Democritus 460 BC - 370 BC

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Early History of the Atom
How could you find the Invisible Man in
Picadilly Circus?.By the reactions of those he
pushed aside. -Ernest Rutherfords advice to
James Chadwick on looking for evidence of
the neutron. 1
Cathy Cobb and Monty L. Fetterolf, The Joy of
Chemistry (Amherst, NY Prometheus Books, 2005),
p.46.
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Democritus460 BC - 370 BC

• went against, Aristotle, who believed that matter
  was composed of four qualities earth, fire, air
  and water
• all matter is composed of small, indivisible
  particles
• coined the term atomos
• John Dalton
• 1766-1844
• developed a model on the atom based on already
  established laws
• the law of conservation of mass (Lavoisier)
• the law of definite proportions (Proust)
• the law of multiple proportions (Dalton)

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Law of Definite Proportions
Mass of Water Mass of Hydrogen Mass of Oxygen MassRatio
18 g 2 g 16 g
9 g 1 g 8 g
36 g
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Summary of Laws Used by Dalton
Law of Conservation of Mass
Mass is neither created nor destroyed in a
chemical reaction.Example 2 g of hydrogen
reacts with 16 g oxygen to produce 18 g water.
Law of Definite Proportions
Elements combine in definite, fixed ratios by
mass to form compounds. Example water is always
1 gram of hydrogen for 8 grams of oxygen.
Law of Multiple Proportions
When 2 or more elements combine to form different
compounds, the ratio of the second element to a
fixed mass of the first is always a small, whole
number ratio. Example Water versus hydrogen
peroxide if 1 g of hydrogen in each, then 8 g of
oxygen in water to 16 g of oxygen in hydrogen
peroxide.This gives 1 to 2 ratio by mass of
oxygen in the two compounds.
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Daltons Atomic Theory1803

• 1) All matter is made up of extremely small
  particles called atoms.
• 2) Atoms of a given element are identical in
  size, mass and otherproperties atoms of
  different elements differ in size, mass and other
  properties.
• 3) Atoms cannot be subdivided, created or
  destroyed.
• 4) Atoms of different elements can combine in
  fixed, small, whole number ratios to form
  compounds.
• 5) Chemical reactions involve the combining,
  separating or rearrangement of atoms to form new
  substances.
• Daltons model of the atom

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Sir William Crookes Cathode Ray Tubes (1870s)
http//members.chello.nl/h.dijkstra19/page7.html
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Crookes Maltese Cross Tube
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Sir J. J. Thomson1856 - 1940

• What were these particles???
• Thomson hypothesis since the beam came from the
  cathode (a negative electrode), and given how the
  ray is deflected by a magnet, the particles have
  a negative charge.

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J. J. Thomson experiments with the CRT1890S
Tried to separate the charge from the particles
by bending the rays with a magnet and then
deflecting with electric plates. He could not
separate the two.

Fig. 2.3b, p.62
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Reverse the electrodes on the cathode ray tube
deflection by positive plate!!
Discovery of a positive component. Thomson used
the hydrogen ion, which is just a single proton,
so he missed that atoms have positively charges
particles - he thought the whole atom was a ball
of positive charge.
Fig. 2.5, p.63
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J. J. Thomson experiments with the CRT1890S

• Calculated the charge to mass ratio of these
  particles by measuring the
• degree of deflection with different strength
  magnetic and electric
• fields.
• No matter which gas he used in the tube, the
  calculated charge to mass
• ratio of the particles was the same (1/1800
  of a hydrogen atom).
• Thomsons experiments provided convincing
  evidence that the rays consisted of charged
  particles and he is credited with the discovery
  of the electron (Nobel Prize 1906)

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Thomsons Plum Pudding Model (1897)

• Thomson concluded that the cathode ray was made
  up of negatively charged subatomic particles,
  later called electrons (e-)
• No matter what metal the electrodes were made of,
  or what gas filled the tube, the same charge to
  mass ratio was calculated. All atoms have them,
  and they are identical in all atoms
• Thomson reasoned that atoms also must have a
  positive component - he reasoned a ball of
  positive charge with the negative charges spread
  throughout, like raisins in plum pudding.

ball of positive charge
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Millikans Oil Drop Experiment 1909

• Determined the charge of an electron. Given
  Thomsons charge to mass ratio calculation, the
  mass of an electron could then also be
  determined.
• Oil drops were given a negative charge by
  attaching electrons to them. By balancing the
  force of gravity with the force from an electric
  field on these droplets, Millikan could determine
  the magnitude of the charge on each oil drop, and
  found that the charge was always some multiple of
  1.6 x 10-19 C.
• Therefore, the charge of a single electron was
  1.6 x 10-19 C.

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Millikans Oil Drop Experiment 1909
Fig. 2.4a, p.63
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Radioactivity

• One of the pieces of evidence for the fact that
  atoms are made of smaller particles came from the
  work of Marie Curie (1876-1934).
• She discovered radioactivity, the spontaneous
  disintegration of some elements into smaller
  pieces.

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Types of Radioactive Emissions
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Types of Radioactive Emissions
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Ernest Rutherford

• New Zealand (1871-1937)

Responsible for the modern, nuclear view of the
atom
Rutherford laboratory
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• Gold Foil Experiment (1909)Rutherford was
  trying to prove the Plum Pudding model.If
  supported, the alpha particles would all pass
  straight through the foil.

02M11AN1.MOV
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Rutherfords Observations

• Most of the alpha particles passed straight
  through, as expected.
• Some were slightly deflected.
• Some were completely deflected!! These
  deflections were unexpected.

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Rutherfords Thoughts
.as if you had fired a 15-inch artillery
shell at a piece of tissue paper and it came
back and hit you.
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Rutherfords Conclusions and Model

• Majority of the atom was empty space
• The positive charge and 99 of the atoms mass
  concentrated in a small volume in the center.
  Discovery of the nucleus!! (dense and positive)
• Electrons orbited around the nucleus. Much
  smaller mass than the nucleus.

Rutherfords Model1911

• Two problems
• 1. The mass of atom did not equal the mass of
  the positive charge alone.
• Why didnt the electrons fall into the nucleus?
  What keeps them in
• motion?

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Rutherfords First Problem

• Rutherfords model suggested atoms composed of
  two subatomic particles with equal but opposite
  charges electrons and protons (see last slide).
• Problem - mass of atom not equal to mass of
  protons alone - must be another subatomic
  particle to make up the difference.
• 1932 - Chadwick discovers the neutron - a
  subatomic particle present in all atoms that has
  about the same mass as a proton but no electrical
  charge!!

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Rutherfords Second Problem

• Why didnt the electrons fall into the nucleus?
  What keeps them in motion?
• ..answered by Bohr..next chapter...stay tuned

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ABOUT PROTONS
In the early 1920's Rutherford and other
physicists made a number experiments, transmuting
one atom into another. In every case, hydrogen
nuclei were emitted in the process. It was
apparent that the hydrogen nucleus played a
fundamental role in atomic structure, and by
comparing nuclear masses to charges, it was
realized that the positive charge of any nucleus
could be accounted for by an integer number of
hydrogen nuclei. By the late 1920's physicists
were regularly referring to hydrogen nuclei as
'protons'. The term proton itself seems to have
been coined by Rutherford, and first appears in
print in 1920.
http//www.physlink.com/Education/askExperts/ae46.
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