Development of Atomic Theory
Physical Science
Chapter 4 - Section1
The Beginnings
• Atomic theory developed slowly over 2000 years
• 4th century B.C., Greek philosopher Democritus
suggested matter consists of indivisible units
– Called these units “Atoms”
– Atomos – Greek for indivisible
• Not possible to acquire experimental evidence
– No tools available during these ancient times
Dalton’s Atomic Theory
• English schoolteacher John Dalton used
experimental observations in 1808
– Some portions are still true today
• Concluded that all atoms of a given element are
identical, and atoms of different elements join to form
compounds
Dalton’s Atomic Theory ( cont. )
• Observed different substances combining in proportions (
in consistent ways )
• Law of Definite Proportions
• A given compound always has elements in exactly the
same proportions by weight or mass
• Water as example ( 11% H and 89% O )
• Atoms combine in whole number ratios ( H2O )
• Dalton’s theory did not fit all observations
– With further experiments by others, some observations
were found to be inaccurate
J.J. Thompson’s Contribution
• In 1897, J.J. Thompson accidently discovered
electrons by experiment
• Conducted experiments with Cathode Ray Tube –
suggested cathode rays consist of negatively-charged
particles that come from atoms
– Cathode Ray Tube consists of two metal plates on either end of a
vacuum tube ( Cathode – negative ; Anode – positive )
– Formed an electron beam ( by applying a voltage across two plates )
and observed that a magnet deflected ( bent ) the beam
• His results also suggested that atoms could be
divided into smaller parts
J.J. Thompson’s Model
• Suggested that atoms can be thought of as plum
pudding ( alternatively, a blueberry muffin )
• In this model, electrons are spread throughout an
atom ( mass and positive charge are evenly
distributed throughout the atom however )
• Imagine a muffin with blueberries dispersed
throughout it.
– Blueberries are electrons
– “Muffin” is the entire atom with the “bread” being the
positive portion of the atom ( evenly distributed )
Earnest Rutherford
• Another Englishmen during 19th-20th century
• Developed experiment to test Thompson’s model
• Found that Thompson’s model needed refining
• In CONTRAST to Thompson’s model, Rutherford
proposed that most of an atom’s mass was
contained in its center ( the nucleus )
• Rutherford discovered the nucleus in a manner
not vastly different than Thompson ( experiment )
Rutherford’s Experiment
• Beam of positively charged alpha particles aimed
at a very thin piece of gold foil
• By Thompson’s Model the positive charge at any
location in an atom would be too small to affect
the paths of incoming particles
• Rutherford predicted that most particles would
remain straight and not be deflected
– Again, assuming Thompson’s model is accurate
– If the positive charge on an atom was concentrated,
alpha articles would be deflected
Rutherford’s Gold Foil Experiment
• Results showed that the alpha particles WERE
DEFLECTED
• Some particles passed through, some were deflected
by a large amount, some bounced straight back (
what did these hit? )
• This indicated:
– That the positively-charged portion of the atom is NOT
dispersed ( or IS concentrated in the atom )
– That Thompson’s model was not totally accurate
– There is something in the middle of the atom!!! ( nucleus )
• Figure 7 ( pg. 118 ) shows model of atom after
Rutherford’s work ( protons, neutrons, electrons )