The History of the ATOM
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The History of the
ATOM
Who are these men?
In this lesson, we’ll learn
about the men whose
quests for knowledge about
the fundamental nature of
the universe helped define
our views.
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
he called…
ATOMA
(greek for indivisible or cannot
be divided)
This theory was ignored
and forgotten for more than
2000 years!
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 Theory
1. All elements are composed
of atoms.
2. Atoms of the same element
are exactly alike.
3. Atoms of different elements
are different.
4. Compounds are formed by
the joining of atoms of two or
more elements.
Dalton’s Atom
HISTORY OF THE ATOM
1898 Joseph John Thomson
• Found that atoms could sometimes
eject a far smaller negative particle
which he called an…
ELECTRON
• To learn about J.J.Thomson’s cathode ray tube
experiment, click here
HISTORY OF THE ATOM
• Thomson develops the idea that an atom was made
up of electrons scattered unevenly within an elastic
sphere.
• The electrons are surrounded by a soup of positive
charge to balance the electron's negative charge like
plums surrounded by pudding.
1904
PLUM PUDDING
MODEL
HISTORY OF THE ATOM
1910 Ernest Rutherford
• Oversaw Geiger and Marsden carrying
out his famous experiment.
• Fired Helium nuclei at a piece of gold
foil which was only a few atoms thick.
• Found that although most of them
passed through. About 1 in 10,000 hit
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
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.
HISTORY OF THE ATOM
1913 Niels Bohr
• Studied under Rutherford at the
Victoria University in Manchester.
• Refined Rutherford's idea by adding
that the negatively charged electrons
were in orbits, like planets orbiting the
sun.
• Each orbit is only able to contain a
set number of electrons.
• Calculated the exact energy levels
for the hydrogen atom.
Bohr’s Atomic Theory
• The Bohr model was a
modification of the
Rutherford model.
• Bohr placed electrons in
certain circular orbits
around the nucleus called
shells.
+
• Proposed the electron cloud.
• Stated that the electron do not
travel in exact orbits but travel in
clouds around the nucleus.
HISTORY OF THE ATOM
1926 Erwin Schrodinger
Electron cloud model
* Although this is an
important model to
recognize, we will
be focusing on the
Bohr Model in
class.
• Scientists could not figure out what made atoms so heavy.
• Chadwick proposed the idea of neutrons—neutral particles that are located in the nucleus with the protons. • The new Bohr model includes these particles.
0
Neutrons
HISTORY OF THE ATOM
1932 James Chadwick
New Bohr Model of a HELIUM ATOM
+ N
N
+
- -
proton
electron neutron
Shell
Today’s Understanding of the Atom
With the help of new technology, such as particle accelerators, scientists now think protons and neutrons are made of smaller particles.
Evolution of
the Atomic
Model
The current and most
accurate model of the
atom shows electrons
are in a around
the nucleus.
However, we will use the in
class to better understand the structure
of the atom.
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