Electromagnetic Radiation

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Electromagnetic Radiation
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Figure 7.1
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Wave motion wave length and nodes
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Short wavelength --gt high frequency high
energy

• Long wavelength --gt
• small frequency
• low energy

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• Rank the following in order of increasing
  frequency
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• microwaves
• radiowaves
• X-rays
• blue light
• red light
• UV light
• IR light

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• Waves have a frequency
• Use the Greek letter nu, ?, for frequency, and
  units are cycles per sec
• All radiation ? ? c
• c velocity of light 3.00 x 108 m/sec
• Long wavelength --gt small frequency
• Short wavelength --gt high frequency

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• What is the wavelength of WONY?
• What is the wavelength of cell phone radiation?
  Frequency 850 MHz
• What is the wavelength of a microwave oven?
  Frequency 2.45 GHz

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Quantization of Energy
Light acts as if it consists of particles called
PHOTONS, with discrete energy.

• Energy of radiation is proportional to frequency

E h ?
h Plancks constant 6.6262 x 10-34 Js
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E h ?
Relationships
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• Rank the following in order of increasing photon
  energy
•  
• microwaves
• radiowaves
• X-rays
• blue light
• red light
• UV light
• IR light

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E h ?
What is the energy of a WONY photon?
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Energy of Radiation

• What is the energy of 1 mole of UV light with
  wavelength 230 nm?

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Energy of Radiation

• What is the energy of 1 mole of IR light with
  wavelength 1200 nm?

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Where does light come from?

• Excited solids emit a continuous spectrum of
  light
• Excited gas-phase atoms emit only specific
  wavelengths of light (lines)

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Light given off by solids
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Light given off by Excited Hydrogen Gas
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The Bohr Model of Hydrogen Atom

• Light absorbed or emitted is from electrons
  moving between energy levels
• Only certain energies are observed
• Therefore, only certain energy levels exist
• This is the Quanitization of energy levels

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Line Emission Spectra of Excited Atoms

• Excited atoms emit light of only certain
  wavelengths
• The wavelengths of emitted light depend on the
  element.

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Line Emission Spectra of Excited H Atoms
High E Short ? High ?
Low E Long ? Low ?
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Line Spectra of Other Elements
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Atomic Absorption and Emission
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Origin of Line Spectra
Balmer series
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• For H, the energy levels correspond to

Constant 2.18 x 10-18 J
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Each line corresponds to a transition

• Example n3 ? n 2

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Name ____________

1. _________
2. _________
3. _________
4. _________

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Quiz

• Q1. Emission line with longest wavelength
• Q2. Absorption line with highest frequency
• Q3. Emission line with lowest frequency
• Q4. Transition that leads to forming H

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Matter Waves

• All matter acts as particles and as waves.
• Macroscopic objects have tiny waves- not
  observed.
• For electrons in atoms, wave properties are
  important.
• deBroglie Equation

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Matter waves
Macroscopic object 200 g rock travelling at 20
m/s has a wavelength
Electron inside an atom, moving at 40 of the
speed of light
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Can see matter waves in experiments
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Heisenberg Uncertainty Principle

• Cant know both the exact location and energy of
  a particle
• So, for electrons, we DO know the energy well, so
  we dont know the location well

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Schrodingers Model of H

• Electrons act as standing waves
• Certain wave functions are allowed
• Wave behavior is described by wave functions ?
• ?2 describes the probability of finding the
  electron in a certain spot
• Also described as electron density

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Example Wavefunction

• Equation slightly simplified

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Its all about orbitals

• Each wavefunction describes a shape the electron
  can take, called an ORBITAL
• Allowed orbitals are organized by shells and
  subshells
• Shells define size and energy (n 1, 2, 3, )
• Subshells define shape (s, p, d, f, )
• Number of orbitals is different for each
  subshell
• s 1
• p 3
• d 5
• f 7

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Quantum Numbers