c18 JWBS043-Rogers September 13, 2010 11:29 Printer Name: Yet to Come
18 Experimental Determination of Molecular Structure
Acceptance, at least provisional acceptance, of atomic theory goes back two and
a half millennia, but it inevitably raises the question of what atoms look like. The
nature of the world around us—air, earth, fire, and water—is governed less by atomic
properties than by the physical and chemical properties of molecules. But acceptance
of molecular theory, a child of the nineteenth century, raises the same question:
What do molecules look like? Experimental evidence for the existence and physical
appearance of molecules comes from interactions of confined pure samples with their
environment, largely through energy transfer by the very broad energy spectrum of
electromagnetic radiation extending all the way from gamma and X rays to low-
energy radio waves.18.1 THE HARMONIC OSCILLATOR
A mass attached to a fixed beam by a spring can be set intooscillationby pulling
it down and letting it go (Fig. 18.1). An ordinary spring obeys Hooke’s law, at least
approximately:f=−kfz(t)wherefis the force exerted by the spring on the moving mass when it is displaced
by a distancezfrom the equilibrium distancez 0. The proportionality constantkfisConcise Physical Chemistry,by Donald W. Rogers
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