Democritus (460-370 B.C.)

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Democritus (460-370 B.C.)
Democritus was one of a few Greek philosophers
who believed that all matter in the world was
made of of indivisible parts which he called
atomos, which means indivisible.
Although this theory was to be discovered to
later be the truth, Democritus ideas faded until
the seventeenth century in Europe.
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John Dalton (1766-1844)

• Dalton is known for his atomic theory
• Theory states that atoms are the smallest
  chemical building block of nature
• His four postulates
• Each element is composed of extremely small
  particles called atoms
• All atoms of a given element are identical the
  atoms of different elements are different and
  have different properties (including different
  masses)
• Atoms of an element are not changed into
  different types of atoms by chemical reactions
  atoms are neither created nor destroyed in
  chemical reactions
• Compounds are formed when atoms of more than one
  element combine a given compound always has the
  same ratio and kind of atoms
• ATOMS ARE SMALL AND CANNOT BE DIVIDED (later
  changed!)

http//en.wikipedia.org/wiki/John_DaltonAtomic_th
eory
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Discovery of Atomic Structure

• By 1850 scientists knew that atoms were composed
  of charged particles.
• Electrostatic attraction
• Like charges repel
• Opposites attract

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J.J. Thomson

• Experimented with cathode rays and found that the
  properties didnt change
• Distinguished charges within atoms, positive and
  negative charges
• Plum-pudding model
• Found electron charge ratio

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Cathode Rays and Electrons

• C.R. 1st discovered in mid-1880s from studies of
  electrical discharge thru partially evacuated
  tubes (CRTs)
• Cathode rays radiation produced when high
  voltage is applied across the tube.
• The voltage causes negative particles to move
  from the negative electrode (cathode) to the
  positive electrode (anode).
• The path of electrons can be altered by the
  presence of a magnetic field.

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Consider cathode rays leaving the positive
electrode through a small hole.

• If they interact with a magnetic field
  perpendicular to an applied electric field, then
  the cathode rays can be deflected by different
  amounts.
• Amount of deflection depends on applied magnetic
  and electric fields.
• Deflection also depends on the charge-to-mass
  ratio of an electron.
• Thomson determined the charge-to-mass ratio of an
  electron in 1897.
• Charge-to-mass ratio 1.76 x 108 C/g
• C Coulomb, SI unit of electric charge

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Robert Millikan

• Millikan built on J.J. Thomsons work on
  electrons by measuring their charge with his
  famous oil-drop experiment.
• Using Thomsons ratio, Millikan calculated
  electrons mass of
• 9.10 x 10-28 g which proved how much smaller
  electrons are than nucleus particles.

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Millikan Oil-Drop Experiment

• Sprayed oil drops over the hole in a positively
  charged plate and measured the electrostatic
  force of attraction.
• Found the charge on the electron to determine its
  mass
• Concluded the charge on the electron must be 1.60
  x 10-19 C
• Mass of electron 1.60 x 10-19 C 9.10 x 10 -28
  g
• 1.76 x 108 C/g

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Radioactivity(Spontaneous emission of radiation)
Type Symbol Charge Mass (amu)
Alpha particle He 2 4.002 60
Beta particle ß or 0-1e 1- 0.000 548 6
Positron ß or 01e 1 0.000 548 6
Gamma ray ? 0 0
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Ernest Rutherford

• -Conducted an experiment using alpha particles to
  bombard gold foil to conclude
• 1. Nucleus of an atom contains positive
  particles that we now call protons.
• The nucleus is a small dense area in the atom.
• -Studied three types of radioactive emissions
• alpha, beta gamma
• -Concluded that
• alpha
• particles were
• He nuclei

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A Positively Charged Nucleus

• Rutherford shot alpha particles though a thin
  piece of gold foil.
• Some of these particles were deflected instead of
  passing straight through
• Recall like repels like.
• When a alpha particle encountered a nucleus of
  a gold atom, it was deflected by the dense
  positively charged nucleus.

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James Chadwick

• James Chadwick suspected there were neutral
  particles when he saw in experiments like
  Rutherfords that some of the particles were
  deflected backwards, meaning that they had no
  charge. Chadwick had discovered the neutron!

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Niels Bohr

• Bohr proposed that an electron circles the
  nucleus in allowed orbits at specific energy
  levels.

orbital
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Scientist Contributions

• Thomson
• Discovered electron (1897)
• Cathode ray experiments
• Plum pudding atomic model
• Millikan
• Mass of electron
• Oil-drop experiment (1909)
• Rutherford
• Positively charged nucleus (1911)
• Gold foil experiments
• Discovered proton (1919)
• Chadwick Discovered neutron (1932)

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Small Numbers

• Electronic Charge 1.609 x 10-19 C
• Charge on an electron -1.609 x 10-19 C
• Charge on a proton 1.609 x 10-19 C
• Atomic Mass Unit (amu) 1.66054 x 10-24 g
• Proton mass 1.0073 amu
• Neutron mass 1.0087 amu
• Electron mass 5.486 x 10-4 amu
• Unit of length used to note atomic dimensions 1
  Angstrom(Å) 1x10-10 m

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• Atomic Number
• Number of protons or electrons in an element
• Identifies the element
• Atomic Mass
• Nucleus contains most of the mass of an atom.
• Protons and neutrons are each 1.67 x 10-24 g.
• Electrons are each 9.11 x 10-28 g.
• Use atomic mass unit (amu) instead of gram.
• The mass of one proton is 1 amu.
• Mass Number
• The sum of the number of protons and number of
  neutrons in the nucleus
• Is approximately equal to the average atomic mass
  shown on periodic table.
• Number of neutrons mass number atomic number

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• Isotopes
• Atoms of the same element with different numbers
  of neutrons
• Have the same number of protons
• Example Carbon-12 and Carbon-14
• Radioactive Isotopes
• Unstable in nature
• Can be used to date fossils and rocks
• The time it takes for half of the radioactive
  atoms in a piece of the fossil to change to
  another element is its half-life.

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Isotopes AX
Z

• Isotopes have the same Z, but different A.
• Isotopes have different numbers of neutrons.
• An atom of a specific isotope is called a
  nuclide.
• Nuclides of hydrogen include
• 1H hydrogen (protium)
• 2H deuterium (heavy hydrogen)
• 3H tritium (3H is radioactive.)

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