Chapter SevenPrentice-Hall ©2002Slide 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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Chapter SevenPrentice-Hall ©2002Slide 4 of 52 Radiation (light) energy is carried through space by WAVES Waves are: oscillations that move outward from a disturbance

Chapter SevenPrentice-Hall ©2002Slide 5 of 52 Simplest Wave Motion

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Chapter SevenPrentice-Hall ©2002Slide 7 of 52 Ways to describe the WAVE FORM (the form of the wave)

Chapter SevenPrentice-Hall ©2002Slide 8 of 52 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.

Chapter SevenPrentice-Hall ©2002Slide 9 of 52 The product of frequency and wavelength give the speed of light (c)

Chapter SevenPrentice-Hall ©2002Slide 10 of 52 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….

Chapter SevenPrentice-Hall ©2002Slide 11 of 52 An Electromagnetic Wave

Chapter SevenPrentice-Hall ©2002Slide 12 of 52 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

Chapter SevenPrentice-Hall ©2002Slide 13 of 52 The Electromagnetic Spectrum

Chapter SevenPrentice-Hall ©2002Slide 14 of 52 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

Chapter SevenPrentice-Hall ©2002Slide 15 of 52 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

Chapter SevenPrentice-Hall ©2002Slide 16 of 52 Infrared absorption spectrum of wood alcohol (methanol).

Chapter SevenPrentice-Hall ©2002Slide 17 of 52 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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Chapter SevenPrentice-Hall ©2002Slide 19 of 52 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.. LINE SPECTRA

Chapter SevenPrentice-Hall ©2002Slide 20 of 52 Emission Spectrum of Helium

Chapter SevenPrentice-Hall ©2002Slide 21 of 52 Visible Spectrum of Hydrogen

Chapter SevenPrentice-Hall ©2002Slide 22 of 52 Line Spectra of Some Elements

Chapter SevenPrentice-Hall ©2002Slide 23 of 52 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 X 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.