History of the Model of the Atom HOTMOTA

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History of the Model of the AtomHOTMOTA

• Chapter 4
• Chem X

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Early Theories

• Ancient Greeks
• Democritus (400 BC)
• Aristotle
• Speculation until 18th century (1700s) when
  scientists began to develop evidence.

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John Dalton

• studied reactions and made careful observations
  measurements
• determined mass ratios of elements in various
  reactions
• this lead to his atomic theory, which was
  generally accepted

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Daltons Atomic Theory (1803)

• All matter is composed of extremely small
  indivisible particles called atoms.
• Atoms of a given element are identical in size,
  mass, and other properties atoms of different
  elements differ in size, mass, and other
  properties.
• Atoms cannot be subdivided, created, or
  destroyed.
• Atoms of different elements combine in simple
  whole-number ratios to form chemical compounds.
• In chemical reactions, atoms are combined,
  separated, or rearranged.
• Dalton turned Democrituss idea into a scientific
  theory that could be tested by experiment.
• Dalton made three mistakes in his theory

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The Atom and Subatomic Particles

• Cathode Ray Tube Experiment
• William Crookes - Cathode Ray Tube
• JJ Thomson

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The Atom and Subatomic Particles

• Cathode Ray Tube Experiment
• Important Observations Conclusions
• 1)
• 2)

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The Atom and Subatomic Particles

• JJ Thomson (1856-1940) English physicist
• Inferences made
• If atoms are electrically neutral then they
  must
• The ratio of the charge of cathode-ray
  particles to their mass it was always the same,
  so he concluded that
• This lead to the _____________ Model

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• Robert Milikan (1868 - 1953)
• _____________ Experiment (1909)

• forced oil through a tiny hole forcing the drops
  to gain/lose electrons and thus carry a charge.

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• Robert Milikan found mass of electron as 1/1840th
  the mass of a hydrogen atom
• (mass electron 9.1 x 10-31 kg mass proton
  1.673 x 10-27 kg)

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The Atom and Subatomic Particles

• Ernest Rutherford (1911) ________________
  experiment

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The Atom and Subatomic Particles

• Important Observations Conclusions
• 1)
• 2)
• He discovered the nucleus! And suggested that
  the electrons surrounded the positively charged
  nucleus like planets around the sun.

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The Atom and Subatomic Particles

• Discovery of the Proton Ernest Rutherford
  (1920)
• Discovery of the Neutron Sir James Chadwick
  (1932)

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Nuclear Symbol and Atomic Mass

• Atomic Number
• Nuclear Symbol
• Isotope
• Atomic Mass
• C-12 is assigned to be exactly 12 a.m.u.
• (thus 1 a.m.u. )

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Nuclear Symbol and Atomic Mass
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Nuclear Symbol and Atomic Mass
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Nuclear Symbol and Atomic Mass
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Unstable Nuclei Radioactive Decay

• Radioactivity
• ordinary reactions vs. nuclear reactions

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Radioactive decay

• Alpha Radiation
• 22688Ra ? 42He ______
• Beta Radiation
• 146C ? 0-1b ______
• Gamma Radiation
• 23892U ? 42He ___ 002 g

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Radioactive decay - Practice

• 8636Kr ? 8737Rb ______
• 21082Pb ? 42He ____ 0-1b
• _____ ? 42He 23694Pu
• 24395Am ? 23993Np ___

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Radioactive decay - Practice

• Whats the daughter nuclide after Pb-210
  undergoes a beta and two alpha decays?
• 21082Pb ?