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Atoms: Development of the Atomic Theory

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Title: Atoms: Development of the Atomic Theory


1
AtomsDevelopment of the Atomic Theory
2
Democritus
  • 460 BC - Greek philosopher proposes the existence
    of the atom
  • His theory
  • all atoms are small hard particles
  • made of a single material formed into different
    shapes and sizes
  • always moving, and that they form different
    materials by joining together

3
John Dalton
  • 1803 - British chemist elements combined in
    specific proportions to form compounds.
  • His theory
  • all substances are made of atoms that cannot be
    created, divided, or destroyed
  • atoms join with other atoms to make new
    substances
  • atoms of the same element are exactly alike, and
    atoms of different elements are different in mass
    and size (elements)

4
Edward Frankland
  • 1852 - English chemist developed the valence
    theory
  • His theory
  • every atom has a fixed number of bonds (chemical
    links) that it can form
  • for the atom to be stable, all of these bonds
    must be used.

5
J.J. Thomson
  • 1897 - English chemist and physicist discovered
    1st subatomic particles
  • His theory
  • negatively charged particles called electrons and
    positively charged matter
  • created a model to describe the atom as a sphere
    filled with positive matter with negative
    particles mixed in
  • Referred to it as the plum pudding model

6
Ernest Rutherford
  • 1912 - New Zealand physicist discovered the
    nucleus
  • His theory
  • small, dense, positively charged particle present
    in nucleus called a proton
  • electrons travel around the nucleus, but their
    exact places cannot be described

7
Niels Bohr
  • 1913 - Danish physicist discovered energy levels
  • His theory
  • electrons travel around the nucleus in definite
    paths and fixed distances
  • electrons can jump from one level to a path in
    another level

8
Erwin Shrodinger
  • 1924 - Austrian physicist developed the electron
    cloud model
  • His theory
  • electrons exact path cannot be predicted
  • regions, referred to as the electron cloud, are
    areas where electrons can likely be found.

9
James Chadwick
  • 1932 - English physicist discovered neutrons
  • His theory
  • neutrons have no electrical charge
  • neutrons have a mass nearly equal to the mass of
    a proton
  • unit of measurement for subatomic particles is
    the atomic mass unit (amu)

10
Modern Theory of the Atom
  • Atoms are composed of three main subatomic
    particles the electron, the proton, and the
    neutron.
  • Most of the mass of the atom is concentrated in
    the nucleus of the atom.
  • The protons and neutrons are located within the
    nucleus while the electrons exist outside of the
    nucleus.
  • In stable atoms, the number of protons is equal
    to the number of electrons.

11
Modern Theory of the Atom
  • The type of atom is determined by the number of
    protons it has.
  • The number of protons in an atom is equal to the
    atomic number
  • The sum of the number of protons and neutrons in
    a particular atom is called the atomic mass
  • Valence electrons are the outermost electrons and
    are where bonding takes place
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