The History of the Atom…
A Summary
Miss Maroldo
Early ideas of the Structure of the Atom
Leucippus, a philosopher of the 5th century BCE, was the first believed to
consider that matter was composed of smaller particles. His philosophy was
called atomism. These smaller particles, he believed, could be divided into
two categories: full and solid, or empty and void.
Democritus, a supposed student of
Leucippus, continued the idea of atomism.
He called the small particles that composed
all matter atoms (meaning indivisible). He
said that all atoms were firm and solid, but
not the same. Atoms would be similar to
the objects they made, i.e. “the atoms of a
needle would be sharp” (J. Nguyen)
Aristotle, a philosopher of the 3rd
century BCE, rejected the ideas of
Leucippus and Democritus and
maintained the a popular idea of the day;
that all matter is composed of four
“elements,” earth, wind, water and fire.
Image courtesy of:
http://www.sixsquare.com/blog/images/0805/airearthfi
rewater2.jpg
Thank you J. Nguyen, J. Lawrence,
K. Porter and M. Jones
John Dalton
Continued the earlier theories of particulate
matter with his atomic theory of matter:
1. Everything is formed of atoms.
2. All atoms of the same element are identical,
but differ from any other element.
3. Atoms can be neither created nor destroyed
4. A given compound always forms in the same
who number ratios of atoms.
Much of Dalton’s contributions to atomic theory
stemmed from his work studying meteorology.
J.J. Thomson
J.J. Thomson, a school teacher, conducted a series of
experiments with a cathode ray tube beginning in 1896.
Check out this video:
http://online.cctt.org/physicslab/content/Phy2HON/lessonnotes/modern/electron
beams.asp
He found that the mysterious glowing ray inside the
cathode ray tube was responsive to the presence of a
magnetic. Thomson concluded that the ray must be
composed of negatively charged particles that he called
electrons.
Although he was unsuccessful at determining the mass of
an electron, he did determine the charge ratio of the
electron to be 1.76 x 108 coulombs/gram.
J.J. Thomson Conn…
After discovering the
electron, Thompson
theorized that an atom
must be composed of
electrons floating in a sea
of positive charge as
shown below. This model
is often referred to as the
plumb-pudding model.
Courtesy of:
http://en.wikipedia.org/wiki/Image:Plum_pudding_at
om.svg
R.A. Millikan
Millikan succeeded in measuring
the charge of an electron in
1909. How you ask???
He sprayed oil droplets into the
apparatus shown. An x-ray
was used to give the droplets a
charge.
As the droplets fell between
charged plates, he measured
the how the different charges
on the plates affected the rate
at which the droplets fell.
Courtesy of:
http://www68.pair.com/willisb/millikan/experiment.html
From this data, Milikan measured that the charge of each droplet
was a multiple of 1.60 x 10-19 coulombs. This meant the each
electron had a charge of 1.60 x 10-19 coulombs. Using Thomson’s
mass-to-charge ratio, Millikan determined the mass of an electron to
be 9.11x10-28 grams (2000 times smaller than a hydrogen atom!)
Rutherford
Rutherford’s gold-foil
experiment lead him
and his team to
discover the nuclear
atom in 1909.
When Rutherford shot a
beam of alpha
particles (which have
a positive charge) at a
piece of thin gold foil,
he found that while
most particles passed
straight through,
others seemed to
bounce off.
Courtesy
of:http://www.dlt.ncssm.edu/TIGER/diagrams/structure/G
oldFoilExperiment.jpg
Rutherford Conn…
Earlier models of the atom hypothesized the atom to be composed of
electrons simply floating in a sea of positive charge. Had this been
true, all of the alpha particles would have passed right through as
shown in diagram a.
Because some alpha particle were deflected, he concluded that there
must be a concentration of positive charge in the center of the
atom (diagram b). He called this this nucleus.
Picture Courtesy of:
http://library.tedankara.k12.tr/chemistry/vol2/the%20structure%20of%20the%20atom/z15.jpg
Check out this web site:
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
Max Plank
Recall that at this point in history, the atom
consists of electrons somehow moving around a
nucleus of positive charge.
Plank suggested that energy emitted or absorbed
by any object is restricted to particular sizes,
which he referred to as quanta, as is expressed
in his equation E = hv. The equation shows us
that the energy emitted by an object is directly
related to the wavelength is emits.
What is quantized energy? It is not continuous
(like an escalator at the mall). Quantized energy
occurs in specific steps (like taking the stairs at
the mall). The size of the steps are unique to
the object absorbing or emitting the energy.
Max Plank
Applying Plank’s theory to atoms tells us that the
energies absorbed or emitted by atoms are
quantized, which means that their values are
restricted to certain quantities and the energy is
not continuous.
Although we now recognize Plank’s discovery as
laying the foundation for advancing the model of
the atom, his discovery did not attract much
attention at first. Even Plank himself was
uncomfortable with his proposed concept and
did not recognize its importance at this time.
Albert Einstein
After studying the work of Max Plank, Einstein saw
a new way of thinking about light. He used
Plank’s theory to explain a puzzling phenomena,
the photoelectric effect.
In the photoelectric effect, electrons are ejected
from the surface of a metal upon exposure to
light. However, it was known that only certain
frequencies of light would induce the
photoelectric effect in certain metals. Red light,
for example, was unable to eject electrons from
potassium metal, however, violet light, even faint
violet light, released electrons easily.
The Photoelectric Effect
Courtesy of: http://hyperphysics.phy-astr.gsu.edu/hbase/imgmod2/pelec.gif
Notice above, that only certain wavelengths of light (or
certain energies of light) can eject electrons from the
surface of this metal.
Albert Einstein Conn…
Einstein extrapolated Plank’s idea and said that light must
consist of quanta of energy that behave like tiny particles
of light.
He called these quanta of light photons, which carry the
amount of energy given by Plank’s equation E = hv.
When a photon strikes the surface of a metal, it transfers its
energy to an electron in a metal atom. The electron can
only absorb the entire photon or none of the photon, but
not part of the photon. Nor can it absorb several
photons to gain the energy of more than one photon.
Therefore, if the energy of the photon is not great
enough, the electron cannot escape the metal atom.
Lastly, if light has the ability to eject electrons from metals,
then light must have the properties not just of waves, but
as particles as well.
Why does green and violet light
eject electrons from the metal?
Courtesy of: http://hyperphysics.phy-astr.gsu.edu/hbase/imgmod2/pelec.gif
Green and violet light have greater energy than red light,
and “result in a great amount of energy per photon
(LeMay).” (Recall, the energy of visible light increases
through the rainbow, ROYGBIV.)
Niels Bohr
The fact that electrons can only absorb or
emit energy in chunks, or quanta, laid the
foundation for theorizing just how
electrons are arranged in an atom. This
and the explanation of another puzzling
phenomena lead Niels Bohr to propose
the first model of the atom that provided
the first illustration the arrangement of
electrons in an atom.
Line Spectra
Early in the century it was observed that samples
of elements emit light when they are vaporized
in an intense flame.
The light that is seen results from the atoms of the
element absorbing the energy of the flame and
releasing the absorbed energy in the form of
light.
When this light is passed through a prism, a
resulting line spectra of specific wavelengths
results. This line spectra is unique for each
element.
Line Spectra
Notice that the color flame produced
from the vaporization of sodium is
yellow. If the yellow light is
separated into its component
wavelengths with a prism, the
resulting spectrum is shown at left.
Courtesy of:
http://upload.wikimedia.org/wikipe
dia/commons/thumb/e/e5/Flamete
st--Na.swn.jpg/200px-Flametest-Na.swn.jpg
Courtesy of: ://www.luc.edu/faculty/spavko1/JCE/line-spectra/SODIUM.GIF http
Niels Bohr Conn…
Bohr was the first to see the connection
between the characteristic line spectra of
each element and atomic structure.
Using Rutherford’s nuclear atom and Plank’s
idea of quantization, Bohr created a model
of the atom that explained line spectra.
His model of the atom still represents the
simplified atom today.
The Bohr Model of the Atom
Courtesy of: http://upload.wikimedia.org/wikipedia/commons/9/9b/Bohratommodel.png
Bohr proposed that the electrons of an atom must be arranged in certain
orbits that correspond to the amount of energy of that electron (these
energies are represented by the letter “n” and increase as the distance from
the nucleus increases.) Here, Hydrogen is represented, since on proton is
present and one electron orbits the nucleus.
How does this Explain Line Spectra?
When an electron absorbs a photon with
high enough energy, it becomes excited
and jumps to the energy level that
corresponds to the new energy of the
electron. When the electron releases
the absorbed energy and returns to its
original position, light is released.
Because the energy levels of the
electrons are quantized, only certain
photons will allow the electron to absorb
enough energy to jump to n=2, n=3 and
so on. Therefore, only certain
wavelengths will be released upon
return to the original energy level.
Although Bohr’s model worked well for the hydrogen atom, it could not
explain spectra of elements with more than one electron.
Louis de Broglie
As you recall, Einstein stated that if light waves
could eject electrons from the surface of a metal,
then light must have some particulate properties.
de Broglie wondered that if light could behave as
particles, could particles behave as waves???
de Broglie theorized that all matter exhibits
properties of both particles and waves. He
derived this mathematical relationship and
showed that the wavelength of an object was
directly related to its velocity and inversely
related to its mass.
Louis de Broglie Conn…
What does this have to do with the model of the atom???
de Broglie showed that electrons, while traditionally thought
of as minute particles, could also behave like waves.
This laid the foundation for Werner Heisenberg who
proposed the Heisenberg Uncertainty Principle which
states that the position and momentum of a moving
object cannot both be known precisely at the same time.
You will learn shortly that we now describe not only the
arrangement of electrons around the nucleus in terms
energy, but also of the probability of finding the electron
in certain locations around the nucleus.
Becquerel
Becquerel became interested in a predecessor's
discovery of a mysterious energy called X-rays.
These rays were found induce fluorescence in
certain minerals and had the ability to pass
through low-density shields.
Since X-rays could induce fluorescence in certain
minerals, Becquerel wanted to see if a
fluorescent mineral was capable of emitting Xrays, specifically, uranium.
Henri Becquerel Conn…
After researching for some time, Bacquerel
becomes frustrated with his work. He wraps the
uranium sample in black paper to prevent light
exposure and throws it into his drawer, where it
would remain for several days in the dark.
Just by chance, Becquerel threw the sample on
top of a photographic plate and just by chance,
he decided to develop the plate after several
days (even though the plate had been
unexposed to light, or so he thought…)
Becquerel Conn…
What do you know???
The photographic plate showed exposure!
This lead Becquerel to the discovery of the
phenomena of radioactivity and the first
radioactive element: Uranium.
Marie Curie
Marie and her husband, colleagues of Becquerel,
are credited with the isolation of two more
radioactive isotopes: radium and polonium.
“As scientists studied radioactivity, they made an
important observation: Radioactivity
accompanies fundamental changes in an
atom…Studying the nature of these changes
gave scientists further clues about the
substructure of the atom.” (LeMay)
Works Cited
LeMay, H., Beall, H., Robblee, K., & Brower,
D. (1996). Chemistry: Connections to Our
Changing World. Upper Saddle River, NJ:
Prentice Hall.