Atomos: Not to Be Cut

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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?
• This presentation discusses six major models of
  what we call atoms. Each new model was
  developed based on new discoveries at the time.

• The Six Atomic Models are
• The Greek Model
• Daltons Model
• Thomsons Model
• Rutherfords Model
• Bohrs Model
• 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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Its only logical!

• Democritus knew this question could not be easily
  answered directly. Lets 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 Daltons Model

• John Dalton proposed his modern atomic theory in
  1803, about 2000 years after Democritus.
• Daltons 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.
  
• 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.

John Dalton 1766-1824
See Sec 4.3 of text.
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Daltons 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 Daltons theory sound
  familiar?
• They should! Daltons atomic theory became the
  foundation of modern chemistry.

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3 Thomsons 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
  negatively-charged 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?vIdTxGJjA4JwlistP
  LD6A43875B9DEC57Eindex24

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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.

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4 Rutherfords 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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• Rutherfords 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 atoms 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.
• Rutherfords 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 Rutherfords.
• 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.)
• Rutherfords model, however, could not explain
  how negatively-charged electrons could maintain
  stable orbits around a positively-charged
  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?vYz1WIKPLXLQt
  

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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 Rutherfords
  model of how electrons stay in orbit around the
  nucleus.

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• These orbits, or energy levels (n1, n2, n3,
  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.
• Bohrs model explained the observed emission
  spectrum data for hydrogen (see fig 10.11, pg
  286).

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• In Bohrs 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 cant 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).
• Schrodingers 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.
• 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.

Electrons can only be present in certain regions
around the nucleus, not just anywhere.
not here
here
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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?vNSAgLvKOPLQ
• 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?vbw5TE5o7JtElist
  PLD6A43875B9DEC57Eindex1

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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