Chapter 7 – The Quantum Mechanical Atom
In chapter 6 we saw the energy can be
transferred as heat.
In this chapter, we shall see that energy
can also be transferred as LIGHT
(RADIATION)
Many chemical reactions emit light as
they occur……
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http://www.lifesci.ucsb.edu/~biolum/chem/
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• Radiation (light) energy is carried
through space by WAVES
• Waves are: oscillations that move
outward from a disturbance
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Simplest Wave Motion
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Ways to describe the WAVE FORM (the form of the wave)
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Wavelength And Frequency
• Wavelength is the distance between any two identical
points in consecutive cycles
• Wavelength is denoted by the Greek letter  (lambda) and
is measured in nanometers and angstroms
• Frequency of a wave is the number of cycles of the wave
that pass through a point in a unit of time
• Frequency is denoted by the Greek letter v (nu) and is
measured in hertz
• Amplitude of a wave is its height: the distance from a line
of no disturbance through the center of the wave peak.
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• The product of frequency and wavelength
give the speed of light (c)
    c  3.00 108 m/s
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•With radiation, the disturbance is a vibrating electric charge
•As the electric charge jiggles, it produces an electric field
around it
•As the electric field pulses, it creates a magnetic field.
•As the magnetic field pulses, it gives rise to another electric
field pulse further away from the source
•The process continues….
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An Electromagnetic Wave
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The Electromagnetic Spectrum
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•
•
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The electromagnetic spectrum is largely invisible to the eye
However, we can feel some radiation through other senses
Sunburned skin is a sign of too much ultraviolet radiation
Materials vary in their ability to absorb or transmit different
parts of the electromagnetic spectrum
– Our bodies absorb visible light, but transmit most X-rays
– Window glass transmits visible light, but absorbs
ultraviolet radiation
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The Electromagnetic Spectrum
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• What we call light is a small slice of the
electromagnetic spectrum with wavelengths
between about 400 and 700 nm
• This is called the visible region because we can
“see” these wavelengths of the electromagnetic
spectrum
• Gamma rays, X rays, and ultraviolet radiation
have wavelengths shorter than the visible region
• Microwaves, infrared radiation, and radio waves
have wavelengths longer than visible light
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• The way a substance absorbs
electromagnetic radiation can be used to
characterize it
• For example, each substance absorbs a
uniquely different set of infrared
frequencies
• A plot of wavelengths absorbed versus the
absorption is called an infrared absorption
spectrum
• It can be used to identify a substance
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Infrared
absorption
spectrum of
wood
alcohol
(methanol).
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Continuous Spectra
• White light passed through a prism produces
a spectrum of rainbow colors in continuous
form. The different colors of light
correspond to different wavelengths and
frequencies.
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LINE SPECTRA
• When light is produced through an element,
a discontinuous spectrum is displayed.
• The pattern of lines produced by the light
emitted by excited atoms of an element is
call a line spectrum.
.
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Emission Spectrum of Helium
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Visible Spectrum of Hydrogen
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Line Spectra of Some Elements
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Planck’s Constant
• Planck proposed that the vibrating atoms in a heated solid
could absorb or emit electromagnetic energy only in
discrete amounts.
• The smallest amount of energy, a quantum, is given by:
E = hv
as Planck’s constant, h, has a value of 6.626 X 10-34 J s.
• Planck’s quantum hypothesis states that energy can be
absorbed or emitted only as a quantum or as whole
multiples of a quantum, thereby making variations
discontinuous, changes can only occur in discrete amounts.
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