Download Atomic History

Atomic History
Dalton’s Atomic Theory – 1803
•  All elements are composed of indivisible units
called atoms.
•  All atoms of the same element are identical.
•  All atoms of different elements are different.
•  Compounds are formed by joining atoms of 2 or
more elements in a definite whole number ratio
•  Changes made to Dalton’s theory in the
modern theory
–  Atoms are divisible (made of smaller
particles)
–  Isotopes of elements are the same
chemically, but have different mass
numbers
•  Isotopes have the same protons and electrons
but different neutrons
Discovery of Electrons
•  William Crooke – invented Crookes tube
or Cathode Ray tube.
–  used electrically charged metal cathodes to
send charged gas molecules across a
vacuum tube. Gases would emit light at the
opposites end of the tube. Believed the
emission was cathode rays.
Discovery of Electrons
•  JJ Thomson – used Crookes tube to show that
cathode rays were actually negatively charged
particles
–  used a wide variety of metals and gases and
determined that all atoms contained these negatively
charged particles
–  discovery of electrons led Thomson to form “plumpudding model”
•  “Plum Pudding Model” – Equal, even distribution
of protons and electrons
•  Rutherford’s Model (Lord Rutherford) – a
small, dense, positively charged nucleus
surrounded by seemingly empty space with
negatively charged electrons orbiting the
nucleus.
–  “Gold Foil Experiment”
•  Projected + Alpha particles at a thin gold foil surrounded
by a ZnS screen that sparked when struck by Alpha
particles
•  Most of the alpha particles passed straight through the
foil, some were slightly deflected, while only a few were
deflected by 90° or more
•  Results indicated that the positive charge was small and
dense and the electrons were spread out in empty
space.
•  *Rutherford could not explain how the electrons could
orbit the nucleus without losing energy
•  Bohr Model (Niels Bohr 1913) – electrons are
found in specific energy levels in which they
can travel w/out radiating energy.
–  The lowest energy electrons are found closest to
the nucleus. The further from the nucleus an
electron is, the more energy it has.
–  When all electrons of an atom are in the lowest
available energy levels the atom is in the ground
state and is stable.
•  Charge – Cloud Model – Electrons are
actually clouds of negative charge that are
moving at speeds close to the speed of light.
–  The model shows the most probable location of
an electron