Models of the Atom
a Historical Perspective
Early Greek Theories
• 400 B.C. - Democritus thought matter
could not be divided indefinitely.
• This led to the idea of atoms in a void.
fire
Democritus
earth
Aristotle
air
water
• 350 B.C - Aristotle modified an earlier
theory that matter was made of four
“elements”: earth, fire, water, air.
• Aristotle was wrong. However, his
theory persisted for 2000 years.
John Dalton
• 1800 -Dalton proposed a modern atomic model
based on experimentation not on pure reason.
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All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine
in constant ratios to form compounds.
• Atoms are rearranged in reactions.
• His ideas account for the law of conservation of
mass (atoms are neither created nor destroyed)
and the law of constant composition (elements
combine in fixed ratios).
Adding Electrons to the Model
Materials, when rubbed, can develop a charge
difference. This electricity is called “cathode rays”
when passed through an evacuated tube.
These rays have a small mass and are negative.
Thompson noted that these negative subatomic
particles were a fundamental part of all atoms.
1) Dalton’s “Billiard ball” model (1800-1900)
Atoms are solid and indivisible.
2) Thompson “Plum pudding” model (1900)
Negative electrons in a positive framework.
3) The Rutherford model (around 1910)
Atoms are mostly empty space.
Negative electrons orbit a positive nucleus.
Thomson Model
• He proposed a
model of the atom
that is sometimes
called the “Plum
Pudding” model.
• Atoms were made
from a positively
charged substance
with negatively
charged electrons
scattered about,
like raisins in a
pudding.
Thomson’s Experiment
• Thomson studied
the passage of an
electric current
through a gas.
• As the current
passed through the
gas, it gave off rays
of negatively
charged
particles.
Video Link
Thomson concluded that the
negative charges came from within
the atom.
A particle smaller than an atom had
to exist.
The atom was divisible!
Thomson called the negatively
charged “corpuscles,” today known
as electrons.
Since the gas was known to be
neutral, having no charge, he
reasoned that there must be
positively charged particles in the
atom.
But he could never find them.
The Gold Foil Experiment
•
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http://chemmovies.unl.edu/ChemAnime/R
UTHERFD/RUTHERFD.html
Ernest Rutherford
• Rutherford shot alpha () particles at gold foil.
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive
substance path of invisible
-particles
Most particles passed through.
So, atoms are mostly empty.
Some positive -particles
deflected or bounced back!
Thus, a “nucleus” is positive &
holds most of an atom’s mass.
The Gold Foil Experiment
• This could only mean that the gold atoms in the
sheet were mostly open space. Atoms were not
a pudding filled with a positively charged
material.
• Rutherford concluded that an atom had a small,
dense, positively charged center that repelled
his positively charged “bullets.”
• He called the center of the atom the “nucleus”
• The nucleus is tiny compared to the atom as a
whole.
Video Link for Discovery of Elementary Particles
Bohr Model
• In 1913, the
Danish scientist
Niels Bohr
proposed an
improvement. In
his model, he
placed each
electron in a
specific energy
level.
Bohr’s model
• Electrons orbit the nucleus in “shells”
• Electrons can be bumped up to a higher
shell if hit by an electron or a photon of light.
There are 2 types of spectra: continuous spectra &
line spectra. It’s when electrons fall back down that
they release a photon. These jumps down from
“shell” to “shell” account for the line spectra seen in
gas discharge tubes (through spectroscopes).
Wave Model
The Wave Model
• In fact, it is impossible to determine the exact
location of an electron. The probable location of
an electron is based on how much energy the
electron has.
• According to the modern atomic model, an atom
has a small positively charged nucleus
surrounded by a large region in which there are
enough electrons to make an atom neutral.
Electron Cloud:
• A space in which
electrons are likely to be
found.
• Electrons whirl about the
nucleus billions of times
in one second.
• However, They are NOT
moving around in random
patterns.
• Location of electrons
depends upon how much
energy the electron has.
Electron Cloud:
• Depending on their energy they are locked into a
certain area in the cloud.
• Electrons with the lowest energy are found in
the energy level closest to the nucleus
• Electrons with the highest energy are found
in the outermost energy levels, farther from
the nucleus.