Chapter 4 Atomic Structure 4.1 Studying Atoms. Early Chemists and Atomic Models Who are the early...
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Transcript of Chapter 4 Atomic Structure 4.1 Studying Atoms. Early Chemists and Atomic Models Who are the early...
Chapter 4Atomic Structure
4.1 Studying Atoms
Early Chemists and Atomic Models
• Who are the early chemists that shaped atomic theory?
• What were the accepted models of the atom?
• How have the models changed over time?
• What experiments were performed to change the models?
Ancient Greek Models
• Democritus was one of first philosophers on record• Believed that matter was made of
extremely small particles• Different particles had different
properties
Ancient Greek Models
• Aristotle was another Greek that described atomic structure• His model was that of the 4
elements :–Fire -Air–Water -Earth
Dalton’s Atomic Theory
• Dalton studied the behavior of gases in air• Discovered that gases consist of
individual particles instead of “one substance”
Evidence for Atoms
• Measured mass of elements when they formed compounds• Showed that the ratio of masses
was always the same
Example• 16 g of Oxygen gas reacts with 24
g of Magnesium • 1.6 g of Oxygen gas reacts with
2.4 g of Magnesium• Ratios are always the same
Dalton’s Theory• All elements are composed of atoms• All atoms of the same element have
the same mass• Compounds contain atoms of more
than one element• Atoms always combine the same way
in each compound
Dalton’s Theory
• Atoms are represented by spheres• Atoms of different elements
would be different size spheres
Thomson’s Model
• Thomson noticed that materials attract or repel other materials when rubbed• Like charges repel, unlike attract• Some charged particles can flow
from one location to another
The Experiment
Thomson used a sealed glass tube with an electric currentA glowing beam is generated between the metal plates at the ends
Video
The Experiment
• Thomson placed charged particles on either side of the glass tube• The stream of particles would
bend toward or away from the charged plates
Evidence for Subatomic Particles
• The particles were negative because they were repelled by negative charged plates• All metal disks reacted the same way, so
he assumed the particles came from inside atoms• Their mass was 1/2000 the mass of
Hydrogen atoms
The Conclusions
• Thomson concluded that Dalton was wrong about atoms being the smallest particles• His experiments showed that
atoms were made of smaller things with charges
Thomson’s Model• Referred to as the “Plum Pudding”
model• Since atoms are neutral, the charges
needed to be spread out• Negative charges were evenly
distributed among the positive particles of the atom
Earnest Rutherford
• He lived from 1871-1937• He was testing some of the ideas
of Thomas with gold foil• He was working with Uranium,
which emits a fast moving particle with a positive charge
The Hypothesis• Rutherford believed that the mass
and charge of particles in gold would be too small to deflect the alpha particles of Uranium• The particles would pass straight
through the gold or only move slightly
The Experiment• A stream of alpha particles was
aimed at gold foil• The particles would hit a screen that
was placed around the foil and would flash when hit• The location of the flash was studied
The Result
• One out of every 20,000 particles would deflect by more than 90˚• Some particles bounced straight
back
The Conclusion
• Something was making the particles deflect• The charge was not evenly
distributed• The positive charged particles
were concentrated in one place
The Nucleus
• The nucleus is the location of all of the positive charges in the atom• Most of an atom is empty space
where the electrons are located