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Introduction to the Atomic
Theory
What is the Atomic Theory?
– The Atomic theory is the study of the nature of
atoms and how they combine to form all types of
matter.
– There have been a number of theories that have
been proposed to explain how atoms work.
Significant contributors are: Democritus 440 BCE,
John Dalton (1766 – 1844), J. J. Thomson (1856 –
1940), Earnest Rutherford (1891 – 1937), James
Chadwick (1891 – 1974), and Niels Bohr (1885 –
1962). (See p176 for a more detailed look at
significant historical figures)
Dalton’s Atomic Theory
• All matter is made of small, dense,
indestructible particles called atoms.
• The atoms of an element are identical to one
another.
• The atoms of different elements have different
properties (mass and size).
• Atoms can join together to form groups of
atoms called molecules. (can form new
substances)
Electrons and Protons discovered by
J.J. Thomson
• J.J. Thomson used a cathode ray to discover
there were negatively charged particles found
in atoms. These negatively charged particles
would be called electrons.
• He then reasoned that if the atoms contained
negatively charged particles there must be
positively charged particles since atoms
themselves do no contain a charge.
J.J. Thomson’s Model
Source) http://www.kutl.kyushu-u.ac.jp/seminar/MicroWorld1_E/Part2_E/P24_E/Thomson_model_E.htm
Video example of Cathode Ray
• http://youtu.be/O9Goyscbazk
• Click on the above link to watch a video
demonstrating how a cathode ray works.
Earnest Rutherford discovers the
nucleus
• Ernest Rutherford discovered the nucleus and
realized it is very very small compared to the
rest of the atom.
•
Source: http://www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/rutherford-model.html
Ernest Rutherford Video
• http://youtu.be/5pZj0u_XMbc
• Click on the above link to watch a video
explaining Ernest Rutherford’s experiment
Chadwick’s discovery of the neutrons
• http://youtu.be/HnmEI94URK8
• Click on the above link to watch a video
explaining Chadwick’s discovery of the
neutron.
Bohr arranges the electrons outside
the nucleus
• Bohn believed that electrons surrounded the
nucleus in specific energy levels. Each shell
contained only a specific number of electrons
with the maximum number of electrons that
can exist in the first shell is 2, second shell 8
and third shell 8.
Examples of a Bohr Diagram
•
http://www.online.bcelearner.ca/login/index.php
Quantum Mechanical Model
• This is the model physicists and chemists use
today.
• Electrons exists in different energy levels and
surround the nucleus in a form resembling a
cloud.
Example of a Quantum Mechanical
Model of an Atom
•
http://pdgusers.lbl.gov/~pslii/uabackup/fundamentals/matter/1300200.html
What is an atom?
• What is an atom?
– An atom is the smallest part of an element that has all
of the element’s properties.
– It contains the following subatomic particles:
electrons (negatively charged), protons (positively
charged), and neutrons (no charge).
– Electrons are the smallest part of an atom, having the
relative mass of 1, while protons have the relative
mass of 1836, and neutrons have the relative mass of
1837.
– Protons and neutrons are contained in the nucleus
while electrons surround the nucleus in different
energy levels.