Transcript Ch 3 Atomic Theory *Atoms *Molecules *Ions

Ch 3 Atomic Theory
Models of the Atom
Pre-Dalton Atom Concepts
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The Greek classical elements are
Fire, Earth, Air, and Water.
Chinese Philosophy had 5
elements that composed nature.
Democritus was a Greek
philosopher who lived between
470-380 B.C. He developed the
concept of the 'atom', Greek for
'indivisible'. Democritus believed
that everything in the universe
was made up of atoms, which
were microscopic and
indestructible.
Ch 3 Atomic Theory
John Dalton
(1766-1844)
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Dalton's model was that the atoms were tiny,
indivisible, indestructible particles and that each
one had a certain mass, size, and chemical behavior
all atoms of an element were identical
compounds consisted of atoms of different
elements combined together
Compounds have constant composition because they
contain a fixed ratio of atoms
chemical reactions involved the rearrangement of
combinations of those atoms
Dalton’s 3 Laws
John Dalton
(1766-1844)
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Law of Definite Proportions
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Law of Multiple Proportions
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Atoms combine in simple, whole number ratios to
form compounds
The mass percentage of elements in a compound is
fixed
Law of Conservation of Mass
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The mass of the reactants is equal to the mass of the
products in any chemical reaction.
J. J. Thomson (1897)
Thomson’s model had the atom as a sea
of positive charge interspersed with
negative “corpuscles” to neutralize the
atom.
 It was called the “Plum Pudding” model.
 His proof was that atoms could be
deflected in a magnetic field due to their
charge.
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Cathode Ray Tube
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Thomson accurately measured the ratio of
mass to charge, but not the actual charge
on the electron.
Robert Millikan (1902)
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By balancing the electrical force on
charged oil droplets in an electric field with
the force of gravity, Robert Millikan was
able to independently determine the
charge of the electron (-1.6x10-19 C) in
his Oil Drop Experiment.
Millikan Oil Drop Experiment
Ernst Rutherford (1911)
Rutherford bombarded atoms (gold Foil) with
Alpha particles (He2+).
 If the atom was as Rutherford had predicted,
they should pass through being slightly
deflected.
 However, some alpha particles were
deflected back indicating a dense, positive
center of the atom.
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Gold Foil Experiment
Rutherford’s Model of the Atom
James Chadwick
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In 1932, Chadwick made a fundamental
discovery in the domain of nuclear science: he
proved the existence of neutrons - elementary
particles devoid of any electrical charge. In
contrast with the helium nuclei (alpha rays)
which are charged, and therefore repelled by the
considerable electrical forces present in the
nuclei of heavy atoms, this new particle need not
overcome any electric barrier and is capable of
penetrating and splitting the nuclei of even the
heaviest elements. Chadwick in this way
prepared the way towards the fission of uranium
235 and towards the creation of the atomic
bomb.