December 13 The wave-mechanical model of the atom

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December 13The wave-mechanical model of the atom

• Homework
• Finish worksheet
• Review book questions

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The Wave Model

• Todays atomic model is based on the principles
  of wave mechanics.
• According to the theory of wave mechanics,
  electrons do not move about an atom in a definite
  path, like the planets around the sun.

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Quantum Model
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• The modern model of the atom is called the
  quantum model or the wave-mechanical model.

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• By 1900 scientist were studying energy and waves.
• It was proposed that energy was made of tiny
  packets called quanta, these packets acted like
  particles.

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• Quantum singular package or bundle or energy.
  If the energy is light is a photon.
• Quanta is the plural of quantum.

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Duality wave -particle

• Louis De Broglie discovered that the electron
  also behaves sometimes as a particle and
  sometimes as a wave

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Orbitals

• In the wave-mechanical model the electrons are
  found not in fixed orbits around the nucleus but
  in ORBITALS.
• ORBITALS are regions in which an electron of a
  particular amount of energy is most likely to be
  found.

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Probability

• The wave mechanical model describes the atom
  using PROBABILITY of finding electrons at
  different distances from the nucleus.

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Quantum theory

• In 1926 Erwin Schrodinger used the quantum theory
  to write an equation describing the location and
  energy of the electron in the atom of hydrogen.

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The quantum model principles

• Electrons are found in orbitals.
• There are different energy levels. Electrons can
  gain or lose fixed amounts of energy (quanta) to
  move to different energy levels.
• When the electrons occupy the lowest available
  orbitals the atom is in the ground state.
• If the atom absorbs a quantum of energy the
  electrons jump to a higher energy level. This
  UNSTABLE CONDITION is called EXCITED STATE.
• When the electrons return to the ground state
  they emit the same amount of energy they absorbed
  as LIGHT of different colors.
• The emitted light can be analyzed with a
  spectroscope and the resulting spectrum can be
  used to identify the atoms.

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Energy levels

• A space in which electrons are likely to be
  found.
• Electrons whirl about the nucleus billions of
  times in one second
• They are not moving around in random patterns.
• Location of electrons depends upon how much
  energy the electron has.

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Energy levelsor shells

• Electrons with the lowest energy are found in the
  energy level closest to the nucleus
• Electrons with the highest energy are found in
  the outermost energy levels, farther from the
  nucleus.

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Waves

• Waves carry energy and have cycles. Each cycle of
  a wave begins at the origin and ends in the
  origin.
• The wavelength (l lambda) of a wave is a
  measurement of how long it is. It is measured in
  length units.
• The longer the wavelength, the smaller amount of
  energy that is associated with.
• Short wavelength high energy.
• Long wavelength low energy.

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Waves Light consists of electromagnetic waves
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How is Light Energy related to Color?

• White light is made up of all colors of the
  spectrum (ROYGBIB)
• When passed through a prism a continuous spectrum
  (rainbow effect) is obtained

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Nature of light

• Light is a form of electromagnetic wave. Each
  color of the visible light is associated with a
  different wavelength.
• R (700 nm)
• O
• Y
• G
• B
• I
• V (400nm)

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Line Spectra

• If high voltage is applied to atoms in gas phase
  at low pressure light is emitted from the gas.
• If the light is analyzed with an SPECTROSCOPE the
  spectrum obtained is not continuous.

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Line Spectra

• This type of spectra is used to identify atoms
  of different elements and were used to explain
  the new model of the atom.

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Bright line spectrum

• When an atom ABSORBS energy, the electrons JUMP
  (LEAP) to a higher energy level. They are in the
  EXCITED STATE.
• When the electrons return to the normal level
  (GROUND STATE) , they emit energy as LIGHT. The
  light emitted light produce the bright line
  spectrum that is characteristic of each element.

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Electrons Surround the Nucleus

• Electrons surround the nucleus and travel at the
  speed of light
• They are found in only certain allowed energy
  levels or orbitals
• Electrons are at the lowest energy level the
  ground state.

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Different Energy States are Possible

• When the electrons in an atom become excited by
  absorbing energy from the surroundings, they jump
  to new higher energy levels.
• The excited state is less stable than the lower
  energy state therefore the electron falls back or
  returns to the lower energy ground state.

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Return to Ground State

• When electrons fall back a wave with a specific
  amount of energy is emitted called a photon. If
  we see color then the emission is in the visible
  range
• We perceive this as unique colors associated
  with particular elements

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Ground state configuration vs excited state
configuration

• 11Na 2-8-1 This is the ground state.
• If atom is excited some possible electron
  configuration would be
• 2-7-2 or 1-8-2 or 1-6-3
• The number of electrons must add to the atomic
  number but the order is not the one that appears
  in the periodic table.

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Indivisible Electron Nucleus Orbit Electron Cloud
Greek X
Dalton X
Thomson X
Rutherford X X
Bohr X X X
Wave X X X