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The History of the Atom
Learning Target: Identify major contributions to the modern theory of atomic structure brought forth by
John Dalton, J.J. Thomson, Robert Millikan, Ernest Rutherford, Niels
Bohr, and James Chadwick.
John Dalton• Born 1766• Did most work from
1808-1827• England• First to develop a
theory for the structure of an atom - Based on five major principles
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Dalton’s Atomic Theory • Five Major Principles:
• Elements are made of indivisible atoms
• All atoms of the same element have the same properties and the same mass
• Compounds are made of atoms of different elements combined together
• Chemical reactions involve the reorganization of those atoms
• Atoms cannot be created nor destroyed
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Dalton’s Atomic Theory • The first and second
principles: Elements are made of indivisible atoms AND All atoms of the same element have the same properties and the same mass
• “Atomos”• His thought was that each
element existed as a simple spherical substance, different in physical and chemical properties from any other element
• This is his MODEL of the ATOM
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Dalton’s Atomic Theory • The third principle:
Compounds are made of atoms of different elements combined together
• When Dalton was able to separate a substance, he called it a compound. Remember, elements could not be separated into smaller parts.
• Unlike a mixture, compounds are chemically combined and can not be separated without a chemical change.
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Dalton’s Atomic Theory • The third principle…
what else does it tell us?• When a particular compound
is created, Dalton stated that it must be created the exact same way each time. Compound #31 must always be made of three empty spheres (oxygen) with one sphere with a cross (sulphur).
• This idea is called the Law of Definite Proportions – when elements combine, they do so in a ratio of whole numbers.
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Dalton’s Atomic Theory • The third principle… can different compounds be
made of the same elements?
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Yes• Take a look at compounds #26 and #27 One represents what Dalton believed to be nitrous oxide and the
other nitric acid. • These are an excellent example of the Law of Multiple
Proportions, which states that if two elements form more than one compound between them, then the ratios of the different compounds must be different from each other.
Dalton’s Atomic Theory • The fourth principle: Chemical reactions involve
the reorganization of those atoms
Dalton’s Atomic Theory • The fifth principle: Atoms cannot be created nor
destroyed • In all chemical reactions the total mass of the original chemicals
equals the total mass of the created chemicals. This is known as the Law of Conservation of Mass.
It’s flawed!Are atoms only
spheres?Or, is there
something else inside?
Dalton’s Atomic Theory
Another flaw – Look at compound #21,
Dalton’s version of water; that’s one oxygen with one hydrogen. Isn’t water made with two hydrogen atoms?
Dalton’s Atomic Theory
So why is it still studied?Despite these major flaws, Dalton opened the
door to enormous understandings of atoms, compounds, and reactions that we still use today. And, three major laws that are also still important – Law of Definite Proportions, Law of Multiple Proportions, and Law of Conservation of Mass.
Dalton’s Atomic Theory
J.J. Thomson
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Lived 1856 – 1940EnglandDid work from 1884 - 1914
Thomson’s ExperimentThomson was fascinated by the idea that electricity could cause a trapped gas to glow. He called his device a cathode ray tube, or Crooke’s tube; these are still used today as standard fluorescent bulbs, neon lights, and spectroscope tubes, to name a few current models.
Thomson’s Experiment• It’s not the cathode ray tube that’s fascinating,
or useful to the development of the atom, however. It’s the fact that magnets held against the tube could deflect (or attract) the light beam inside.
Thomson’s Atomic ModelThe Plum Pudding Model
The electrons were thought to be positioned throughout the atom, but the atom was also was said to have had a "cloud" of positive charge.
Thomson won the Nobel prize for the discovery of the electron in 1906.
Robert Millikan• Lived 1868 - 1953• University of Chicago!• Experimenting with the input
of Harvey Fletcher
Millikan’s Experiment of 1909
Millikan’s Results
Millikan’s Oil Drop Experiment, published in 1913, helped the atom in two ways.
Millikan’s Results
First, it showed the charge on the electron…
1.6x10-19 coulombs.– And second, it showed
the mass of the electron was 1/2000th of a proton.
Millikan’s Results
And second, it showed the mass of the electron was 1/2000th of a proton.
Thomson had already determined the charge-to-mass ratio for the electron, but the actual values were unknown.
Millikan was able to calculate it.
Ernest Rutherford• Lived 1871 - 1937• New Zealand origin, England lab• One of Thomson’s students
Rutherford’s Experiment
Rutherford’s Gold Foil Experiment, in 1909-1911, predicted that all particles would go straight through the atom described by the Plum Pudding Model.
Alpha Particle
4
4 +2
2
He
Rutherford’s Experiment
That’s not what happened, though!Some particles went through the foil while others were deflected and still others bounced back.
Rutherford’s Atomic ModelThe Planetary Model
Rutherford proved that there was a positively charged, very dense, very heavy center, called the nucleus. While the rest of the atom was empty space.
He is credited with the discovery of the proton.
Niels Bohr
• Lived 1885 - 1962• 1913 published theory on
atomic structure• Denmark
Bohr’s Experiment
Bohr’s Hydrogen Energy Level Experiment used a spectroscope to separate the colors of a heated hydrogen gas. What could cause specific colors of light to be created, rather than a full continuous spectrum?
Bohr’s Experiment
Specific colors of light have specific wavelengths of specific energy. This led Bohr to theorize that there are levels, called energy levels, around the atom’s nucleus for electrons to travel between
Bohr realized that electrons must absorb energy as the hydrogen gas is heated. When the electron absorbs the energy, it moves to a higher energy level.
Bohr’s Experiment
The electron eventually releases that energy, as a wavelength of color, falling to a lower energy level.
Bohr’s Atomic Model
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The Orbital Model
Electrons travel in orbits, now called energy levels, around the atom's nucleus. The variable “n” represents the energy levels.
Bohr’s Atomic ModelBohr’s Orbital Model is still not the current model of the atom. It did, however, determine some important information about atoms. • The chemical properties of each element can
largely be determined by the number of electrons in the outer orbits of its atoms.
• Bohr’s work became the basis for quantum mechanics which is the current model of the atom.
James Chadwick
• Lived 1891 - 1974• Major discovery: 1932• England
Chadwick’s Experiment
Alpha Particle BombardmentAlpha particles (the same particles that Rutherford used) were used to bombard a sample and it was discovered that neutral particles were released that changed the mass of the atom.
Chadwick’s Atomic Model
The neutron was the last subatomic particle discovered.
Chadwick won the Nobel Prize for his discovery of the neutron in 1935.
Erwin Schrödinger
• Lived 1887 - 1961• Germany
Schrödinger’s Atomic Model• Louis de Broglie had determined that
electrons moved in wave-like motion.
Schrödinger’s Atomic Model• This meant that electrons were not moving
around the nucleus in orbits, as described by Bohr, but, rather, in clouds.
• The Cloud Model
Schrödinger’s Atomic Model• Joining this idea with the theories of the
scientists before him, Schrödinger developed the Schrödinger equation. The Schrödinger equation and it’s use of quantum mechanics is the current model of the atom.
