History of the Atomic Model
• Around 460 B.C., he was the first to suggest
• Believed that atoms were indivisible and
• His research lacked experimental support
atom was a tiny, hard,
Famous Greek Philosopher
All substances made up of 4 elements:
Fire - Hot
Air - light
Earth - cool, heavy
Water - wet
Blend these in different proportions to get all
Did not experiment.
Greeks settled disagreements by
Aristotle was a better debater
His ideas carried through to the
Age of Alchemists - tried to change lead to gold.
scientist • Joseph Proust
led Dalton • Law of Definite Proportions
to the first
atomic • Law of Conservation of Mass
theory. • Gay-Lussac
• Gases react in small whole number ratios
Law of Definite Proportions
contain the same elements in
exactly the same proportions
by mass regardless of size of
the sample or source of the
Law of Conservation of Mass
Mass is neither created or
destroyed during chemical
reactions or physical changes
Law of Multiple Proportions
If 2 or more different
compounds are composed of
the same 2 elements, they
combine in different ratios.
• 1803 (2000 years after Democritus)
Dalton's "modern" atomic theory includes:
1. All matter is composed of extremely small particles called atoms.
2. Atoms of the same element are identical in size, mass and other
properties; atoms of different elements differ in size, mass and
3. Atoms cannot be subdivided, created or destroyed in chemical
4. Atoms of different elements combine in simple whole-number
ratios to form chemical compounds.
5. In chemical reactions, atoms are combined, separated or
ELEMENT A ELEMENT B
Atoms of element A Atoms of element B
are identical are identical, but
different that atoms
of element A
Atoms of element and
Atoms of element A B can chemically
and element B can combine to form a new
physically mix together compound
• The first evidence for sub-atomic particles came from experiments
with the conduction of electricity through gases in sealed glass
tubes at low pressures, called Cathode Ray Tubes.
• J.J. Thomson demonstrated that the rays consist of a stream of
negatively charged subatomic particles which he called electrons.
Cathode Ray Tube Experiments
•In Thomson’s experiments,
the cathode ray is deflected
by a both a magnet and also
by an electrically charge metal
•Thomson believed the ray
was made of negatively
charged particles because a
positively charged plate
attracted the ray and the
negative plate repelled it
•These negatively charged
particles were later named
- Voltage source
Passing an electric current makes a beam
appear to move from the negative to the
By adding an electric field he found that the
moving pieces were negative.
Thomson’s (Lord Kelvin’s)Plum
• The particles making up the rays acted as if they
were negatively charged.
• Since elements are neutral, there must be positive
particles balancing out the negative particles
• Thomson theorized that an atom consisted of a
“pudding” of evenly distributed positive and
“Plum Pudding Model”
• Millikan and co-workers studied the motion of charged
oil droplets in an electric field
• Through their work, they determined values for the
charge and mass of an electron: the electron carries one
unit of negative charge and its mass is 1/1840 the mass
of a hydrogen atom
• Millikan’s values are very similar to those we use today
Believed the plum pudding model of the atom
Used radioactivity to experiment with
this idea in mind..
Alpha particles - positively charged
pieces given off by uranium.
Shot them at gold foil which can be
made a few atoms thick.
The alpha particles to pass through without
changing direction very much.
The positive charges were spread out
evenly. Alone they were not enough to stop
the alpha particles.
What he expected
What he got
How he explained it
Atom is mostly empty.
are deflected by
it if they get close
• Based on his experiments, he concluded that the atom was made
of mostly empty space
• The mass and positive charge are concentrated in a small area call
• Nucleus: the tiny central core of an atom and is composed of
protons and neutrons
• Rutherford’s model was also known as the nuclear model.
• Analogy: If an atom is the size of a football field, the nucleus is
the size of a marble
• In 1920, Ernest Rutherford postulated that there
were neutral, massive particles in the nucleus of
• In 1932, James Chadwick discovered the neutron
He spent more than a decade searching.
• Won the Nobel Prize in 1922 for his work on
atomic structure suggesting that atoms emit
electromagnetic radiation as the electrons move
from one fixed energy level to another
• Electrons in energy levels called orbits.
Wave Mechanical Model
• The three men most responsible for the model are
Louis de Broglie, Erwin Schrodinger, and Werner
• Louis de Broglie was the first man to openly
support the idea that electrons exhibit wave
Wave Mechanical Model
• Erwin Schrodinger developed a wave
equation for the electrons in an atom. This
equation accurately predicted the behavior of
every electron in every atom.
Wave Mechanical Model
• Werner Heisenberg, through his famous
uncertainty principle, showed that it is impossible
to know both the exact location and the exact
speed of an electron
Wave Mechanical or Electron Cloud