Introductory Chemistry, 2nd Edition Nivaldo Tro

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Bohr and Introduction To Electron
Structure Part I
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Big Picture

• Lets look at three consecutive elements
• Chlorine atomic 17
• Argon atomic 18
• Potassium - atomic 19
• How are they similar and different?

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• Chlorine
• yellow-green gas at room temperature
• Highly reactive
• Argon
• Gas at room temperature
• Gas used in light bulbs
• Extremely unreactive
• Potassium
• Solid metal at room temperature
• Highly reactive

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Essential questions

• What accounts for the elements differences?
• What role do electrons play in all of this?

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Lets start by looking back at our atomic theory
of the atom
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Rutherfords Model

• Described the nucleus, but
• It did not explain similarity and differences in
  elements chemical properties
• It did not explain why the how the electrons are
  arranged around the nucleus.
• Nor why the negatively charged electrons did not
  fall into the positively charged nucleus and
  collapse the atom.

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Emission Spectra

• In the early 1900s, scientists observed that
  different elements emitted visible light when
  heated.
• Analysis of this emitted light reveals that
  chemical behavior is related to electron
  arrangement.
• (flame test demo)

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Emission Spectra

• Atoms can absorb energy, but they must eventually
  release it
• When atoms emit energy, it is released in the
  form of light
• Atoms dont absorb or emit all colors
• The spectrum of colors emitted can identify the
  element

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Emission Spectra
Lithium chloride Red
Sodium chloride Yellow
Potassium chloride Violet
Calcium chloride Red-orange
Strontium chloride Bright red
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Line Spectra specific wavelengths are emitted
characteristic of atoms
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The Bohr Model of the Atom

• Bohr postulated that electrons do not fall into
  the nucleus because they can only travel in
  certain allowable orbits or energy levels.
• Proposed a model of the atom that explained the
  light given off by the heated or excited atoms.
• When atoms are excited, electrons jump up energy
  levels and then emit light as they fall down to
  their ground states.

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Bohr Model of Atom Electron Orbits

• In the Bohr Model, electrons travel in orbits or
  energy levels around the nucleus
• The farther the electron is from the nucleus the
  more energy it has.

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The Bohr Model of the AtomOrbits and Energy

• Each orbit (energy level) has a specific amount
  of energy
• Energy of each orbit is symbolized by n, with
  values of 1, 2, 3 etc the higher the value the
  farther it is from the nucleus and the more
  energy an electron in that orbit has

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The Bohr Model of the AtomEnergy Transitions

• Electrons can move from a lower to a higher
  (farther from nucleus) energy level by absorbing
  energy
• When the electron moves from a higher to a lower
  (closer to nucleus) energy level, energy is
  emitted from the atom as a photon of light

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The Bohr Model of the AtomGround and Excited
States

• Ground state atoms with their electrons in the
  lowest energy level possible this lowest energy
  state is the most stable.
• Excited state a higher energy state electrons
  jump to higher energy levels by absorbing energy
• Atom is less stable in an excited state it will
  release the extra energy to return to the ground
  state

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The limits of Bohrs model

• The Bohr Model very accurately predicts the
  spectrum of hydrogen with its one electron
• It is inadequate when applied to atoms with many
  electrons
• It has did not explain the chemical behavior of
  atoms.
• Even though the model is incorrect, it laid the
  groundwork for future atomic models.

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Ever wonder how glow sticks work?

• Similar to how we use fire to excite our atoms
  earlier, glow sticks use a chemical reaction to
  excite electrons
• The chemical reaction causes the electrons to
  rise to a higher energy level
• When the electrons return to their ground state,
  they emit light in the form of chemiluminescence.

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Check for Understanding

• Now work with a partner to complete
• Electron Energy and Light
• Bohr Atomic Models Questions