Transcript Development of Atomic Theory Notes

The Development of Modern
Atomic Theory
In the beginning…
• Democritus- In the 4th century BC, he
proposed that the universe was made of tiny
indivisible units.
• He called these atoms, from the Greek word
atomos, meaning unable to be divided or cut.
• He did not have any evidence to support his
theory, but nonetheless, some people
believed.
Dalton’s Atomic Theory
• In 1808, he proposed a revised atomic
theory.
• According to Dalton, all atoms of a given
element were exactly alike, and atoms of
different elements could join to form
compounds.
• Theory was based on experimental evidence.
• However, some parts of his work turned out
to be incorrect.
Thomson’s Model of the Atom
• 1897- Experiments with electricity led to the
discovery that atoms were NOT indivisible!
• He conducted a famous cathode ray tube
experiment which suggested that these rays
were made of negatively charged particles
that came from inside of atoms.
Results from the Cathode Ray Tube
Experiment
• Thomson knew that a magnet deflected
charges, and he reasoned that because the
tube was a vacuum the beam must have
come from the cathode (positively charged
plate) or anode (negatively charged plate).
• The direction of deflection indicated a
negatively charged beam, and electrons were
discovered!!!!
Thomson’s Plum Pudding Model
• Based on the outcome of his experiment, he
proposed a new model of the atom.
• He reasoned that the electrons were spread
throughout the atom, just like in plum
pudding, a dessert in his time.
• We might use the chocolate chip cookie
model to better visualize it today.
Rutherford’s Model of the Atom
• Ernest Rutherford proposed his model shortly
after Thomson.
• He found that most of the mass of an atom
was concentrated at the atom’s center, or the
nucleus.
• To understand this conclusion, you must
learn about his famous gold foil experiment.
Gold Foil Experiment
• A beam of positively charged alpha particles
were aimed at a thin sheet of gold foil.
• Because of Thomson’s model, Rutherford
predicted that there would not be a large
enough mass of positive charge to cause
many particles to bounce back. He thought
they would travel in a straight path through
the foil.
Gold Foil Experiment
• But, the observations did not match his
hypothesis. Most passed straight through,
but a few were deflected by a large amount.
• A few even bounced back!
• He said, “It was quite the most incredible
event that has ever happened to me in my
life. It was almost as incredible as if you
fired a 15- inch shell at a piece of tissue
paper and it came back and hit you.”
Gold Foil Experiment
Results of Rutherford’s experiment
• An atom’s positive
charge in concentrated
in the center of the
atom, called the
nucleus.
• He is also credited with
the discovery of the
proton.
• Here is his atomic
model: