chapter sevenprentice-hall ©2002slide 1 of 52 in chapter 6 we saw the energy can be transferred as...
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Chapter SevenPrentice-Hall ©2002 Slide 1 of 52
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……
Chapter 7 – The Quantum Mechanical Atom
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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)
m/s 10 3.00 8 c
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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
• 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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• 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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LINE SPECTRA
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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.