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THE ROAD TO THE
ATOM
Democritus
• Democritus a fifth
century B.C. Greek
philosopher proposed
that all matter was
composed of indivisible
particles called atoms
(Greek for uncuttable).
JOHN DALTON (1803)
• Dalton viewed the atom as a small
solid sphere.
• Each element was composed of the
same kind of atoms.
• Each compound was composed of
different kinds of atoms.
J.J. THOMSON (1897)
• Developed a primitive
electron gun.
• Experimented with beams
of particles accelerated in
the electron gun.
• Found that beam gets attracted by
a positively charged plate.
• New Discovery!!! The atom is NOT the
smallest, as it consists of much smaller
particles.
• Lead to the discovery of the electron, a
negatively charged sub-atomic particle.
• Now scientist knew the atom
consists of positive and negative
particles.
• But where were they in the atom?
• 1904: Thomson suggested that there is a
cloud of positive charge in the atom
embedded with negative particles in it. This
became known as the Plum Pudding model.
ERNEST RUTHERFORD (1911)
• Rutherford tested
Thomson’s model. He
experimented with alphaparticles and made a
remarkable discovery.
• What are alpha-particles?
• Helium nuclei. They are positively
charged.
Rutherford’s Experiment:
• He bombarded a thin gold foil leaf with
alpha-particles.
• Why gold foil?
• Gold is a very heavy atom, therefore it
has a high number of protons and
electrons.
• Why thin gold foil?
• The thin foil acted as if the alphaparticles were being fired at a single
row of atoms.
Diagram of Gold Foil experiment
Rutherford’s findings:
• Most alphaparticles traveled
straight through.
• Some were
deflected by
large angles.
• A few were
rebounded
directly.


This indicated that
most of the atom
was empty space.
This indicated that
there was
something small
and dense inside
the atom, and it
repelled positive
particles.
• Rutherford called the small, dense
center the Nucleus.
• He knew the Nucleus was positively
charged, as it deflected some alphaparticles.
• If the nucleus only contains positive
particles, then the electrons must orbit
the nucleus to keep it neutral.
Rutherford’s MODEL of the Atom:
• A dense,
positively
charged nucleus.
• Negatively
charged
electrons
orbiting the
nucleus.
• Later Rutherford measured the mass of a
particle from the nucleus and found it
much less than that of an alpha-particle.
• These positively charged particles were
then named PROTONS.
• Millikan determined the mass of an
electron through his oil drop experiment,
and it was known that the proton’s mass
is about 1835 times that of an electron.
Neils Bohr (1919)
• Electrons orbit the nucleus in fixed
positions, according to their energy
levels.
The Rutherford-Bohr model of the
atom:
James Chadwick (1932)
• Discovered that there was very
high radiation coming from
particles in the nucleus.
• These were not deflected by
electric or magnetic fields,
therefore could not be protons
or electrons.
• He discovered the neutron; large
neutral particles in the nucleus with
mass about the same as for protons.
• All three sub-atomic particles as we
know them at school level, were now
discovered.
• Atom masses and isotope masses could
now be determined.
• The atom model for use at school level
was now complete.
• Examples:
Further interesting facts that lead
from these early scientists:
• Bohr’s analysis of the energy given off when
an electron dropped from a higher energy
orbit to a lower energy orbit didn't hold up for
atoms bigger than hydrogen
• In 1924, a French physicist named Louis de
Broglie suggested that, like light, electrons
could act as both particles and waves
• Another question quickly followed de
Broglie's idea. If an electron traveled as a
wave, could you locate the precise position of
the electron within the wave? A German
physicist, Werner Heisenberg, answered no
in what he called the uncertainty principle
• an Austrian physicist named Erwin
Schrodinger derived a set of equations or
wave functions in 1926 for electrons.
According to Schrodinger, electrons confined
in their orbits would set up standing waves
and you could describe only the probability of
where an electron could be. The distributions
of these probabilities formed regions of
space about the nucleus were called
orbitals. Orbitals could be described as
electron density clouds
• Atom model with electron clouds:
• Soon after the shape of the electron clouds
were determined and a refined model for the
atom came about.
• The orbitals were also named : s, p, d and f.
s - orbitals
p - orbitals
• And so, until the next model, this is how it
happened: