Transcript atomichistorychemx

Atoms: Brainstorm
Is Matter Infinitely Divisible
• Is time infinitely divisible?
• Space?
• Atoms?
4.1
Democritus (460B.C.-370B.C.)
Atom: a = un, tomos = cuttable
Atoms are indivisible/indestructible
Aristotle: believed all things were
made of 4 elements:
Dalton’s View of Atoms (1766-1844)
Dalton and Democritus
• What is wrong with Dalton’s atomic theory?
Billiard Ball Model
J.J. Thomson (1856–1940)
Cathode Ray Tube Experiments
A Cathode Ray is Deflected by
electrically charged plates
Inferring If a cathode ray is attracted to a positively charged plate, what can
you infer about the charge of the particles that make up the cathode ray?
A Cathode Ray is Deflected by a
magnet as well
Are electrons magnetic?
Lenz’s Law
• Electricity and
Magnetism are the
same force.
• A moving B (magnetic
field) induces an electric
current.
• That moving electric
current can induce a
magnetic field.
Because like charges repel and opposite
charges attract: Thomson concluded that a
cathode ray is a stream of negative particles
now called electrons and that all atoms must
have electrons.
Plum-Pudding Model of the Atom
• Protons and Neutrons
were also discovered
shortly after J.J.
Thompson’s experiments.
A new vision of the atom
emerged.
• The atom was a positively
charged sphere with
negatively charged
particles embedded in it.
Blueberry Muffin Model
Milikan & Fletcher 1909 (oil drop)
•
The experiment entailed balancing the
downward gravitational force with the
upward drag and electric forces on tiny
charged droplets of oil suspended between
two metal electrodes. Since the density of
the oil was known, the droplets' masses, and
therefore their gravitational and buoyant
forces, could be determined from their
observed radii. Using a known electric field,
Millikan and Fletcher could determine the
charge on oil droplets in mechanical
equilibrium. By repeating the experiment for
many droplets, they confirmed that the
charges were all multiples of some
fundamental value, and calculated it to be
1.5924(17)×10−19 C, within 1% of the
currently accepted value of
1.602176487(40)×10−19 C.
•
They proposed that this was the charge of a
single electron.
Rutherford’s Gold-Foil Experiment
(1911)
Alpha particles scatter from the gold foil.
The Rutherford Atomic Model
• Rutherford concluded that the atom is mostly
empty space. All the positive charge and
almost all of the mass are concentrated in a
small region called the nucleus.
• In the nuclear atom, the protons and neutrons
are located in the nucleus. The electrons are
distributed around the nucleus and occupy
almost all the volume of the atom.
Nuclear Atom
Rutherford’s atomic model could
not explain why objects change
color when heated
Chemistry is such a Bohr (1913)
• Bohr devised the
Planetary Model of the
Atom based upon the
work of Max Planck.
• Electrons orbit the
Nucleus like planets
orbit the sun.
• Electron energies are
quantized (they exist at
fixed levels).
Electron Orbits and Energy
• Orbitals are like swinging
a paddleball.
• The more energy the
further away from a
nucleus an electron will
be.
•
When excited the
electrons jumps up to
another level.
Bohr’s Strange Laddder
• The higher the energy
level occupied by an
electron, the less
energy it takes to move
from that energy level
to the next higher
energy level.
Heisenberg Uncertainty Principle
• Werner Heisenberg in
1920 says we can’t
know both the position
and momentum of an
electron in at atom.
• We can only know the
probability of finding an
electron in a given
space.
1926 Schrödinger Wave-Quantum
Mechanical Model of the Atom
• The probability of finding an
electron within a certain
volume of space
surrounding the nucleus can
be represented as a fuzzy
cloud. The cloud is more
dense where the probability
of finding the electron is
high.
• Electrons are quantized but
they have very specific
orbital shapes and
configurations.
1932 Chadwick Discovers Neutron
What does a duck with an advanced
degree in science say?
• Protons and Neutrons
are made of quarks.
• Quarks STRINGS???
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Standard model physics
Leptons
Hadrons
Baryons
Mesons
Supercollider!
Atomic Mystery
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Like charges repel
Opposite charges attract.
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Why don’t electrons rush into the nucleus?
Why doesn’t the nucleus pull itself apart?
Atomic history: Nature of Science
Four hundred years before Christ, from the philosophical musings of Democritus came the solid, uniform,
indivisible and indestructible atom. Over 2000 years later the English schoolteacher John Dalton published his
atomic theory which said that atoms combine in simple whole number ratios and that atoms of one element are
completely identical but different than atoms of another element. Atoms were still considered indivisible and
uniform. JJ Thomson came along passing current through tubes of gas. Because opposite charges attract, he
surmised via his Cathode Ray experiment that the atom had negative particles now called electrons. This led to
the blueberry muffin model of the atom. A dilute positive sea of cake embedded with negative blueberries. The
atom is no longer uniform or indivisible at this time. After Thomson came Rutherford with his famous gold foil
experiment. He was shocked when a few of the positive alpha particles aimed at the foil came back in almost the
opposite direction. Most of the particles did go straight through and this led him to conclude that there must be a
small, dense, central region of positive charge in an atom. The nucleus was found and it was determined that most
of the atom was completely empty with electrons whizzing around the nucleus. After Rutherford things get
Bohring or more difficult to understand. Bohr came up with the planetary model of the atom. Electrons orbit the
atom in fixed, quantized orbits just like the planets orbit the sun. If the electrons become excited, they can jump
up an orbital and when they “hop back down”, light or “photons” thanks to Einstein's photoelectric effect, are
emitted. In the last stage of atomic history we have the equations of Schrödinger and quantum mechanics telling
us that electron energies are quantized as Bohr said, but that they exist in fuzzy clouds or probabilistic regions in
an atom. Oppositely spinning electron pairs reside in these dumbbell, spherical and oddly shaped p,s,d, and f
orbitals. No one can know exactly the location and momentum of an electron. We only know where it might be.
Chadwick discovered the neutron in the 1930s and standard model physics has unearthed a whole host of other
subatomic particles and we now know that baryons such as protons and neutrons are made up of quarks. The
wheels of science keep turning and our models of the atom keeps getting refined.
Timeline of the Atom
Timeline of the Atom
Research Paper
• Format: APA
• Length:3 pages
• Topic: How does the history of atomic theory
relate to the nature of science.
• You should explain the history of atomic theory in
the paper and also what science is and how they
relate. Remember in class we learned that
science doesn’t “prove” things and the scientific
method is cyclical meaning that theories are
continually refined…