
  the phlogiston theory to a modern theory of
  combustion.
• 1799
• Prousts observation of the fact that specific
  substances always contain elements in the same
  ratio by mass led to the law of definite
  proportions.
• Law of Definite Proportions/Law of Multiple
  Proportions
• http//www.youtube.com/watch?v3dWdMqZ2UOU

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History of the Atom

• 1803
• Dalton, known as the Father of Modern Atomic
  Theory, gathered experimental evidence
  supporting and explaining the theories of
  Lavoisier and Proust his was the first atomic
  theory based on experimental evidence.
• Daltons atomic theory
• (1) All matter is composed of atoms that are
  indivisible.
• (2) Atoms of the same element are identical.
• (3) Atoms of different elements are dissimilar.
• (4) Atoms of the same element can unite in more
  than one ratio with another element to form more
  than one compound. Atoms can unite with other
  atoms in simple numerical ratios to form
  compounds (law of multiple proportions).

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History of the Atom
John Dalton used simple symbols to represent
elements and because he was colorblind, he chose
to use black and white. Element table to the
right, graphic representation of Daltons atomic
theory below
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History of the Atom

• 1811
• Avogadro hypothesized that equal volumes of
  gases, at the same temperature and pressure, have
  the same number of molecules.
• http//www.youtube.com/watch?v13WUqWd_Y
  k8
• 1865
• Mendeleev arranged elements into 7 groups with
  similar properties. He discovered that the
  properties of elements were periodic functions
  of their atomic weights. This became known as
  the Periodic Law.

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History of the Atom
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History of the Atom
Mendeleev's Periodic Table (1871)
                                          
                                                  
                                                  
                                                  
            
http//www.youtube.com/watch?vkuQ0Um4Wc
z0
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History of the Atom
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History of the Atom

• 1870s
• Crookes experiments were some of the first
  evidence that electrons exist. He discovered
  Cathode rays had the following properties
  travel in straight lines from the cathode cause
  glass to fluoresce impart a negative charge to
  objects they strike are deflected by magnets to
  suggest a negative charge cause pinwheels in
  their path to spin indicating they have mass. His
  experiments were a model for other scientists to
  use in advancing the knowledge about electrons.
• http//www.youtube.com/watch?vu_rljVre-
  G4

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History of the Atom

• 1896
• Becquerel found that matter containing uranium
  exposes sealed photographic film. This led to
  the discovery that rays are given off by uranium
  and radium.
• 1898
• Marie and Pierre Curie discovered radium and its
  ability to give off rays. They called the
  spontaneous decay of these elements
  radioactivity.

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Radioactivity
Alpha Particles consist of 2 protons and 2
neutrons (same as the helium nucleus) and are
emitted by a radioactive source. They are large,
heavy and positively charged.
Beta Particles high-energy, high-speed electrons
or positrons emitted by a radioactive source.
They are much smaller and lighter than Alpha
Particles and are negatively charged.
Gamma Radiation electro magnetic radiation of
very high frequency produced by sub-atomic
particle interactions. It is energy, so it has no
mass or charge.
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History of the Atom

• 1897-1904
• J.J. Thomson, Thomson discovered the electron,
  the first known particle that is smaller than the
  atom, and experimentally determined its
  charge-mass ratio by deflecting them by magnetic
  and electric fields. He also showed that canal
  rays consist of positively-charged particles and
  calculated the mass of the proton to be at least
  a thousand times that of the electron. He also
  discovered isotopes by using a type of mass
  spectrometer. Thomson developed the plum-pudding
  model of atoms.
• 
  http//www.youtube.com/watch?vJwdGF
  ZA3WOsfeaturerelated
• 
  http//www.youtube.com/watch?vIdTxGJjA4J
  wfeaturerelated

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History of the Atom

• 1900
• - Planck introduced the idea that
  (electromagnetic) energy is
  radiated in small packets called quanta.
• 1911
• Millikan used an oil drop experiment to determine
  the charge on an electron (1.602 x 10-19 coulomb)
  and then used Thomsons e/m value to calculate
  the mass of an electron (9.11 x 10-28 gram).
• http//www.youtube.com/watch?v
  XMfYHag7Liw
• 1911
• Rutherford predicted the existence of the neutron
  
• and demonstrated that the nucleus is a small,
  positively
• charged heavy core in an atom, which consists
  mostly
• of space. (Gold foil experiments) Suggested the
  name
• proton for the fundamental positively charged
  particle,
• the nucleus of the hydrogen atom.
• http//www.youtube.com/watch?v5pZj0u
  _XMbc

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History of the Atom
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History of the Atom

• 1922
• Bohr used and modified Rutherfords model of the
  atom. Bohr originated the idea that electrons
  travel in a definite energy level around the
  nucleus with no loss of energy from their motion.
  He applied Plancks idea of small packets or
  quanta of energy to his electron model.
• Electrons in energy levels further from the
  nucleus have greater energy.
• An electron can give off a photon of energy
  equivalent to the difference in energy levels if
  it falls from one energy level to a lower one.
• An electron cannot fall to a lower energy level
  if it is full (an energy level can hold only a
  certain number of electron.)
• Normally every atom is in its ground state,
  meaning the lowest energy levels are full and no
  electron can lose energy.
• Atoms can absorb energy from an outside source
  and an electron can jump to a higher energy
  level, an excited state. It rapidly falls back
  and energy is emitted.

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History of the Atom
The Hydrogen Atom
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History of the Atom

• Schrodinger, 1930 Viewed electrons as
  continuous clouds and introduced wave mechanics
  as a mathematical model of the atom.
• Chadwick, 1932 Using alpha particles Chadwick
  discovered a neutral atomic particle with a mass
  close to a proton. He is credited with the
  discovery of the neutron.

                
                
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