The History of the Atom

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The History of the Atom
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Atomic Theory

• Because we can not see atoms, we use models to
  teach and learn about atoms.
• The atomic theory has changed over time as new
  technologies have become available.
• Remember Scientific knowledge builds on past
  research and experimentation.

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Ancient Greece

• Aristotle
• There are four elements
• Earth, air, water, fire
• The four elements combine in various ways to make
  all matter.
• Matter is continuous. (There is not a smallest
  particle.)

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Ancient Greece

• Democritus
• Matter is made of tiny particles called atoms.
• The atom is the smallest piece. It is
  indivisible. (It cant be divided any further.)
• Atoms of an element have specific properties
  (smooth, spiky, etc.) that give the element its
  properties.

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John Dalton- Father of the Atomic Theory

• Matter is made of indivisible particles called
  atoms.
• Atoms of the same element are identical, but
  differ from atoms of other elements.
• Atoms cannot be created or destroyed.
• Atoms of different elements can combine in
  simple, whole number ratios to form compounds.
  (The Law of Definite Proportions)
• Atoms of same element can combine in more than
  one ratio to form two or more compounds. (The Law
  of Multiple Proportions)
• The atom is the smallest unit of matter that can
  take part in a chemical reaction.

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John Dalton

• The Billiard Ball model
• An atom is a solid, indivisible sphere

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J.J. Thompson

• The cathode ray tube experiment

• Energizing matter in the tube removes charged
  particles and makes a cathode ray (beam of
  light).
• Magnets can deflect the cathode ray in a way that
  shows it is made of negative particles.
• The amount of deflection tells us how massive the
  particles are.

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J.J. Thompson

• Thomson discovered that
• The atom is NOT indivisible
• A small negatively-charged particle (the
  electron) can be removed from the atom.
• He concluded that
• The atom must have negative particles (electrons)
  in it (its not like a billiard ball).
• The electrons are evenly distributed amongst
  positive background stuff that makes up most of
  the atom.

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J.J. Thompson

• This led to the Plum Pudding model

(The electrons are like the plums in a pudding
or the chips in a chocolate chip cookie.)
Plum pudding plum-flavored pudding with plums
distributed throughout it used to be a popular
thing people ate. We still call this the Plum
Pudding Model even though we dont eat plum
pudding.
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Ernest Rutherford

• The Gold Foil Experiment

• Alpha particles (which are positive) were shot at
  a piece of gold foil.
• They were expected to go straight through.
• Some particles were (unexpectedly) deflected.
• What deflected them??

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• (Positive) alpha particles are deflected by other
  positives.
• (Like charges repel.)
• The nucleus must be positive!
• The nucleus must have most of the mass, or it
  would get pushed around by the alpha particles.
• The nucleus must be ridiculously small because
  10,000 alpha particles pass straight through for
  each one that is deflected.

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Ernest Rutherford

• The Gold Foil Experiment Conclusions
• Atoms have a NUCLEUS
• It contains all the positive charge
• It contains almost all the mass
• It is TINY (1/10000 of the atom volume)
• Everything else is mostly empty space

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Ernest Rutherford

• The nuclear model separates the atom into two
  parts
• Nucleus positive massive tiny
• Electrons negative, tiny, practically weightless.

• The Nuclear Model
• This picture is not to scale!
• The nucleus should be WAY smaller than the rest.

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Neils Bohr

• Since opposites attract, electrons are attracted
  to the (positive) nucleus.
• Something must prevent electrons from falling
  into the nucleus.

• Electrons are located in orbits around the
  nucleus.
• Like planets orbiting the sun, they are
  attracted, but if they stay at the correct
  distance, they dont fall in.
• The distance from the nucleus determines the
  energy.

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Neils Bohr

• Bohr quantized the atom.
• Only certain energies are allowed, which means
  only certain orbits are allowed.
• All other places are forbidden to the electrons.

Electrons are allowed to be here.
Or here!
But not here. (This is forbidden because no
orbit for the electron to land on.)
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Niels Bohr

• Moving from one energy level to another requires
  the electrons to absorb or emit energy

• Because only certain orbits are allowed
• The energy comes in specific colors of light for
  each element.

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Albert Einstein

• The photoelectric effect
• Electrons can be ejected from metal by light
  (energy) of a certain frequency.

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Photoelectric Effect

• If the light isnt high enough energy, nothing
  happens.
• If it does have enough energy, electrons are
  removed.

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Albert Einstein

• The photoelectric effect proved
• Different colors of light are worth different
  amounts of energy.
• Electrons can be moved to different orbits (or
  out of the atom altogether) by energy in the form
  of light.
• Light has momentum therefore it is a particle
  (even though it has no mass).
• Light particles are called photons.

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Thomas Young

• The double-slit experiment
• Electrons were sent through two small openings
  and collected on a screen on the other side.

• The electrons created interference patterns.
• Interference patterns come from waves overlapping
  constructively and destructively.

Therefore electrons are a wave (?!)
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Louis de Broglie

• Particle-Wave duality
• Einstein proved light could act like a particle
• Young proved electrons could act like a wave
• De Broglie concluded that wave and particle
  arent mutually exclusive. The electron can act
  as either one.
• Electrons have particle-wave duality.

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Louis de Broglie

• Electrons-as-waves explains the fact that only
  certain orbits are allowed for each element.

• The orbits circumference has to be a multiple of
  the wavelength in order for it to be allowed.

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Heisenberg

• The Uncertainty Principle
• Electrons can be moved by light.
• We see things because light bounced off of them.
• So every time you see an electron, the light
  that made it visible to you probably also caused
  it to move.
• So it isnt actually there anymore!
• It is impossible to know the location and
  velocity of an electron at the same time.

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The Uncertainty Principle

• Means that electrons cannot be traveling in
  predictable circular orbits around the nucleus.
• They move randomly and unpredictably.
• We can estimate the probability of finding an
  electron somewhere, but we cannot say where it is
  with certainty.
• (Lets face it if de Broglie is correct and the
  electron is partly a wave, it doesnt necessarily
  travel as a particle anyway.)

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Erwin Schrödinger

• Developed an equation to solve for the locations
  the electron probability is greatest.
• This basically describes a cloud-like region
  where the electron is most likely to be found.
• It cannot say with any certainty where the
  electron actually is at any point in time, but it
  describes where the electron could be.

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Erwin Schrödinger

• The probable locations of the electron predicted
  by Schrödinger's equation happen to coincide with
  the locations specified in Bohr's model.
• The difference is that now everything is fuzzy
  because of the lack of certainty.

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Modern Atom the Quantum model

• Combining Heisenberg Schrodinger (with all the
  previous discoveries) gives us

• Orbitals, not orbits
• Cloud-like regions (fuzzy borders) where the
  electron probability is highest.

• Orbitals are 3-D (orbits were 2-D).
• Quantized energies
• Electrons can only exist at specific energies
  allowed by the orbitals.
• Electrons can absorb or emit energy to move to
  higher or lower energy orbitals.

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The current (modern) atomic model