Atoms are not the smallest thing

1
Atoms are not the smallest thing

• Growing evidence for the divisibility of the
  indivisible

2
Atoms are not the smallest thing

• Growing evidence for the divisibility of the
  indivisible

3
Learning Objectives

• Describe the three particles in the atom
• Define atomic number and mass number
• Describe isotopes
• Write symbols for elements
• Determine the numbers of particles in any atom
  from the element symbol

4
Electrostatics and electricity

• Static electricity was observed by Thales (300
  BC). Some charged objects repel and others
  attract
• The voltaic cell (Volta, 18th century) generated
  electrical current from chemical reactions
• Mechanical electrical generation was achieved in
  1825
• The point Atoms are neutral. If indivisible,
  where do electrical charges come from?

5
Faradays prescience

• Although we know nothing of what an atom is, we
  cannot resist forming some idea of a small
  particle and though we are in equal ignorance of
  electricity, there is an immensity of facts which
  justify us in believing that the atoms of matter
  are associated with electrical powers to which
  they owe their most striking qualities, and
  amongst them their chemical affinity.

6
Ray of hope

• 1858
• Discovery of cathode rays by Julius Plucker
• Application of a large voltage across an
  evacuated tube causes a current to flow. The
  current flow is accompanied by radiation from the
  excited gas molecules
• How does the neutral and indivisible atom create
  a charge?

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Cathode rays are electrons1897. J.J. Thomson
demonstrates that cathode rays are negatively
charged particles, which have a much smaller mass
than an atom. The first sighting of the electron.

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The Thomson Plum Pudding model

• "I regard the atom as containing a large number
  of smaller bodies which I will call corpuscles,
  these corpuscles are equal to each other.... In
  the normal atom, this assemblage of corpuscles
  forms a system which is electrically neutral.
  Though the individual corpuscles behave
• like negative ions, yet when they are
• assembled in a neutral atom the negative
• effect is balanced by something which
• causes the space through which the corpuscles are
  spread to act as if it had a charge of positive
  electricity equal in amount to the sum of the
  negative charges of the corpuscles

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X-rays and atoms invisible rays

• 1895
• Wilhelm Conrad Roentgen discovers X-rays while
  doing experiments with cathode rays

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Radioactivity invisible rays and unstable atoms

• 1896
• Henri Becquerel discovered radioactivity, which
  suggested that some atoms were capable of
  decomposing to give smaller particles.
• 1903
• Frederick Soddy and William Ramsey demonstrated
  that uranium decayed to give helium. Direct
  proof that atoms were divisible.

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1909 Ernest Rutherfords gold foil experiment

• The atoms inner secrets exposed by its own
  offspring

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Hierarchy of discovery

• The path to knowledge occurs in steps, each
  depending on a previous advance

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The nucleus

• Tiny
• Incredibly dense contains all the mass of the
  atom
• Positively charged
• Contains protons (charged) and neutrons (neutral)
  not discovered until much later

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Summary of Atom Pictures

• Dalton Indivisible atom
• Thomson Electrons
• Rutherford Nucleus

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Comparison of subatomic particles
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Atoms are neutral electrons protons

• Atomic number (Z) number of protons in nucleus
• Each element has unique atomic number
• For neutral atom, atomic number equals number of
  electrons around nucleus.

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Isotopes and the mass number

• Mass number protons plus neutrons
• Isotopes have same atomic number, different mass
  number

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Element notation Atomic number and mass number
Mass number number of protons neutrons
Element symbol
Atomic number number of protons

• Counting particles
• Number of electrons number of protons 6
• Number of neutrons mass number atomic number
  (13 6 7)

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Ions losing and gaining electrons

• Atoms can lose or gain electrons
• Atomic number remains the same
• Loss Positive ions have fewer electrons than
  protons
• Gain Negative ions have more electrons than
  protons

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What of the electrons?

• We now understand atom contains tiny positively
  charged massive nucleus surrounded by vast empty
  space containing electrons
• When atoms combine the electrons must interact
• We need to understand the arrangement of
  electrons in the atom