Atomos: Not to Be Cut
The History of Atomic Theory
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Atomic Models
• At right is the image most people have
of what an atom looks like. It shows a
nucleus of protons and neutrons with
electrons occupying various orbits
around the nucleus.
• Models use familiar ideas to explain
unfamiliar facts observed in nature.
Models can be changed as new
information is collected.
• So, is this the most accurate model of
the atom? Or is it far from reality?
The Six Atomic Models are:
1. The Greek Model
2. Dalton’s Model
3. Thomson’s Model
• This presentation discusses six major
models of what we call “atoms”. Each
new model was developed based on
new discoveries at the time.
4. Rutherford’s Model
5. Bohr’s Model
6. Wave-Mechanical Model
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1. The Greek Model
• Democritus was a Greek
philosopher who began the
search for a description of
matter more than 2400 years
ago (around 400 BC).
• He asked: could matter be
divided into smaller and
smaller pieces forever, or
was there a limit to the
number of times a piece of
matter could be divided?
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It’s only
logical!
• Democritus knew this question could not
be easily answered directly. Let’s say you
were breaking up a piece of rock -- how
would you know when the smallest piece
of rock was actually reached?
• Democritus logically concluded that a
piece of matter could not be divided into
smaller pieces forever. Eventually the
smallest possible piece would be obtained.
• Since this piece is the smallest possible it
would be indivisible, meaning it could not
be further divided.
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• He named the smallest possible piece of matter “atomos,”
which means “not to be cut.”
•
To Democritus, atoms were small, hard particles that were
all made of the same material but were different shapes
and sizes. They were infinite in number, always moving,
and capable of joining together.
• However, Democritus’ theory was ignored and forgotten for
more than 2000 years! Why?
• The ancient Greeks did not experiment, but tried to win
arguments through logic and debate. This would cause a
problem for Democritus.
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• The famous philosopher Aristotle had a different
theory of matter. He proposed that matter was
composed of four elements: earth, fire, air and
water. These four elements were supposedly
blended in different proportions to make up all the
various types of substances in the world.
• Aristotle had such a great reputation as a debater
and logical thinker that many of his ideas were
simply accepted as true without any sort of
experimental evidence.
• Aristotle may have been a great thinker and
philosopher, but his theory of matter was not
scientifically based, to say the least.
• Democritus had the right idea about the nature of
matter, but it was ignored. His theory of the atom
was not popular again until the 19th century.
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2: Dalton’s Model
• John Dalton proposed his modern atomic theory in
1803, about 2000 years after Democritus.
• Dalton’s atomic theory explained several
observations about matter that became known since
the time of Democritus:
– Most natural materials are mixtures of pure
substances .
– Pure substances are either elements or compounds.
John Dalton
1766-1824
– A given compound always contains the same
proportions (by mass) of the elements, no matter
where it came from – known as the Law of Constant
Composition.
See Sec 4.3 of
text.
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Dalton’s Atomic Theory
1. All elements are composed of atoms.
2. Atoms of the same element are identical.
3. Atoms of a given element are different
from those of another element.
4. Compounds are formed by the joining of
atoms of two or more elements. A given
compound always has the same relative
number and types of atoms.
5. Atoms are indivisible and may not be
created or destroyed in chemical
reactions. Chemical reactions simply
rearrange how atoms are grouped
together.
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• Dalton thought of atoms as
little more than very tiny
spheres – much like billiard
balls. He knew nothing about
any subatomic parts.
• However, do the points of
Dalton’s theory sound familiar?
• They should! Dalton’s atomic
theory became the foundation
of modern chemistry.
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3: Thomson’s Plum Pudding Model
• See section 4.5 in text.
• In 1897 the English scientist J.J.
Thomson proposed that atoms
themselves are made of even
smaller, sub-atomic, particles.
• Thomson (among others)
experimented with cathode ray
tubes. When such a tube is
evacuated of air and an electric
current is passed through it, a
glowing region (called cathode
rays) was observed.
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• Cathode rays are so named because they are emitted at
the negative (cathode) end and travel to the anode
(positive) end of the tube.
• Cathode rays are deflected towards a positively charged
plate which showed they are composed of negativelycharged particles that have mass.
• Regardless of the type of metal used at the cathodes
and anodes of the vacuum tubes, the cathode rays were
seen and had the same properties.
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• From these observations Thomson concluded that cathode rays
were actually a stream of negatively charged particles that came
from within the atom.
• If particles smaller than the atom existed, then atoms themselves
were divisible, not indivisible.
• Thomson called these negatively-charged particles “corpuscles”.
We, of course, know them today as electrons.
• Since atoms were neutral he reasoned that there must also be
positively-charged parts in the atom, but he could never identify
them.
• Check out the video at:
https://www.youtube.com/watch?v=IdTxGJjA4Jw&list=PLD6A43875B9DE
C57E&index=24
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• Thompson proposed a model
of the atom that became
popularly known as the “Plum
Pudding” model.
• Atoms were made from a
positively-charged substance
with negatively-charged
electrons scattered about, like
raisins in a pudding.
• The “Plum Pudding” model was
also proposed around 1900 by
William Thompson (better
known as Lord Kelvin), who
was no relation to J. J.
Thompson.
4: Rutherford’s Model
• See sections 4.5, 10.1
in text
• In 1910, New Zealand
physicist Ernst
Rutherford performed a
series of experiments
to study the structure of
atoms.
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• Rutherford’s experiment (see next slide) involved firing a
stream of positively (+) charged “bullets” – called alpha
particles (actually, helium nuclei) -- at a thin sheet of
gold foil about 2000 atoms thick.
• Most of the positively charged alpha particle “bullets”
passed nearly straight through the gold atoms in the
sheet of foil.
• However, some of the alpha particles bounced away
from the gold sheet as if they had hit something solid.
• How could something like this be explained given the
current model of what atoms were?
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• Rutherford concluded that the gold atoms in the sheet
were mostly open space, and not like a solid “pudding.”
• He further concluded that both the mass and the positive
charge of an atom are concentrated within a tiny fraction
of the atom’s volume, which he called the nucleus.
• The few alpha particle “bullets” that were deflected were
bouncing off the tiny, dense nucleus at the center of the
mostly empty atom. Those not deflected were passing
through empty space!
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What Rutherford expected: alpha particle paths virtually
uninterrupted as they went through the atom because at
the time it was believed that atoms had the same
consistency throughout.
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What he observed: scattering of some alpha particles as they
bounced off the dense nucleus at the center of the atom.
Most of the alpha particles simply passed through the empty
space of the atoms.
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• These observations certainly changed
the idea of how atoms could be pictured.
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• Rutherford’s atomic model contained
positive charges in a very tiny, very
dense nucleus.
• The negatively charged electrons filled
the remaining much larger volume of the
atom and orbited outside the nucleus.
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• By 1920 Rutherford established that the nucleus of an atom consisted
of positively charged particles, called protons, and, for stability, must
contain neutral particles with the same mass as protons.
– The neutral particle, called the neutron, was eventually discovered in 1932
by James Chadwick, a student of Rutherford’s.
– Neutrons are found in the nuclei of all atoms except hydrogen, which has
a single proton in its nucleus. (The isotope of hydrogen with a single
proton accounts for 99.98% of all hydrogen in the universe. There are
rare isotopes of hydrogen that have one and two neutrons, respectively.)
• Rutherford’s model, however, could not explain how negativelycharged electrons could maintain stable orbits around a positivelycharged nucleus. Since opposite charges attract, the orbiting
electrons should be drawn into the nucleus. But this is not observed.
So what was keeping electrons in stable orbits – and atoms in
existence?
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• Another example of the Rutherford model of the
atom in which electrons can orbit the atom in an
infinite number of paths.
• A good video on the Rutherford model can be
seen at:
https://www.youtube.com/watch?v=Yz1WIKPLXLQt
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5: Bohr Model
• See section 10.5 in text.
• In 1913, the Danish scientist
Niels Bohr proposed that
electrons are restricted to
certain fixed (quantized) orbits
around the nucleus. This
appeared to solve the problem
with Rutherford’s model of
how electrons stay in orbit
around the nucleus.
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• These orbits, or energy levels
(n=1, n=2, n=3, etc.), are only
located at specific distances from
the nucleus.
• If electrons gain energy (become
energized) they can jump to a
higher fixed energy level.
• When electrons lose energy they
drop back down to a lower energy
level. The energy lost is released
(emitted) as a photon of light of a
specific frequency (color).
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• A quantum (fixed amount) of energy is required
to move electrons to the next highest level.
Likewise, the same fixed amount of energy is
emitted when an electron drops to a lower
energy.
• Bohr’s model explained the observed emission
spectrum data for hydrogen (see fig 10.11, pg
286).
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• In Bohr’s model, the position of electrons around
the nucleus is analogous to the steps of a stair
(or rungs of a ladder).
• Electrons cannot exist between energy levels,
just like you can’t stand between steps on a stair
(or rungs on ladder).
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6: The Wave Mechanical Model
• See sections 10.6, 10.7, 10.8 in text.
• Also called the quantum mechanical model, or cloud model.
• By the mid-1920s the Bohr model was shown to be incorrect
because it only worked well for the hydrogen atom. Any model of
the atom has to work well for all atoms, not just hydrogen.
• A new model suggested that electrons exhibit wave-like as well as
particle-like behavior (just as photons do).
• Electrons do not “orbit” the nucleus as in the Bohr model, but are
located within regions of space around the nucleus.
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• Scientists responsible for the model:
– Louis de Broglie was the first to support the idea
that electrons exhibit wave characteristics.
– Erwin Schrodinger developed a mathematical
wave formula that described electrons as waves.
– Werner Heisenberg showed that it is impossible
to know both the exact location and the exact
speed of an electron (the Uncertainty Principle).
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• Summary of wave-mechanical model:
– The nucleus remains as defined by Rutherford.
– Electron states are described as orbitals, not orbits.
– Orbitals are regions around a nucleus where
electrons have a probability of being found. The
precise location and speed of electrons within
orbitals cannot be accurately determined (see firefly
example on pg 289).
– Schrodinger’s wave equations describe orbitals of
various shapes – such as spherical or dumbbell
(see pg 291 in text).
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– Orbitals are also described as electron cloud regions.
– Electron clouds are visual models that map the
possible location of electrons in an atom.
Electrons can only be
present in certain
regions around the
nucleus, not just
anywhere.
not here
here
– The edge of an orbital or cloud is “fuzzy,” meaning it
does not have an exact size.
– Electron clouds are denser closer to the nucleus
where electrons are more likely to be found.
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– Location of an electron depends upon how much
energy the electron has.
– Electrons closer to the nucleus have less energy than
those further away.
– Energy states of electrons correspond to orbitals with
different shapes: s, p, d, f (see below).
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Some links to videos on the web
• Atomic timeline video:
http://www.youtube.com/watch?v=NSAgLvKOPLQ
• A good short history of Dalton to Bohr
https://www.youtube.com/watch?v=-4Us5PTb4J8
• Lots of interesting videos! Pick the ones that interest
you.
http://www.youtube.com/watch?v=bw5TE5o7JtE&list=PLD6A43875B9DEC
57E&index=1
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Summary of the Six Models
•
Greek Model
•
Dalton Model
•
Thompson Model (“Plum Pudding”)
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•
Rutherford Model
•
Bohr Model
•
Wave-Mechanical Model
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