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Atomic Structure
Section 4-1
Democritus
Greek philosopher 4th Century BC
• First to come up with
“atom”.
• Matter is composed of
tiny particles called
atoms.
• These atoms are
invisible, indestructible
fundamental units of
matter.
John Dalton
English (1766-1844)
• School teacher.
• Studied the ratios in which
elements combine in
chemical reactions.
• Formulated first modern
Atomic Theory.
Dalton’s Atomic Theory
1. All elements are composed of
indivisible particles called atoms
which cannot be divided.
2. Atoms of the same element have
the same mass. Atoms of
different elements have different
masses.
Dalton’s Atomic Theory
3. Compounds contain atoms of more
than one element.
4. In a particular compound, atoms of
different elements always combine
in the same way.
Dalton’s Atomic Model
Based on Dalton’s theory, each
type of atom is represented by a
tiny, solid, sphere with a different
mass.
Wooden spheres made
by Dalton
Atomic Theory
• Much of Dalton’s theory still
holds true today
• However, we now know that
atoms are not indivisible
(atoms CAN be divided)
Joseph John (JJ) Thomson
(1856-1940) English
• 1897 discovered the electron while
experimenting with cathode rays.
• Thomson proved that the rays consisted of
charged particles. This was the first evidence
that atoms are made of smaller particles.
• This discovery revised Dalton’s model of the
atom proving it was divisible.
JJ Thomson with the CRT
Thomson’s Discovery
• Thomson found that cathode rays were
attracted to metal plates with a positive
electrical charge.
• Plates with a negative charge repel the
ray.
• Knowing that like charges repel and
opposite charges attract, he determined
that the ray was made of negatively
charged particles – electrons.
Deflection of Cathode Ray
CATHODE RAY
• Thompson showed that the production of
the cathode ray was not dependent on the
type of gas in the tube, or the type of metal
used for the electrodes.
• He concluded that these particles were part
of every atom.
Applications of the Cathode Ray Tube
Televisions
Computer Monitors
Thomson’s Atomic Model
An atom was believed to be a sphere of
uniform positive charge into which the
negatively charged electrons were embedded.
This model was like a plum pudding dotted
with raisins and was thus called the 'plum
pudding' model of the atom.
The evenly spread
out “negative
charges” balance
out the “positive
charges”.
Ernest Rutherford
(1871-1937)England
Tested Thomson’s theory of atomic structure
with the “gold foil” experiment in 1910.
The Gold Foil Experiment
• Bombarded thin gold foil with a beam of
‘alpha’ particles.
• If the positive charge was evenly spread
out, the beam should have easily passed
through.
Rutherford's Experiment
• Expected
• Found
Rutherford's Experiment
Rutherford and coworkers aimed a
beam of alpha particles at a sheet
of gold foil surrounded by a
florescent screen.
Rutherford's Experiment
Most particles passed through with no deflection, while some
were highly deflected.
Rutherford concluded that most particles passed through
because the atom is mostly empty space.
Rutherford’s Conclusions
• All of the positive charge, and most of the
mass of an atom are concentrated in a small
core.
• This region would have a positive charge
capable of causing the deflection observed.
• This region is called the nucleus.
How big is the nucleus?
If the Houston Astrodome were a model
for an atom, a marble at its center could
represent the nucleus.
Review what you learned …
• What was Dalton’s theory about the atom?
• What did Dalton’s model of the atom look like?
• Which part of Dalton's theory was later revised?
• What subatomic particle did J.J. Thomson
discover?
Review what you learned …
• What was the name of the device J.J,
Thomson used in his experiment?
• What was the charge of the beam in
Thomson’s experiment?
• What was Thomson’s model called?
Review what you learned …
• What was the name of Rutherford’s
experiment?
• What was the difference in the expected
results versus what actually occurred?
• During Rutherford’s experiment why were
some particles deflected and bounce
back?
Review what you learned …
• What subatomic particle did Rutherford
discover?
• According to Rutherford where is all of the
positive charge of an atom located?