Download History of the Atom

History of the Atom
• 440 BC – Democritus developed the original
atomic concept
– There exist indivisible particles called
atoms (“a” meaning not; “tomos” meaning
cut
• There exist empty space between atoms
• Atoms are completely solid
• Atoms are homogeneous with no internal
structure
• Atoms differ in size, shape and weight
1789 Antoine Lavoisier
• Law of conservation of matter – matter can
be neither created no destroyed, merely
changed from one form to another
1800’s Joseph Proust
• Law of Definite Proportions
• Observed that specific substances always
have the same ration by mass
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1808 John Dalton – Atomic theory explains
and verifies the Law of Conservatio of Matter
• Theory of atoms
– All elements are made up of tiny particles
called atoms
– The atoms of a given element are identical, but
are different from any other element
– Atoms of different elements combine to form
compounds
– A given compound always has the same
relative number and types of atoms
– In a chemical reaction atoms are reorganized,
resulting in changes in the way they are bound
together; the atoms themselves are not changed
1810 John Dalton
• Law of Multiple Proportions
– Elements in a compound combine in whole
number ratios by mass
• Thus he says atoms react as whole units
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1859 – Julius Plucker
• Sealed gases in tubes with electrodes on
each end
– One end – the anode – was positively charged,
thus attracted negative ions
– The other end – the cathode – was negatively
charged, thus attracte positive ions
1897 JJ Thomson
• Used a cathode ray tube with a beam
that glowed between the electrodes
when they were connected to an
electrical charge – traveling from the
cathode to the anode
• The beam was attracted to a positive
charge, leading Thomson to propose
that there were small negatively charged
particles that he called electrons
1907 JJ Thomson
• Develops the “plum pudding” model of the
atom
– A sphere of positive electricity (the pudding_
with electrons (the plum bits) scattered in it
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1906 Ernst Rutherford
• Announced that alpha particles can be
scattered by air
• Discovers the presence of the nucleus
1913 Robert Millikan
Determined the charge on the electron
1913 Neils Bohr
Working with Rutherford proposed the
planetary model of the atom
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Late 1910’s
Based on earlier work by E. Goldstein
(1886) Millikan, Thomson and
coworkers proposed the presence of a
positively charged particle called the
proton
Goldstein observed what he called canal
rays while using a cathode ray tube with
the rays traveling in the opposite
direction of Thomson’s
1920 Rutherford
Proposes the presence of a third particle,
found in the nucleus when the total mass of
the atom cannot be attributed to the proton
and electron
1924 Wolfgang Pauli
States the quantum exclusion principle
No two particles can occupy the same position at
the same time in an atom
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1926 Werner Heisenberg
Proposes a theory of the movement of
electrons in the atom – matrix
mechanics
States it is impossible to accurately
predict both the position and the
momentum of any object (including
an electron) at the same time – the
Heisenberg uncertainty principle
1926 Erwin Schrodinger
Developed the wave equation to explain
electron activity
Considered the electron’s behavior as like that
of a wave rather than a particle
1926 Max Born
Suggested that the wave function of the
Schrodinger equation was a measure of the
probability of finding an electron at a given
location
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Quantum Mechanical Model of
the Atom
Based on Shrodinger’s wave equation,
describes the electrons as a cloud
surrounding the nucleus
The higher the energy of the electron, the further
it can travel from the nucleus
1932 James Chadwick
Finds high energy particles with essentially
the same mass as a proton and no charge –
the neutron
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