Post on 15-Mar-2018
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HISTORY OF THE ATOM
460 BC Democritus develops the idea of atoms
he pounded up materials in his pestle and
mortar until he had reduced them to smaller
and smaller particles which could not be
divided and called these:
ATOMA
(greek for indivisible)
Historic Models of the Atom
• Aristotle (384-322 BC) didn’t think there
was a limit to the number of times matter
could be divided.
• He knew there were small particles. Air,
Fire, Earth, Water
HISTORY OF THE ATOM
1808 John Dalton
suggested that all matter was made up of
tiny spheres that were able to bounce around
with perfect elasticity and called them
ATOMS
Dalton’s Atomic Theory
1. All elements are composed of atoms (which
can’t be divided).
2. Atoms of the same element have the same
mass and atoms of different elements have
different masses.
3. Compounds contain atoms of more than one
element.
4. In a compound, atoms of different elements
always combine in the same way.
Dalton’s Atomic Theory
Most of Dalton’s statements are now known to be flawed.
As we continue to study the atom, we will talk about discoveries that disproved Dalton’s statements.
Scientists have revised the theory due to new discoveries!
HISTORY OF THE ATOM
1898 Joseph John Thomson
found that atoms could sometimes eject a far
smaller negative particle which he called an
ELECTRON
J.J. Thomson, 1897• Discovered the electron, the subatomic particle
with a negative charge
• His experiments involved the use of a cathode
ray tube
Animation of
Thomson’s model
HISTORY OF THE ATOM
Thomson develops the idea that an atom was made up of
electrons scattered unevenly within an elastic sphere surrounded
by a soup of positive charge to balance the electron's charge
1904
like plums surrounded by pudding.
PLUM PUDDING
MODEL
Thomson’s Atomic Model
• Developed the Plum
Pudding model.
(think of a chocolate
chip cookie)
• Electrons evenly
distributed throughout
a positively charged
material.
HISTORY OF THE ATOM
1910 Ernest Rutherford
oversaw Geiger and Marsden carrying out his
famous experiment.
they fired Helium nuclei at a piece of gold foil
which was only a few atoms thick.
they found that although most of them
passed through. About 1 in 10,000 hit
something
HISTORY OF THE ATOM
gold foil
helium nuclei
They found that while most of the helium nuclei passed
through the foil, a small number were deflected and, to their
surprise, some helium nuclei bounced straight back.
helium nuclei
Ernest Rutherford, 1911
• Tested theory that electrons were evenly
distributed throughout the atom within
positively charged material
• Performed the Gold-Foil Experiment
Ernest Rutherford
• If Thomson’s model was correct, most of
the alpha particles should pass through
with a little deflection
Rutherford’s Model
• Proposed that atoms contain a nucleus, a
small, dense, positively-charged sphere in
the center of the atom.
• Atom contains mostly empty space.
• The nucleus is tiny compared with the
atom as a whole.
HISTORY OF THE ATOM
Rutherford’s new evidence allowed him to propose a more
detailed model with a central nucleus.
He suggested that the positive charge was all in a central
nucleus. With this holding the electrons in place by electrical
attraction
However, this was not the end of the story.
• The Houston
Astrodome occupies
more than nine acres
and seats 60,000
people.
• If the stadium were a
model for an atom, a
marble could
represent its nucleus!
HISTORY OF THE ATOM
1913 Niels Bohr
studied under Rutherford at the Victoria
University in Manchester.
Bohr refined Rutherford's idea by adding
that the electrons were in orbits. Rather
like planets orbiting the sun. With each
orbit only able to contain a set number of
electrons.